EXAM 2

Question 1

Brainstem and Its Functions

Answer 1

The brainstem consists of three main components:
* Midbrain
* Pons
* Medulla oblongata (most relevant in human physiology)The medulla oblongata is crucial in many physiological processes:
* It is the point where most of the parasympathetic output of the autonomic nervous system exits via the 10th cranial nerve (vagus nerve).
* It serves as the integration center for short-term blood pressure regulation (baroreceptor reflex), which plays a role in homeostasis.
* It contains the neuronal circuits responsible for generating basic respiratory rhythms.The brainstem is essential for life-supporting functions, including:
* Cardiovascular regulation
* Respiratory control
* Other autonomic functionsThe reticular formation:
* A diffuse collection of neurons spread throughout the brainstem, rather than a single discrete structure.
* It is involved in regulating consciousness and arousal.
* Damage to this region is fatal, emphasizing its role in fundamental physiological processes.
* Because the neurons are scattered, the reticular formation is not easily visualized in anatomical textbooks, making it distinct from clearly defined nuclei in the brain.

Question 2

The Cerebellum – “The Little Brain”

Answer 2

The cerebellum, also called the “little brain”, is located at the back of the brain and has multiple lobes with distinctive folds.Primary function:
* The cerebellum is not responsible for initiating movement (that is the role of the primary motor cortex).
* Instead, it modifies and fine-tunes movements, ensuring smooth coordination of skeletal muscle contractions.
* It is crucial for executing complex, multi-muscle movements in the correct sequence and intensity.Damage to the cerebellum can result in:
* Intention tremors – uncoordinated, shaky movements that worsen as an individual attempts to reach for an object.
* Difficulty in performing precise motor tasks, such as picking up an object smoothly.Rare cases of cerebellar agenesis (complete absence of the cerebellum):
* Some individuals are born without a cerebellum.
* This condition impacts not only movement but also other functions, including speech and cognition.
* NPR has documented cases of such individuals, showing how they adapt despite the absence of a cerebellum.Neural connections:
* The cerebellum receives sensory input and information from:
◦ The thalamus
◦ Various cortical regions
* It integrates these inputs to improve movement execution without directly initiating behavior.

Question 3

Answer 3

D

Question 4

Spinal Cord Structure and Organization

Answer 4

• The spinal cord is an elongated structure located within the vertebral canal, surrounded by the vertebrae for protection.
• It is a crucial part of the central nervous system (CNS), responsible for transmitting signals between the brain and the body.
  Key Structural Features:
• The spinal cord proper ends at the second lumbar vertebra (L2).
  ◦ Below this, nerve projections (cauda equina) extend further down the vertebral column.
• Gray vs. White Matter Arrangement:
  ◦ Unlike the brain, where gray matter is on the outermost layer and white matter is underneath, the spinal cord has the reverse arrangement:
  ▪ Gray matter is found in the interior in an H-shaped (or butterfly-shaped) structure.
  ▪ White matter surrounds the gray matter and consists of myelinated axons that transmit signals up and down the spinal cord.
• Functions of Gray vs. White Matter:
  ◦ Gray matter (interior): Contains neuronal cell bodies and synapses, responsible for processing and integrating information.
  ◦ White matter (exterior): Composed of myelinated axons, forming tracts that carry sensory (ascending) and motor (descending) signals between the brain and peripheral nervous system.

Question 5

Spinal Nerves and the Cauda Equina

Answer 5

• The spinal cord gives rise to 31 pairs of spinal nerves, which exit at various levels:
  ◦ Cervical nerves (topmost)
  ◦ Thoracic nerves
  ◦ Lumbar nerves
  ◦ Sacral nerves
  ◦ Coccygeal nerves (bottommost)
  Spinal Cord Termination at L2:
• The spinal cord proper ends at the second lumbar vertebra (L2).
• Above L2: Both neuronal cell bodies and axons are present.
• Below L2: Only myelinated axons remain, forming nerve projections.
  Cauda Equina (Latin: “Horse’s Tail”)
• Below L2, the spinal cord does not contain neuronal cell bodies, only long axons that extend downward.
• These axons exit at different levels (sacral and coccygeal regions) to innervate muscles in the legs, ankles, and feet.
• The cauda equina gets its name because the bundle of axons resembles the fine hairs of a horse’s tail.
• In anatomical displays (e.g., Body Worlds), the cauda equina can be seen splayed out like fibers, highlighting its distinct structure.

Question 6

Cauda Equina and Medical Procedures

Answer 6

• Cauda Equina (Latin: “Horse’s Tail”) is the bundle of neuronal axons below L2.
• This region contains no neuronal cell bodies, only axons extending to lower body muscles.
  Medical Importance:
• Spinal Tap (Lumbar Puncture):
  ◦ Used to extract cerebrospinal fluid (CSF) for analysis (e.g., bacteria, viruses).
  ◦ A needle is inserted into the cauda equina region to access CSF without damaging neurons.
• Epidural Injection:
  ◦ Delivers drugs directly into cerebrospinal fluid for pain relief (e.g., during childbirth).
  ◦ The needle moves past axons, which shift aside due to CSF, minimizing nerve damage.
  Why Below L2?
• No neuronal cell bodies, so the risk of permanent CNS damage is minimal.
• Higher spinal cord punctures could destroy neurons, leading to irreversible damage since CNS neurons have little to no regenerative ability.

Question 7

Answer 7

B

Question 8

Structure and Function of Spinal Cord Gray and White Matter

Answer 8

• The spinal cord gray matter has an “H” or butterfly shape.
• It consists of dorsal horns (toward the back) and ventral horns (toward the front).
  Dorsal Horns
• Contain sensory neurons responsible for receiving and processing sensory input.
• Function as the entry point for sensory information into the spinal cord.
  Ventral Horns
• Contain motor neurons responsible for movement.
• The cell bodies of motor neurons are located here.
• These neurons innervate skeletal muscles, enabling voluntary movement.
  White Matter
• Surrounds the gray matter and consists of axon tracts that transmit information.
• Ascending tracts (e.g., green-labeled regions): Carry sensory information up toward the brain.
• Descending tracts (e.g., violet-labeled regions): Transmit motor commands down from the brain to the body.
• Located primarily in the ventral region of the spinal cord.

Question 9

Dorsal Root Ganglia and Sensory Neuron Pathways

Answer 9

• Dorsal Root Ganglia (DRG): Clusters of neuronal cell bodies located in the peripheral nervous system (PNS).
• Function: Contain the cell bodies of sensory neurons, which relay sensory information to the spinal cord.
  Sensory Neuron Structure & Pathway
  1. Sensory receptors in the periphery (e.g., under the skin) detect stimuli.
  2. The sensory neuron’s axon extends toward its cell body located in the dorsal root ganglion.
  3. After passing the cell body in the DRG, the axon continues toward the spinal cord.
  4. It enters the dorsal horn of the spinal cord to synapse and transmit sensory information.
  Dorsal Roots
• Definition: Bundles of axons carrying sensory signals from the periphery into the spinal cord.
• Found in the dorsal root ganglia region.
• These sensory axons travel through the dorsal roots before synapsing in the dorsal horn of the spinal cord.

Question 10

What is the function of the ventral and dorsal roots, mixed spinal nerves, and how does the brainstem contribute to vital physiological functions?

Answer 10

• Ventral Roots:
  ◦ The ventral roots contain axons of motor neurons that carry information from the central nervous system (CNS) to the skeletal muscles.
  ◦ These axons extend outward to activate muscle contractions and execute voluntary movements.
• Dorsal Roots and Mixed Spinal Nerves:
  ◦ The dorsal root ganglia contain sensory neurons that send signals from the body to the CNS.
  ◦ A mixed spinal nerve is formed when the dorsal and ventral roots join. These mixed nerves contain both sensory and motor axons, enabling two-way communication between the body and the CNS.
  ◦ The spinal nerves transmit both sensory (afferent) and motor (efferent) signals, which allow for complex body functions.
• Brainstem and Homeostasis:
  ◦ The brainstem, particularly the medulla, is responsible for maintaining key physiological functions vital for survival, such as regulating blood pressure and respiration.
  ◦ For example, the baroreceptor reflex is an important homeostatic mechanism for regulating short-term blood pressure. The integration center for this reflex is located in the medulla, which processes sensory input and coordinates the appropriate motor response to maintain stable blood pressure.
  ◦ Basic respiratory rhythm is also generated in the medulla, coordinating the breathing cycle necessary for life.
  ◦ The brainstem contains a diffuse arrangement of neurons that helps regulate and calm overall brain activity, contributing to the broad regulation of vital life functions.
  Key Concepts:
• Ventral Roots: Motor neuron axons sending signals to muscles.
• Mixed Spinal Nerves: Spinal nerves that contain both sensory and motor axons, facilitating two-way communication.
• Brainstem: Essential for regulating vital functions like blood pressure and respiration; integrates reflexes like the baroreceptor reflex and generates respiratory rhythms.

Question 11

Answer 11

True

Question 12

Are motor neurons part of the CNS or the PNS, and what is the role of the dorsal root ganglia in the nervous system?

Answer 12

• Motor Neurons:
  ◦ Motor neurons are often described in the context of the Peripheral Nervous System (PNS) because most of their axons are located peripherally, extending to muscles and organs.
  ◦ However, their cell bodies are located in the Central Nervous System (CNS), specifically in the ventral horns of the spinal cord.
  ◦ This creates a debate among scientists regarding whether motor neurons should be classified as part of the CNS or PNS. Some argue that because the cell bodies are in the CNS, they should be considered part of it. However, there is no definitive answer, and the focus should be on understanding where the neuron components (axons and cell bodies) are located, rather than labeling them strictly as CNS or PNS.
• Dorsal Root Ganglia:
  ◦ The dorsal root ganglia contain the cell bodies of afferent neurons (sensory neurons), not motor neurons.
  ◦ These neurons transmit sensory information from the body to the CNS, carrying input from the periphery (e.g., touch, pain, temperature).
  ◦ The confusion arises from the term “dorsal root,” but it is important to note that the ganglia associated with the dorsal roots primarily house afferent neuron cell bodies.
  Key Concepts:
• Motor Neurons: Cell bodies in CNS, axons in PNS.
• Dorsal Root Ganglia: Contain cell bodies of afferent (sensory) neurons

Question 13

What is the structure and function of the somatic nervous system, and how does it relate to the autonomic nervous system (ANS), including its subdivisions and neurotransmitter activity?

Answer 13

• Somatic Nervous System:
  ◦ The somatic nervous system (SNS) includes all the motor neurons responsible for innervating skeletal muscles.
  ◦ These motor neurons are essential for voluntary movement control, and their function is critical for activities such as muscle contraction.
  ◦ A key feature of the SNS is that motor neurons extend a singular axon from their cell body in the ventral horn of the spinal cord directly to a skeletal muscle, where they form a chemical synapse. There are no intervening synapses in this pathway.
  ◦ The axons can be extremely long, especially in tall individuals (e.g., an NBA player), where the motor neurons innervating the feet extend from the spinal cord at L2 all the way down the body to the legs.
  ◦ Neurotransmitter: The neurotransmitter released at the synapse between motor neurons and skeletal muscles is acetylcholine (ACh). This neurotransmitter is excitatory, meaning that when it is released, it triggers the muscle to contract.
• Autonomic Nervous System (ANS):
  ◦ The ANS controls involuntary functions and is divided into two primary subdivisions:
  1. Sympathetic Division:
  * Active during states of excitement or exercise, and is responsible for the fight or flight response.
  2. Parasympathetic Division:
  * Active when the body is at rest, supporting activities such as digestion and kidney filtration.
  ◦ Both divisions of the ANS regulate functions like heart rate, digestion, and respiratory rate, but the sympathetic division tends to prepare the body for action, while the parasympathetic division promotes rest and recovery.
• Inhibition of Muscle Contraction:
  ◦ To inhibit a muscle from contracting, the motor neuron itself must be inhibited in the CNS.
  ◦ For example, to relax the triceps while performing a bicep curl, the motor neuron innervating the triceps is inhibited.
  ◦ There is no inhibitory neurotransmitter released directly onto the muscle itself. Inhibition is controlled at the CNS level by modulating the motor neurons.
  Key Concepts:
• Somatic Nervous System: Controls voluntary skeletal muscle movement via direct innervation by motor neurons.
• Neurotransmitter in SNS: Acetylcholine (ACh) is excitatory and triggers muscle contraction.
• ANS Subdivisions: Sympathetic (fight or flight) and Parasympathetic (rest and digest).
• Muscle Relaxation: Achieved by inhibiting motor neurons in the CNS, not by a separate inhibitory neurotransmitter at the muscle.

Question 14

How does the autonomic nervous system (ANS) differ from the somatic nervous system (SNS), and what are the functions and neurotransmitter roles in the sympathetic and parasympathetic divisions?

Answer 14

• Difference Between SNS and ANS:
  ◦ The somatic nervous system (SNS) is simpler as it involves a single motor neuron extending from the spinal cord directly to the target (skeletal muscle) for voluntary control.
  ◦ In contrast, the autonomic nervous system (ANS) involves two neurons in series: the pre-ganglionic fiber and the post-ganglionic fiber.
  ▪ The pre-ganglionic fiber has its cell body in the CNS (spinal cord) and releases acetylcholine (ACh) onto the post-ganglionic fiber, which extends out to target tissues.
  ▪ The post-ganglionic fiber innervates various targets like smooth muscle, cardiac muscle, glands, and neurons in the enteric nervous system.
• Autonomic Nervous System Divisions:
  ◦ Sympathetic Division (Fight or Flight):
  ▪ Activated during stress, excitement, or exercise.
  ▪ Releases epinephrine (adrenaline) from the post-ganglionic fibers to increase heart rate, cardiac muscle contraction, and blood flow to essential muscles.
  ▪ It also inhibits digestion, lowers blood flow to the gut, and suppresses reproductive behaviors.
  ◦ Parasympathetic Division (Rest and Digest):
  ▪ Activated during restful states.
  ▪ Releases acetylcholine from post-ganglionic fibers to lower heart rate, decrease heart contraction force, and increase digestion and nutrient absorption from the gut.
• Dual Innervation:
  ◦ Most organs receive input from both the sympathetic and parasympathetic branches, which generally work in opposition to maintain homeostasis.
  ◦ Example: The heart and lungs are controlled by both divisions.
  ▪ The sympathetic system increases heart rate and respiratory rate, while the parasympathetic system decreases them during rest.
• Neurotransmitters and Effects:
  ◦ The pre-ganglionic fibers release acetylcholine (ACh), which always has an excitatory effect on the post-ganglionic fiber.
  ◦ The post-ganglionic fibers release different neurotransmitters depending on the division:
  ▪ Sympathetic Division: Releases epinephrine (adrenaline), which can be excitatory, affecting various target organs.
  ▪ Parasympathetic Division: Releases acetylcholine (ACh), which can be either excitatory or inhibitory, depending on the context and target tissue.
  Key Concepts:
• SNS vs ANS: SNS uses a single motor neuron; ANS uses two neurons in series (pre-ganglionic and post-ganglionic).
• Sympathetic Division (Fight or Flight): Uses epinephrine for stress-related body functions.
• Parasympathetic Division (Rest and Digest): Uses acetylcholine for rest-related functions, including digestion.
• Dual Innervation: Both branches work together to regulate body systems in a balanced manner.

Question 15

What are the physiological effects of activating the sympathetic and parasympathetic nervous systems, and how can drugs influence these systems?

Answer 15

• Sympathetic Nervous System Activation (Fight or Flight):
  ◦ Increases Heart Rate: To pump more blood and oxygen to muscles.
  ◦ Dilates Pupils: To take in more light, improving vision to detect threats.
  ◦ Inhibits Digestion: Energy is redirected to essential functions like muscle activity.
  ◦ Inhibits Nasal Secretion and Saliva Production: Reduces less critical bodily functions during stress.
  ◦ Stimulates Other Functions: Increases blood flow to muscles and decreases blood flow to the gut.
• Parasympathetic Nervous System Activation (Rest and Digest):
  ◦ Decreases Heart Rate: Reduces energy consumption when the body is at rest.
  ◦ Stimulates Digestion: Increases digestive function, allowing nutrient absorption.
  ◦ Promotes Saliva Production: Supports the digestive process.
  ◦ Enhances Restorative Processes: Promotes recovery and energy conservation.
• Drugs and Their Effects on the Nervous System:
  ◦ Certain drugs can upregulate or downregulate the functions of the sympathetic or parasympathetic systems.
  ◦ Example: A drug may slow down the heart rate but also produce other effects, such as stimulating digestion or reducing saliva production, depending on the neurotransmitters it affects.
  ◦ Sympathetic Drugs: Typically speed up heart rate, inhibit digestion, and increase alertness.
  ◦ Parasympathetic Drugs: Typically slow heart rate, enhance digestion, and promote relaxation.
• Neurotransmitters and Receptors:
  ◦ The full effects of these drugs and systems will be understood in more detail when focusing on neurotransmitters and the receptors involved in these responses.
  Key Concepts:
• Sympathetic System: Prepares the body for stress, increasing heart rate, dilating pupils, and inhibiting digestion.
• Parasympathetic System: Promotes relaxation, decreases heart rate, stimulates digestion.
• Drugs: Can modulate these systems by targeting specific neurotransmitters, leading to various physiological effects.

Question 16

What are the key anatomical differences between the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS)?

Answer 16

• Sympathetic Division:
  ◦ Output Location: Pre-ganglionic neurons exit between the first thoracic (T1) and second lumbar (L2) regions of the spinal cord.
  ◦ Ganglia: Post-ganglionic neuron cell bodies are located close to the spinal cord, in structures called the sympathetic trunk, which runs alongside the spinal cord. These ganglia give the trunk a beaded appearance due to the clusters of cell bodies.
  ◦ Sympathetic Trunk: The sympathetic trunk extends the length of the spinal cord and is involved in the fight-or-flight response.
• Parasympathetic Division:
  ◦ Output Location: Pre-ganglionic neurons exit from either the brainstem (via cranial nerves) or the sacral region of the spinal cord.
  ◦ Ganglia: The post-ganglionic neuron cell bodies are located near or within the target organs (e.g., heart, lungs, digestive organs). Unlike the sympathetic division, parasympathetic ganglia do not form chains alongside the spinal cord.
  ◦ Cranial Nerve X (Vagus Nerve): A major output route, responsible for about 75% of parasympathetic output, innervating multiple target organs throughout the body.
  Key Differences:
• Sympathetic: Pre-ganglionic neurons originate in the T1-L2 spinal cord regions, and ganglia are near the spinal cord.
• Parasympathetic: Pre-ganglionic neurons originate from the brainstem or sacral spinal cord, with ganglia located close to the target organs.
  Key Concept:
• The sympathetic system prepares the body for stressful or emergency situations, while the parasympathetic system promotes rest and recovery. The sympathetic nervous system uses a two-neuron pathway with ganglia close to the spinal cord, while the parasympathetic system uses ganglia near target organs.

Question 17

How do the sympathetic and parasympathetic divisions of the autonomic nervous system interact, and how does their activity change depending on the body’s needs?

Answer 17

• Constant Activity: Both the sympathetic and parasympathetic divisions are always active to some degree. Their activity levels fluctuate depending on the body’s needs and the situation at hand.
• Fight or Flight Response:
  ◦ During stress or danger (fight or flight), the sympathetic division becomes more active. However, it’s not that the parasympathetic division is turned off completely; instead, there is a shift in activity between the two systems.
  ◦ The sympathetic division predominates, triggering physiological responses like increased heart rate, dilated pupils, and enhanced blood flow to muscles.
• Rest and Digest:
  ◦ At rest or during relaxed states, the parasympathetic division becomes more dominant. It helps the body focus on processes like digestion, lowering the heart rate, and promoting restorative functions.
• Relative Predominance:
  ◦ The balance between the two systems is dynamic, with one division predominating over the other depending on the body’s current needs, but both are never completely inactive at any time.
  Key Concept:
• The sympathetic and parasympathetic branches are always active, and their relative activity levels adjust based on environmental and physiological demands.

Question 18

Answer 18

Question 19

Answer 19

Question 20

What is the role of the adrenal medulla in the sympathetic nervous system, and how does it contribute to the fight or flight response?

Answer 20

• Adrenal Medulla: The adrenal medulla is a modified sympathetic ganglion located inside the adrenal gland. It contains specialized neurons called chromaffin cells, which lack axons, making them unique. These cells are part of the autonomic nervous system but release neurohormones rather than neurotransmitters.
• Catecholamine Release:
  ◦ The chromaffin cells receive input from pre-ganglionic sympathetic neurons, which release acetylcholine onto the cells. In response, the chromaffin cells release catecholamines, primarily epinephrine (adrenaline) (about 80%) and norepinephrine (20%).
  ◦ These catecholamines enter the bloodstream and circulate to act on various target cells, including those in the heart, blood vessels, and muscles, to mediate the fight or flight response (e.g., increased heart rate and force of contraction, vasoconstriction).
• Difference between Neurotransmitter and Neurohormone:
  ◦ Neurotransmitters like acetylcholine act on local target cells via synapses.
  ◦ Neurohormones, like epinephrine and norepinephrine, circulate through the blood to act on distant target cells.
• Epinephrine vs. Norepinephrine:
  ◦ Both epinephrine and norepinephrine have similar effects on target cells, such as increasing heart rate and blood flow to muscles, but they differ by one enzymatic step in their synthesis.
• Why “Epinephrine” is Preferred:
  ◦ Epinephrine is the preferred scientific term, as it accurately refers to the catecholamine produced in the adrenal medulla, which is located above the kidneys. The term “adrenaline” is more colloquial but still refers to the same substance.
  Key Concept:
• The adrenal medulla contributes to the fight or flight response by releasing epinephrine and norepinephrine, which circulate through the bloodstream to activate physiological changes.

Question 21

What is the role of acetylcholine and catecholamines in the autonomic nervous system?

Answer 21

• Acetylcholine (ACh):
  ◦ First Synapse in Both Divisions: Acetylcholine is the neurotransmitter released by pre-ganglionic fibers onto post-ganglionic cells in both the sympathetic and parasympathetic divisions of the autonomic nervous system.
  ◦ Parasympathetic Division: In the parasympathetic branch, acetylcholine is released by post-ganglionic fibers onto targets like smooth muscle, cardiac muscle, and neurons in the gastrointestinal (GI) tract. This contributes to rest and digest activities.
• Catecholamines (Epinephrine & Norepinephrine):
  ◦ Sympathetic Division: In the sympathetic division, chromaffin cells of the adrenal medulla and post-ganglionic fibers release catecholamines, specifically epinephrine (adrenaline) and norepinephrine (noradrenaline). These hormones circulate through the bloodstream to activate the fight or flight response, affecting targets like the heart, blood vessels, and muscles.
  ◦ Key Difference: The release of catecholamines is distinctive of the sympathetic division, whereas the parasympathetic division relies solely on acetylcholine for both its pre- and post-ganglionic neurotransmission.
  Key Concept:
• Acetylcholine plays a role in both divisions of the autonomic nervous system, while catecholamines (epinephrine and norepinephrine) are released exclusively in the sympathetic division.

Question 22

Which neuronal cells of the peripheral nervous system (PNS) do not synthesize and secrete acetylcholine, and what neurotransmitters are released by these cells?

Answer 22

• The sympathetic post-ganglionic neurons of the peripheral nervous system do not release acetylcholine. Instead, they predominantly release norepinephrine (a catecholamine) as their neurotransmitter to target tissues.
• Additionally, chromaffin cells (found in the adrenal medulla) release epinephrine (adrenaline) directly into the bloodstream, contributing to the body’s stress response.
• Acetylcholine is primarily used by pre-ganglionic fibers in both sympathetic and parasympathetic divisions, as well as by parasympathetic post-ganglionic neurons.
  Key Concepts:
• Sympathetic Post-ganglionic Neurons: Release norepinephrine, not acetylcholine.
• Chromaffin Cells: Release epinephrine into the bloodstream.
• Acetylcholine: Released by pre-ganglionic neurons in both divisions and post-ganglionic neurons in the parasympathetic division.

Question 23

What are the types of acetylcholine receptors, and how are they distinguished?

Answer 23

There are two main types of acetylcholine receptors:
1. Nicotinic Acetylcholine Receptors
◦ Found on skeletal muscle at neuromuscular junctions and on dendrites and cell bodies of neurons in parasympathetic and sympathetic ganglia.
◦ Activated by nicotine (an agonist) and stimulate signaling.
◦ Antagonists (e.g., chemicals that bind but don’t stimulate receptors) can block signaling, such as curare.
2. Muscarinic Acetylcholine Receptors
◦ Found on target cells (e.g., cardiac muscle, smooth muscle) innervated by post-ganglionic parasympathetic neurons.
◦ Activated by acetylcholine but not by nicotine.
◦ Antagonist example: Atropine (blocks muscarinic receptors, used to dilate pupils in eye exams, causing discomfort).
Adrenergic Receptors:
* These are activated by catecholamines (e.g., epinephrine and norepinephrine) and are found on target tissues in the sympathetic nervous system.
* They are divided into alpha and beta subtypes, each with further variations, important for fine-tuning the body’s response to stress (e.g., fight or flight).
Key Terms:
* Nicotinic receptors: Activated by nicotine.
* Muscarinic receptors: Activated by acetylcholine but not nicotine; involved in parasympathetic responses.
* Adrenergic receptors: Activated by epinephrine and norepinephrine, involved in sympathetic responses.

Question 24

What type of receptors are found on the post-ganglionic neurons in the parasympathetic division of the autonomic nervous system?

Answer 24

• Receptors on Postganglionic Parasympathetic Neurons:
  ◦ Postganglionic neurons in the parasympathetic division of the autonomic nervous system have nicotinic acetylcholine receptors located on their dendrites and cell bodies.
  ◦ These receptors are nicotinic because they are activated by acetylcholine (ACh), which is released from the preganglionic neuron.
  ◦ The presence of nicotinic receptors is important because these receptors mediate the synaptic transmission between the preganglionic and postganglionic neurons in both the sympathetic and parasympathetic divisions of the autonomic nervous system.
• Key Concept:
  ◦ In the parasympathetic division, the neurotransmitter acetylcholine (ACh) is released by preganglionic neurons, which then binds to nicotinic receptors on the postganglionic neurons to propagate the signal further down the line.
  Takeaway:
• The correct receptor type on postganglionic parasympathetic neurons is nicotinic acetylcholine receptors, which are activated by acetylcholine released by the preganglionic neurons.

Question 25

Why do we have receptors for nicotine, even though nicotine isn’t a normal neurotransmitter in the body?

Answer 25

Nicotine as an Acetylcholine Receptor Agonist:
* Nicotine, the active compound in tobacco products, acts as an agonist at acetylcholine receptors. This means nicotine binds to these receptors and activates them in a similar way to acetylcholine, the body’s natural neurotransmitter.
* Nicotine specifically activates nicotinic acetylcholine receptors (nAChRs), which are normally activated by acetylcholine, particularly in the autonomic nervous system and neuromuscular junctions.Why Do We Have Receptors for Nicotine?
* Our bodies evolved to have acetylcholine receptors for important physiological processes like muscle contraction and neurotransmission within the nervous system.
* Nicotine just happens to be a substance that mimics acetylcholine and can activate these receptors, even though it’s not a natural neurotransmitter in the body.Important Point:
* Although nicotine can activate acetylcholine receptors, it is not used by the body as a normal neurotransmitter. Instead, it’s an external substance that affects the same receptors, leading to various effects such as increased alertness and addiction in individuals who use tobacco products.Takeaway:
* The presence of acetylcholine receptors is a result of evolutionary processes, and nicotine’s ability to activate these receptors is simply a quirk of chemistry, not a normal part of our biological processes.

Question 26

Why are receptor subtypes important in determining the effects of catecholamines like epinephrine on target tissues?

Answer 26

Receptor Subtypes and Their Role:
* The receptor subtype is the key factor that determines the physiological response when a catecholamine (like epinephrine) interacts with a target tissue. Different tissues can have different responses to the same chemical signal, based on the specific subtype of adrenergic receptors present on their cells.
* Adrenergic receptors are divided into two main categories: alpha and beta receptors, each having further subtypes (like alpha-1, alpha-2, beta-1, beta-2, etc.).
* The subtype of receptor on a target tissue will decide whether the tissue responds by constricting (e.g., smooth muscle contraction) or relaxing (e.g., smooth muscle relaxation).Fight or Flight Example:
* For instance, when the body is under stress or preparing for fight or flight, epinephrine is released. The way it affects different tissues depends on the receptor subtype:
◦ In vascular smooth muscle leading to the legs, epinephrine may activate a receptor subtype that causes relaxation, allowing more blood flow to the legs for quick action.
◦ Conversely, in the gut vasculature, epinephrine might activate a receptor subtype that causes constriction, reducing blood flow to the gut, as digestion is less of a priority during stressful situations.Key Point:
* Epinephrine doesn’t cause a uniform response in all tissues. Its effects are context-dependent and receptor-specific. The same neurotransmitter can cause different responses depending on the type of receptor it interacts with.Takeaway:
* Understanding the specific receptor subtypes on different tissues is essential for explaining how the body adapts to various situations. The ability to both vasoconstrict and vasodilate based on receptor activation is a crucial aspect of the body’s adaptive response to stress, exercise, and other demands.

Question 27

Answer 27

B

Question 28

Structure and Function of the Cauda Equina and Spinal Cord Anatomy

Answer 28

Cauda Equina:
* Below L2, the spinal cord transitions into the cauda equina, a bundle of neuronal axons rather than a solid cord.
* It consists of lumbar (L3-L5), sacral, and coccygeal spinal nerves.Clinical Relevance:
* This region is targeted for epidural injections or lumbar punctures to access cerebrospinal fluid (CSF) for drug delivery or diagnostic testing (e.g., for infections like bacterial or viral meningitis).
* A spinal tap works by inserting a needle into this region, where CSF pushes axons aside, preventing direct neural damage.
* There are no neuronal cell bodies here, so inserting a needle does not kill neurons, reducing the risk of permanent CNS damage.Spinal Cord Anatomy:
* The gray matter of the spinal cord has an H-shaped or butterfly-like structure.
◦ Dorsal horns (posterior): Contain sensory neurons (afferent input).
◦ Ventral horns (anterior): Contain motor neurons (efferent output).
* White matter contains axonal tracts:
◦ Ascending tracts (green): Carry sensory signals to the brain.
◦ Descending tracts (violet): Carry motor commands from the brain.Dorsal Root Ganglia & Sensory Pathways:
* Dorsal root ganglia (DRG): Contain cell bodies of sensory neurons (afferent neurons), which carry information toward the CNS.
* Dorsal roots: Contain axons of sensory neurons, transmitting signals to the spinal cord.
* Ventral roots: Contain axons of motor neurons, transmitting signals from the CNS to muscles.
* Mixed spinal nerves: Formed where dorsal and ventral roots merge, containing both sensory and motor axons.True/False Concept Check:
* True: Spinal nerves contain both afferent (sensory) and efferent (motor) neurons.
* False: Dorsal root ganglia contain efferent neurons. They actually contain afferent (sensory) neurons.Motor Neurons & Classification Debate:
* Motor neurons are often classified in the peripheral nervous system (PNS) because their axons extend outside the CNS.
* However, their cell bodies reside in the CNS, leading some to argue they should be considered part of the CNS.Neural Fragility & Protection:
* Spinal cord neurons are highly delicate; injuries can cause permanent paralysis.
* Protected by vertebral bones to minimize damage.Peripheral Nervous System (PNS) Organization:
* Includes somatic nervous system, which consists of motor neurons that innervate skeletal muscles.

Question 29

What is the basic anatomy and structure of the spinal cord, including the arrangement of gray and white matter, spinal nerves, and the concept of the “cauda equina”?

Answer 29

• The spinal cord is an elongated structure located within the vertebral canal, which is a fluid-filled region surrounded by vertebrae.
• The spinal cord proper ends at the second lumbar vertebra (L2). Below L2, projections (axons) continue down without neuronal cell bodies.
• The gray matter in the spinal cord is located centrally in an H- or butterfly-shaped structure, and it contains the cell bodies of neurons.
• The white matter surrounds the gray matter and consists of myelinated axon tracts that carry signals up and down the spinal cord.
• The spinal cord has 31 pairs of spinal nerves that exit from different regions, from top to bottom: cervical, thoracic, lumbar, sacral, and coccygeal nerves.
• These spinal nerves are paired, with each pair branching off the spinal cord at their respective intervals.
• Below the L2 level, there are no more neuronal cell bodies, only axons. This area is called the “cauda equina” (Latin for “horse’s tail”) because the axons resemble fine tail-like hairs when spread out.
• The axons below L2 extend to intervene muscles of the legs and ankles, with the neuronal cell bodies located at higher levels of the spinal cord.
  Key Concepts:
• Spinal Cord Structure: Gray matter inside, white matter outside.
• Spinal Nerves: 31 pairs exiting from cervical to coccygeal regions.
• Cauda Equina: Axons below L2 without neuronal cell bodies, named for its resemblance to a horse’s tail.

Question 30

What is the function and anatomy of the reticular formation, cerebellum, and the effects of cerebellum damage?

Answer 30

• Reticular Formation:
  ◦ The reticular formation is a collection of neurons spread throughout the brainstem.
  ◦ This structure plays a critical role in maintaining consciousness and regulating vital functions.
  ◦ Damage to the reticular formation can be fatal, as it is involved in life-sustaining processes like breathing and heart rate regulation.
  ◦ Unlike a distinct nucleus, the reticular formation does not have a specific anatomical location; it is a widespread group of neurons whose axons connect with various structures in the brainstem.
• Cerebellum (Little Brain):
  ◦ The cerebellum, often referred to as the “little brain,” is located at the back of the brain and has multiple lobes and folds.
  ◦ It is primarily known for its role in modifying and smoothing out complex movements.
  ◦ While the primary motor cortex initiates movement, the cerebellum helps coordinate and refine movements that involve multiple muscles working together.
  ◦ Intention Tremors: Damage to the cerebellum can result in intention tremors, which are tremors that worsen as a person tries to perform a movement.
  ▪ For example, when reaching for an object like a phone, the person may experience tremors due to a lack of coordination in muscle contraction, making it difficult to execute smooth, controlled movements.
• Absence of the Cerebellum:
  ◦ Some individuals are born without a cerebellum. While this condition severely impacts their motor coordination, the cerebellum also has a wider range of functions beyond movement.
  ◦ The cerebellum integrates input from various brain regions, including the thalamus and cortical regions, to improve the execution of behaviors.
  ◦ Damage to or absence of the cerebellum can affect more than just movement, influencing several aspects of daily life and behavior.
  Key Concepts:
• Reticular Formation: Vital for consciousness and life-sustaining functions; widespread neuronal network.
• Cerebellum: Smooths out and coordinates complex movements; damage leads to tremors and clumsy movement.
• Function beyond Movement: The cerebellum integrates sensory and cortical inputs to optimize behavior, not just movement.

Question 31

Answer 31

Question 32

Answer 32

agonist

Question 33

Answer 33

Step-by-Step Guide to Break Down the Question:

1. Identify the Key Symptoms:
   
   • Severe asthma attack: This means bronchoconstriction is happening.
   • Wheezing, rapid breathing, cyanosis: These are signs that the airways are constricted, leading to difficulty breathing and inadequate oxygenation.
2. What is the Goal of Treatment?
   
   • Bronchodilation: The goal is to relax the muscles around the airways to open them up and allow more air to pass through, easing the breathing process.
3. Identify the Types of Receptors Involved:
   
   • Adrenergic receptors (specifically beta-2 receptors): Activation of these receptors causes bronchodilation, which is what you want in asthma.
   • Muscarinic receptors: These receptors, when activated, generally cause bronchoconstriction, which would make asthma worse.
   • Nicotinic receptors: These are involved in muscle contraction (especially skeletal muscles), not specifically for regulating airway smooth muscle.
4. Go Through Each Option:
   
   • A) An adrenergic receptor agonist:
     
     • Adrenergic agonists (specifically beta-2 agonists) relax smooth muscle, causing bronchodilation and improving airflow. This is the correct choice for asthma.
   • B) A muscarinic AChR agonist:
     
     • Muscarinic agonists cause bronchoconstriction (contraction of the smooth muscle), which would worsen the asthma attack. This is incorrect.
   • C) A nicotinic AChR antagonist:
     
     • Nicotinic antagonists block nicotinic receptors, which primarily affect skeletal muscle and are not relevant for asthma. This would not help with bronchodilation. This is incorrect.
   • D) An adrenergic receptor antagonist:
     
     • Adrenergic antagonists (such as beta-blockers) would block the beta-2 receptors, which are responsible for bronchodilation. This would lead to bronchoconstriction, making the asthma worse. This is incorrect.
   • E) A nicotinic AChR agonist:
     
     • Nicotinic agonists would stimulate muscle contraction, but these do not affect airway smooth muscles in the same way adrenergic or muscarinic receptors do. This is incorrect for asthma treatment.

###

Answer 34

Question 35

Answer 35

Question 36

Answer 36

Question 37

Answer 37

Question 38

Why is the fight-or-flight (sympathetic) response more widespread compared to the parasympathetic response?

Answer 38

The sympathetic nervous system produces a more widespread and long-lasting response compared to the parasympathetic nervous system due to the following key reasons:

1. Role of the Adrenal Medulla:
   
   • The adrenal medulla secretes catecholamines (epinephrine and norepinephrine) directly into the bloodstream.
   • These act as neurohormones, circulating throughout the body, leading to a more widespread and prolonged effect.
   • The parasympathetic nervous system does not have an analogous endocrine structure to release hormones systemically.
2. Neurotransmitter Release and Effect:
   
   • Sympathetic responses involve both neurotransmitters (norepinephrine at synapses) and hormones (epinephrine and norepinephrine from the adrenal medulla).
   • Parasympathetic responses rely only on acetylcholine, which is released at local synapses, leading to more discrete, localized effects.
3. Dual Innervation Misconception:
   
   • While most organs receive both sympathetic and parasympathetic innervation, the widespread effect of the sympathetic system is not due to greater innervation, but rather due to hormonal circulation and neurotransmitter properties.
4. Neurotransmitter Characteristics:
   
   • Catecholamines (epinephrine and norepinephrine) are hydrophilic and circulate in the bloodstream, contributing to the widespread and longer-lasting response.
   • Acetylcholine, used by the parasympathetic system, is only released at chemical synapses, limiting its effects to localized target tissues.

Thus, the key reason for the widespread effect of the sympathetic system is the release of neurohormones by the adrenal medulla, which circulate through the bloodstream, unlike the localized neurotransmitter release in the parasympathetic system.

Question 39

How do neurotransmitters differ between the somatic, sympathetic, and parasympathetic nervous systems?

Answer 39

The release and function of neurotransmitters vary between the somatic, sympathetic, and parasympathetic nervous systems:

1. Somatic Nervous System:

• Neurotransmitter: Acetylcholine (ACh)
• Target: Skeletal muscle
• Effect: Binds to nicotinic receptors, causing muscle contraction.

2. Autonomic Nervous System (ANS):

The ANS consists of the sympathetic and parasympathetic divisions, each with distinct neurotransmitter release patterns.

A. Parasympathetic Nervous System:

• Neurotransmitter: Acetylcholine (ACh)
• Pathway:
  
  • Pre-ganglionic neuron → Post-ganglionic neuron: ACh (acts on nicotinic receptors).
  • Post-ganglionic neuron → Target organ: ACh (acts on muscarinic receptors).
• Effect: Localized and restorative functions (e.g., decreased heart rate, digestion).

B. Sympathetic Nervous System:

• Neurotransmitters: Norepinephrine (NE) and Epinephrine (EPI)
• Pathway:
  
  • Pre-ganglionic neuron → Post-ganglionic neuron: ACh (acts on nicotinic receptors).
  • Post-ganglionic neuron → Target organ: Norepinephrine (NE) (acts on adrenergic receptors).
  • Adrenal Medulla → Bloodstream: Epinephrine (EPI) is released from chromaffin cells, acting as a neurohormone.
• Effect: Widespread and long-lasting responses (e.g., increased heart rate, bronchodilation, energy mobilization).

Key Takeaways:

• Acetylcholine (ACh) is the primary neurotransmitter in the somatic and parasympathetic nervous systems and is also used in sympathetic ganglia.
• Norepinephrine (NE) is released by post-ganglionic neurons in the sympathetic system, while the adrenal medulla releases epinephrine (EPI) into the bloodstream.
• The parasympathetic system uses ACh at all synapses, leading to localized effects, whereas the sympathetic system uses both neurotransmitters and neurohormones, creating widespread activation.

Question 40

What are the key receptor types in the autonomic nervous system, and how are they activated?

Answer 40

The autonomic nervous system (ANS) utilizes different receptor types based on neurotransmitter binding. These receptors can be classified as nicotinic, muscarinic, and adrenergic receptors:

1. Nicotinic Receptors (Nicotinic Acetylcholine Receptors - nAChRs)

• Location: Found in autonomic ganglia (both sympathetic and parasympathetic) and the neuromuscular junction.
• Activated by:
  
  • Acetylcholine (ACh) (the natural ligand)
  • Nicotine (an exogenous agonist)
• Function: Excitatory—causes depolarization and rapid signal transmission in both autonomic divisions.

2. Muscarinic Receptors (Muscarinic Acetylcholine Receptors - mAChRs)

• Location: Found on target organs of the parasympathetic nervous system.
• Activated by:
  
  • Acetylcholine (ACh) (the natural ligand)
  • Muscarine (a mushroom toxin, exogenous agonist)
• Function: Modulates parasympathetic responses, such as decreasing heart rate and stimulating digestion.
• Not activated by: Nicotine (unlike nicotinic receptors).

3. Adrenergic Receptors (Activated by Catecholamines - Epinephrine & Norepinephrine)

• Location: Found on target organs of the sympathetic nervous system.
• Activated by:
  
  • Norepinephrine (NE) (released by postganglionic sympathetic neurons)
  • Epinephrine (EPI) (released by the adrenal medulla into the bloodstream)
• Subtypes:
  
  • Alpha receptors (α1, α2) – Typically cause vasoconstriction and increased blood pressure.
  • Beta receptors (β1, β2, β3) – Typically cause increased heart rate, bronchodilation, and metabolic effects.
• Function: Modulates sympathetic “fight-or-flight” responses, such as increasing heart rate and dilating airways.

4. Receptor Antagonists (Blocking Agents):

• Some substances can bind to receptors without activating them, effectively blocking neurotransmitter action.
• Example: Nicotinic and muscarinic receptors can be blocked by specific antagonists, preventing normal autonomic function.

Key Takeaways:

• Nicotinic receptors are found in ganglia and activated by ACh & nicotine.
• Muscarinic receptors are found on parasympathetic targets and activated by ACh & muscarine (not nicotine).
• Adrenergic receptors are found on sympathetic targets and activated by norepinephrine & epinephrine.
• Alpha and beta adrenergic subtypes exist, but memorizing their individual functions is less important than recognizing their general role in sympathetic signaling.

Question 41

Why do different tissues respond differently to epinephrine, and how does this relate to adrenergic receptor types?

Answer 41

Different tissues respond differently to epinephrine because they express different adrenergic receptor subtypes, which trigger distinct physiological effects when activated.

Key Adrenergic Receptor Types & Effects:

• Alpha-1 (α1) Receptors: Found in blood vessels of the gut, skin, and kidneys → Vasoconstriction (reduces blood flow).
• Alpha-2 (α2) Receptors: Found in presynaptic nerve terminals → Inhibits norepinephrine release (negative feedback mechanism).
• Beta-1 (β1) Receptors: Found in the heart → Increases heart rate and contractility (supports fight-or-flight response).
• Beta-2 (β2) Receptors: Found in bronchioles and skeletal muscle blood vessels → Bronchodilation & vasodilation (enhances oxygen delivery).

Example – Fight-or-Flight Response:

• Skeletal muscles need more blood flow → β2 receptors dilate blood vessels.
• Digestive system needs less blood flow → α1 receptors constrict blood vessels.
• Heart needs to pump faster → β1 receptors increase heart rate.

Example – Asthma Treatment:

• Asthma attacks involve airway constriction due to excessive parasympathetic tone.
• Adrenergic receptor agonists (e.g., albuterol, ephedrine) activate β2 receptors, leading to bronchodilation and easier breathing.

Key Takeaways:

• Different adrenergic receptor subtypes allow epinephrine to trigger specific effects in different tissues.
• Beta-2 activation (β2) is key for bronchodilation, which is why adrenergic receptor agonists help in asthma attacks.
• Beta-1 activation (β1) increases heart rate, which explains why ephedrine (a general adrenergic agonist) can also cause increased heart rate.

Question 42

Electrical Disequilibrium and Ion Distribution in Cells

Answer 42

• Cells maintain an electrical disequilibrium due to differences in ion distribution across the plasma membrane.
• Major Ions in Different Compartments:
  
  • Intracellular Fluid (ICF):
    
    • Major Cation: Potassium (K⁺)
    • Major Anions: Phosphate groups and negatively charged proteins
  • Extracellular Fluid (ECF):
    
    • Major Cation: Sodium (Na⁺)
    • Major Anion: Chloride (Cl⁻)
• Some anions inside the cell do not have matching cations, and some cations do not have matching anions. This results in a net negative charge inside the cell compared to the exterior.
• The interior of the plasma membrane is slightly negative, while the exterior is slightly positive, creating an electrical gradient.
• This charge difference is measured in millivolts (mV).
• Convention for Measurement:
  
  • The charge outside the cell is set to 0 mV by convention.
  • Any measured charge in millivolts reflects the difference inside the cell relative to the outside.
  • This does not mean the extracellular fluid lacks charge—it is just used as the reference point.

Question 43

Fundamental Principles of Electricity in Physiology

Answer 43

• Law of Conservation of Electrical Charges:
  
  • The human body is electrically neutral overall.
  • The net electrical charge produced by any process is zero.
• Basic Electrical Principles:
  
  • Opposite charges attract, and like charges repel.
  • Separating positive and negative charges requires energy input.
• Conductors vs. Insulators in the Body:
  
  • Body fluids (salty water) are excellent conductors for electrical charges.
  • The plasma membrane of cells is an insulator, preventing charges from freely moving across.
• Application in Physiology:
  
  • These principles apply to resting membrane potential, skeletal muscle physiology, and cardiovascular physiology due to the role of electrically excitable cells.
• Thought Experiment - Artificial Cell:
  
  • Unlike real cells, this artificial cell lacks ion channels and consists only of a phospholipid bilayer.
  • The intracellular fluid (yellow) and extracellular fluid (violet) contain equal concentrations of cations and anions.
  • No charge imbalance exists because the bilayer is impermeable to ions, meaning the system is in chemical and electrical equilibrium.
  • In real cells, membrane proteins confer selective permeability, allowing certain ions to cross and creating electrical gradients.

Question 44

Formation of Electrochemical Gradients & Resting Membrane Potential

Answer 44

• Energy-Driven Ion Transport:
  
  • Moving a positively charged ion from the interior to the exterior of the cell requires energy.
  • A transporter consuming ATP facilitates this movement.
• Disrupting Electrical Equilibrium:
  
  • The anion left behind inside the cell cannot follow due to the membrane’s impermeability to anions.
  • This results in:
    
    • Exterior of the membrane becoming more positive
    • Interior of the membrane becoming more negative
• Formation of an Electrochemical Gradient:
  
  • The movement of cations creates a charge imbalance, leading to an electrical gradient.
  • The difference in ion concentration across the membrane creates a chemical gradient.
  • Together, these gradients form an electrochemical gradient, influencing ion movement across the membrane.
• Resting Membrane Potential (RMP):
  
  • The charge difference across a resting membrane is called the resting membrane potential.
  • It is measurable and expressed in millivolts (mV).
  • By convention, the extracellular compartment is set to 0 mV, and the measured value represents the difference in charge between the intracellular and extracellular compartments.

Question 45

Understanding Membrane Potential and Electrochemical Gradients

Answer 45

• Membrane Potential (-100 mV Example):
  
  • The negative charge indicates that the interior of the membrane is more negative than the exterior.
  • This value is relative, meaning the extracellular fluid is set to 0 mV by convention.
• Calcium (Ca²⁺) and Electrochemical Gradients:
  
  • Electrical Gradient: Calcium (+2 charge) is attracted to the negatively charged interior, so it moves inward.
  • Chemical Gradient: There is 1000x more calcium outside than inside, favoring calcium entry.
  • Both forces favor calcium entering the cell.
• Equilibrium Potential:
  
  • A balance where electrical force and chemical force are equal in magnitude but opposite in direction, leading to no net ion movement.
  • This equilibrium potential can be calculated and helps determine ion flow across a membrane.
• Recording Membrane Potential:
  
  • Measured by comparing intracellular charge to a reference electrode outside the cell (set at 0 mV).
  • Example: In an absolute scale, moving one cation out leaves an interior charge of 1 and an exterior of +1, creating a charge difference of 2 mV.
  • In real measurements, the value represents the difference between intracellular and extracellular charge, not absolute charge.

Question 46

Intracellular Recording & Equilibrium Concepts

Answer 46

• Intracellular Recording Setup:
  
  • Requires a recording electrode carefully inserted into a cell without damaging it.
  • Uses a reference electrode in the extracellular fluid for charge comparison.
  • A voltmeter measures the charge difference, and a recording device captures the data.
  • Typical resting membrane potentials:
    
    • Neurons: ~ 70 mV
    • Muscle cells: ~ 70 to -90 mV
• Electrical vs. Chemical Equilibrium:
  
  • Electrical Equilibrium: The total positive and negative charges inside the cell equal those outside.
  • Chemical Equilibrium: The types of ions inside and outside are balanced.
  • Example:
    
    • Artificial Cell Setup:
      
      • Inside: Potassium (K⁺) and proteins (negatively charged).
      • Outside: Sodium (Na⁺) and chloride (Cl⁻).
      • Result: Electrical equilibrium exists, but chemical equilibrium does not since different ions are separated.
• What Happens if the Membrane Becomes Fully Permeable?
  
  • Ions move along their chemical gradients until they are evenly distributed.
  • Example: 2 K⁺ ions swap with 2 Na⁺ ions, and 2 proteins swap with 2 Cl⁻ ions.
  • Final state: Both chemical and electrical equilibrium are achieved.

Question 47

Potassium Leak Channels & Electrochemical Equilibrium

Answer 47

• Potassium Leak Channels:
  
  • Function like a “straw” allowing K⁺ to move across the membrane.
  • K⁺ follows its chemical gradient and leaves the cell.
  • Proteins (negatively charged) remain inside, making the intracellular space more negative.
  • This buildup of negative charge creates an electrical gradient that pulls K⁺ back in.
• Electrochemical Equilibrium:
  
  • At some point, the chemical gradient (K⁺ leaving) and the electrical gradient (K⁺ being pulled back in) are equal in magnitude but opposite in direction.
  • This is called the equilibrium potential for potassium (Eₖ), meaning no net K⁺ movement occurs.
• Ion Movement Across a Selective Membrane:
  
  • If a membrane is only permeable to K⁺, K⁺ will diffuse down its concentration gradient.
  • Cl⁻ cannot follow, so a negative charge builds up on the side losing K⁺.
  • Eventually, the negative charge is strong enough to counteract further K⁺ movement, leading to electrochemical equilibrium.
• Key Takeaway:
  
  • Both chemical and electrical forces determine ion movement.
  • Ions do not diffuse until concentrations are equal—they stop moving when electrochemical equilibrium is reached.

Question 48

What is the equilibrium potential and how is it calculated?

Answer 48

The equilibrium potential is the charge on a membrane that is permeable to a specific ion, where the electrical force (charge) in millivolts exactly balances the concentration gradient for that ion. It can be calculated using the Nernst equation, which gives the membrane potential that would prevent any net movement of the ion. The equation is:

E = (61 / Z) * log([Ion]extracellular / [Ion]intracellular)

Where:

• E is the equilibrium potential in millivolts.
• 61 is a constant based on physiological temperature (37°C), Faraday’s constant, and the universal gas constant.
• Z is the valence (charge) of the ion.
• [Ion]extracellular is the concentration of the ion in the extracellular fluid.
• [Ion]intracellular is the concentration of the ion inside the cell.

The equilibrium potential tells you the electrical charge needed to counteract the concentration gradient of a given ion. For example, when calculating the equilibrium potential for potassium (K+), the concentration of potassium outside the cell might be 5 mM, while inside it’s 140 mM. The equilibrium potential would be:

E(K+) = 61 * log(5 / 140) ≈ -90 mV

At -90 mV, there is no net flow of potassium ions because the electrical and concentration gradients are balanced. However, resting neurons typically have a resting potential of about -70 mV, which is more positive than the potassium equilibrium potential of -90 mV. This is because the membrane is permeable to other ions, such as sodium (Na+), which influences the overall resting membrane potential.

The Nernst equation helps calculate the equilibrium potential for a cell permeable to a single ion, explaining the electrochemical forces acting on that ion.

Question 49

Calculating Equilibrium Potential with the Goldman-Hodgkin-Katz Equation

Answer 49

• Equilibrium Potential for Chloride Ions:
  
  • When a membrane is permeable only to chloride ions, its equilibrium potential can be calculated using the Nernst equation, yielding a value of 61 millivolts for chloride.
  • Equilibrium Potential Concept:
    
    • The equilibrium potential is the charge at which the electrical force and chemical force (concentration gradient) are equal in magnitude but opposite in direction.
    • When these forces balance out, no net ion movement occurs, even though the opposing forces are acting on the ion.
    • At +61 mV, chloride ions would not move across the membrane because the electrical and chemical forces are equal and opposite.
• Resting Membrane Potential (RMP) and Ion Permeability:
  
  • In a real cell (e.g., neuron), the RMP often differs from the Nernst potential for a single ion.
  • For example, the RMP for potassium in a real neuron might be 70 mV, which is more positive than the equilibrium potential of 90 mV for potassium.
  • This indicates the membrane is permeable to more than just potassium ions.
• Goldman-Hodgkin-Katz (GHK) Equation:
  
  • The GHK equation is used to calculate the equilibrium potential when a membrane is permeable to multiple ions, such as potassium (K⁺), sodium (Na⁺), and chloride (Cl⁻).
  • The GHK equation considers the permeability (P) of the membrane to each ion.
    
    • The more permeable the membrane is to a particular ion (due to more leak channels), the greater the contribution of that ion to the membrane potential.
    • If the membrane is less permeable to an ion, that ion still contributes to the overall membrane potential, but to a lesser degree.
  • The concentration of ions in the intracellular and extracellular spaces is factored into the equation, with special attention to chloride’s opposite valence compared to sodium and potassium.
    
    • The chloride concentration is treated differently in the equation:
      
      • Intracellular chloride concentration is in the numerator
      • Extracellular chloride concentration is in the denominator
  • The effects of chloride ions on membrane potential are opposite to those of sodium and potassium due to their opposite charge.
• Key Points to Remember:
  
  • The equilibrium potential is a balance between electrical and chemical forces.
  • The GHK equation adjusts for the permeability of the membrane to multiple ions, helping to calculate the realistic membrane potential in a cell.

Question 50

Resting Membrane Potential and Ion Pumping

Answer 50

• Resting Membrane Potential and Ion Permeability:
  
  • The resting membrane potential is primarily determined by the relative permeability of the membrane to different ions.
  • Potassium (K⁺) ions contribute the most to the resting membrane potential because there are more potassium leak channels in the membrane.
  • Sodium (Na⁺) ions, although present, contribute less to the resting membrane potential due to fewer sodium leak channels.
  • In most cells, the resting membrane potential is around 70 mV.
• Goldman-Hodgkin-Katz Equation:
  
  • The GHK equation is used to calculate the membrane potential when multiple ions (e.g., K⁺, Na⁺, Cl⁻) contribute to the overall charge.
  • The permeability of each ion is factored in, with more permeable ions having a greater influence on the membrane potential.
  • The presence of chloride (Cl⁻) ions, with a negative valence, has an opposite effect on membrane potential compared to potassium and sodium ions.
• Sodium-Potassium Pump (Na⁺/K⁺ ATPase):
  
  • The Na⁺/K⁺ ATPase pump plays a critical role in maintaining the resting membrane potential by actively pumping sodium ions out of the cell and potassium ions into the cell, consuming ATP in the process.
  • This creates and maintains the concentration gradients for Na⁺ and K⁺ across the membrane, essential for establishing a negative resting membrane potential inside the cell.
• Importance of Membrane Charge Separation:
  
  • The charge difference across membranes is crucial for cellular processes such as muscle contraction, nerve signaling, and heart function.
  • The ability to manipulate the permeability of the membrane to specific ions allows for the movement of ions in response to charge and concentration gradients.
  • This charge separation is also vital for the proper functioning of the cardiovascular system and cognitive abilities.
  • Without charge separation and ion movement, functions like breathing, muscle contraction, and neuronal activity would be impossible, leading to failure of critical bodily functions.
• Clinical Relevance:
  
  • The Na⁺/K⁺ ATPase pump and ion concentration gradients are essential for the function of the nervous system, heart, and muscle cells.
  • Disruption of these processes can lead to conditions like muscle paralysis, arrhythmias, and neurological deficits.

Question 51

What is Membrane Potential?

Answer 51

• Definition:
  
  • Membrane potential is the difference in electrical charge across the plasma membrane of a cell.
  • All cells have a membrane potential, but excitable cells (neurons, muscle cells) can use it to generate action potentials.
• Why is Membrane Potential Important?
  
  • Charge separation across the membrane allows cells to perform work (e.g., signaling in neurons, muscle contraction).
  • Changes in membrane potential allow for the transmission of information through afferent and efferent pathways.
• Resting Membrane Potential:
  
  • Neurons at rest typically have a membrane potential of 70 mV.
  • This is different from the equilibrium potential of potassium (-90 mV) because of sodium leak channels.
  • Sodium (Na⁺) ions leak into the cell, making the membrane potential more positive than the K⁺ equilibrium potential.
• Ion Permeability and Charge Movement:
  
  • Ions move across membranes due to chemical (concentration) and electrical (charge) gradients.
  • Changing membrane permeability to different ions alters the membrane potential, which is key to neural signaling.

Question 52

What are Graded Potentials and How Do They Lead to Action Potentials?

Answer 52

• Graded (Synaptic) Potentials:
  
  • Small, local changes in membrane potential that occur when ligand-gated ion channels open.
  • Caused by neurotransmitter binding to receptors on the dendrites and cell body of a neuron.
  • These increase or decrease membrane potential, depending on which ions enter or leave.
• How Graded Potentials Trigger Action Potentials:
  
  1. At rest: Ligand-gated ion channels are closed, preventing ion flow.
  2. Neurotransmitter release: Nearby neurons release neurotransmitters, which bind to ligand-gated channels.
  3. Ion movement: The channels open, allowing ions to cross the membrane.
  4. Graded potential spreads: If enough graded potentials summate, they can depolarize the trigger zone of the neuron.
  5. Voltage-gated channels open: If the depolarization reaches threshold, voltage-gated ion channels in the trigger zone open, leading to an action potential.
• Key Differences Between Graded & Action Potentials:
  
  • Graded potentials are local, short-lived, and vary in strength.
  • Action potentials are all-or-nothing signals that propagate down the axon to communicate with effectors (e.g., muscles).

Question 53

Equilibrium Potential and Electrochemical Gradient

Answer 53

• Chemical vs. Electrical Gradient:
  
  • Chemical gradient moves ions from high to low concentration.
  • Electrical gradient moves ions based on charge attraction or repulsion.
  • At equilibrium potential, these forces are equal and opposite.
• Direction of Electrical Gradient:
  
  • If Z is concentrated outside and moving inward, the electrical gradient must be outward to counteract it.
  • If Z is a cation (+), a positive membrane potential repels it outward.
  • If Z is an anion (-), a negative membrane potential repels it outward.
• Magnitude of Equilibrium Potential:
  
  • If the chemical gradient increases, the electrical gradient must also increase to maintain equilibrium.
  • This means the magnitude of the equilibrium potential (measured in mV) increases.
• Comparing Two Cells with Different Ion Charges & Membrane Potentials:
  
  • Given ions A and B with known charges, we analyze how their movement is influenced by the membrane potential.
  • If an ion’s equilibrium potential is close to the membrane potential, little movement occurs.
  • If the membrane potential is far from an ion’s equilibrium potential, strong ion movement occurs to restore balance.

Question 54

Equilibrium Potential and Ion Charge

Answer 54

• Equilibrium potential is determined by the chemical and electrical gradients.
• If the chemical gradient is inward, the electrical gradient must be outward to counteract it.
• Why do equilibrium potentials differ?
  
  • Cell 1: The ion is a cation (+) → the membrane potential must be positive to repel it outward.
  • Cell 2: The ion is an anion (-) → the membrane potential must be negative to repel it outward.
• If the membrane potential were opposite in charge, ions would be attracted inward, and equilibrium would be lost.
• Key takeaway: The equilibrium potential’s charge depends on the ion’s charge to ensure proper repulsion and balance.

Question 55

Equilibrium Potential and Chemical Gradients

Answer 55

• Key Concept: The equilibrium potential depends on both the chemical gradient and the electrical gradient.
• Opposite Equilibrium Potentials?
  
  • If two cells have opposite equilibrium potentials for the same ion, their chemical gradients must be in opposite directions.
• Membrane Potential vs. Equilibrium Potential:
  
  • Membrane potential: The overall charge difference between the inside and outside of a cell.
  • Equilibrium potential: The specific membrane potential where the electrical and chemical forces on an ion are balanced, preventing net ion movement.
• Charge & Ion Flow:
  
  • A cation (+) will move inward if the cell is negative and outward if the cell is positive.
  • An anion (-) will move inward if the cell is positive and outward if the cell is negative.
• Application to Magnesium Example:
  
  • Magnesium (Mg2+) is repelled by a positive intracellular charge.Mg2+Mg^{2+}
  • Since its chemical gradient favors leaving the cell and the electrical gradient also pushes it out, magnesium moves outward, not inward when channels open.

Question 56

Membrane Potential and Electrical Signaling

Answer 56

• Membrane Potential: All cells have a membrane potential, but not all are excitable.
• Polarization: A membrane is polarized if there is a charge difference between the interior and exterior of the cell.
• Chemical vs. Electrical Forces:
  
  • Cations move inward if the membrane is negative (opposite charges attract).
  • Cations move outward if the membrane is positive (like charges repel).
• Resting Membrane Potential (RMP): ~ -70mV in neurons.
• Depolarization: When the membrane potential becomes less negative (e.g., -70mV → -50mV), moving toward zero.
• Equilibrium Potential: The point at which the chemical and electrical forces balance, preventing net ion movement.
• Neuronal Communication: Ion channel activity allows for electrical signals, which neurons use to transmit information.

Question 57

Polarization, Depolarization, Repolarization, and Hyperpolarization

Answer 57

• Polarized Membrane: The membrane has a charge difference between the interior (negative) and exterior (positive).
• Depolarization: When the membrane potential becomes less negative (e.g., -70mV to -50mV), losing its original polarity.
• Overshoot: A brief reversal of membrane potential, where it becomes positive during an action potential, but this is not considered depolarization.
• Repolarization: When the membrane potential returns toward the resting membrane potential (e.g., -50mV back to -70mV).
• Hyperpolarization: When the membrane potential becomes more negative than the resting membrane potential, further increasing polarity.
• Action Potentials and Graded Potentials: Changes in membrane potential in excitable cells (e.g., neurons, muscle cells) described as depolarization, repolarization, and hyperpolarization.

Question 58

Neuron Activation and Signal Transmission

Answer 58

• Graded Potentials and Action Potentials: Neurons transmit electrical signals via graded potentials (from ligand-gated ion channels) and action potentials (from voltage-gated ion channels).
• Graded Potentials: These occur when ligand-gated ion channels open in the dendrites or cell body, leading to a localized change in membrane potential (depolarization).
• Action Potentials: Once the graded potentials are strong enough, they trigger voltage-gated ion channels at the trigger zone, initiating an action potential that propagates along the axon.
• Neurotransmitters: These are chemical signals (e.g., glutamate) that bind to ligand-gated ion channels, allowing ions like sodium to flow into the cell, creating a small depolarization.
• Ion Movement and Depolarization: Sodium enters the cell, making the inside more positive. This depolarizes the membrane, increasing the likelihood of an action potential.
• Channel Locations:
  
  • Ligand-gated ion channels are in dendrites and cell bodies, receiving input.
  • Voltage-gated ion channels are concentrated at the trigger zone and along the axon.

Question 59

Experimental Setup: Potassium Movement and Electrical Potential Measurement

Answer 59

• Experimental Setup Overview:
  
  • Two chambers are separated by a selectively permeable membrane that allows only potassium ions (K⁺) to pass through.
  • The recording electrode (in red) is placed in one chamber, and the reference electrode (in gray) is placed in the other.
  • Reference Electrode Convention: The side with the reference electrode is set at 0 mV by convention, and we measure the potential difference between the reference (extracellular) and the recording electrode (intracellular).
• Ion Distribution and Concentration Gradient:
  
  • The concentration gradient of potassium ions favors movement from the right chamber (higher concentration) to the left chamber (lower concentration).
  • As potassium ions move across the membrane, more positively charged K⁺ ions accumulate on the left side, creating a negative charge behind (because K⁺ carries a positive charge).
• Effect on Electrical Potential:
  
  • As potassium ions continue to diffuse, the left chamber will accumulate a positive charge.
  • This leads to a positive electrical potential recorded by the voltmeter, not a negative one. Thus, the voltage measured in the left chamber will be positive, ruling out the option of a negative potential (C).
• Nernst Equation:
  
  • The Nernst equation helps determine the equilibrium potential for potassium:
    E_K = (61 / z) × log([K⁺]extracellular / [K⁺]intracellular)
  • For potassium (K⁺), with a valence (z) of +1, and concentration values of 120 mM outside and 12 mM inside:
    E_K = (61 / 1) × log(120 / 12) = 61 mV
  • Therefore, the voltmeter will record a potential of 61 mV when the membrane is perfectly selective for potassium.
• Conclusion: The correct result is a positive potential in the left chamber, and the Nernst equation confirms the expected voltage difference of 61 mV.

Question 60

Homeostatic Reflex Arc, Graded Potentials, and Action Potentials

Answer 60

• Homeostatic Reflex Arc Overview:
  
  • The efferent pathway in a reflex arc connects the integration center to the effector. This pathway is responsible for sending information in a specific direction toward effectors.
• Generation of Graded Potentials:
  
  • Graded potentials are generated by the stimulation of ligand-gated ion channels.
  • These channels are found in the membrane of dendrites and the cell body of neurons.
  • If the graded potential is strong enough, it can lead to depolarization and trigger the opening of voltage-gated sodium (Na⁺) and potassium (K⁺) channels. This results in the generation of an action potential.
  • The action potential is first generated in the trigger zone and then propagated along the axon toward the axon terminals.
• Graded Potentials vs. Action Potentials:
  
  • Graded Potentials can be either excitatory or inhibitory.
    
    • Excitatory graded potentials make the membrane potential less negative, increasing the likelihood of firing an action potential.
    • Inhibitory graded potentials make the membrane more negative (hyperpolarization), making the cell less likely to fire an action potential.
  • Action Potentials are all-or-nothing signals that travel down the axon after the threshold potential is reached.
• Ligand-Gated Ion Channels and Depolarization:
  
  • Ligand (neurotransmitter) binds to a ligand-gated ion channel, causing it to open.
  • When the channel opens, it becomes permeable to cations (positively charged ions), which flow into the cell due to the concentration gradient.
  • The inside of the cell is slightly negative, attracting the positive cations, leading to local depolarization.
  • This creates a small local current around the open channel, where positively charged ions stick to nearby negatively charged regions in the membrane.
• Distance from the Open Channel:
  
  • As the distance from the open ligand-gated ion channel increases, the effect of the current decreases, and the local depolarization diminishes.

Question 61

Characteristics of Graded Potentials and Their Differences from Action Potentials

Answer 61

• Graded Potentials and Their Amplitude:
  
  • The strength (or amplitude) of a graded potential decreases as the distance from the open ion channel increases.
  • This is because as sodium ions (Na⁺) enter the cell, they diffuse away from the channel and are attracted to more negative areas of the membrane.
  • As sodium ions move, some may leak back through sodium channels, reducing the depolarization effect over distance.
  • This phenomenon is similar to water flowing through a leaky hose, where water pressure decreases the further it travels because of leakage.
• Leaky Nature of Graded Potentials:
  
  • Due to this “leakiness,” graded potentials are detrimental because the signal weakens as it travels from its point of origin.
• Comparison to Action Potentials:
  
  • Unlike graded potentials, action potentials are non-decremental: they are reinforced and propagate with the same strength down the axon.
  • The regenerative process of action potentials allows them to travel long distances without losing signal strength.
• Summation of Graded Potentials:
  
  • Graded potentials can add up or summate, meaning the changes in membrane potential from multiple channels can combine to create a larger depolarization.
  • For instance, if multiple ligand-gated ion channels open and allow more sodium into the cell, the cumulative effect can cause a larger depolarization event.
  • This is different from action potentials, which are all-or-nothing events and do not add up in this way.
  • Therefore, graded potentials are additive, while action potentials are separate and distinct events.

Question 62

Key Differences Between Voltage-Gated Sodium and Potassium Channels

Answer 62

• Voltage-Gated Sodium Channels:
  
  • Quick Response: Voltage-gated sodium channels react quickly to changes in membrane voltage, opening rapidly when the membrane reaches the threshold value.
  • Inactivation Gate: These channels have an inactivation gate, which blocks the channel once it’s open.
  • States: Voltage-gated sodium channels cycle through three states: closed, open, and inactivated. Once inactivated, the channel cannot reopen until it first returns to the closed state after a delay.
  • Unable to Reopen from Inactivation: When the sodium channel is inactivated, it cannot immediately reopen even if the membrane voltage changes. It must first return to the closed state before it can open again.
• Voltage-Gated Potassium Channels:
  
  • Slower Response: Potassium channels react slowly to changes in membrane voltage compared to sodium channels.
  • No Inactivation Gate: Voltage-gated potassium channels do not have an inactivation gate and only transition between open and closed states.
  • No Delay to Reopen: Potassium channels can open and close without the need for an inactivation state.

Important Concept:

• Voltage-gated sodium channels are responsible for the rapid depolarization phase of an action potential, while voltage-gated potassium channels facilitate repolarization.
• Action Potential Direction: The inactivation of sodium channels and the delayed opening of potassium channels ensure that action potentials travel in one direction down the axon.

Question 63

What are the stages and processes involved in generating an action potential in a neuron?

Answer 63

An action potential in a neuron is a complex event that involves several stages and mechanisms, each requiring specific changes in membrane potential and the activity of different ion channels. Here’s a detailed breakdown of the process:

1. Resting Membrane Potential:
   
   • The membrane potential of a resting neuron is typically around -70 millivolts (mV). This is the stable, baseline electrical state of the cell when it’s not actively transmitting signals.
2. Initial Depolarization (Graded Potentials):
   
   • In order to generate an action potential, the neuron’s membrane potential must first change. This is caused by excitatory graded potentials, which occur when ligand-gated ion channels open, allowing ions (typically sodium or calcium) to flow into the cell.
   • These graded potentials cause small, localized depolarizations, which are not yet action potentials but are the initial steps toward one.
3. Threshold Potential:
   
   • The neuron has a specific voltage value called the “threshold potential,” which is usually around -50 to -55 mV. This is the minimum membrane potential that must be reached for an action potential to be triggered.
   • Graded potentials that do not reach this threshold are considered subthreshold and do not result in an action potential.
4. Reaching Threshold:
   
   • Once the membrane potential reaches the threshold potential, both voltage-gated sodium and potassium channels are triggered. However, it is the voltage-gated sodium channels that open first.
5. Sodium Influx (Depolarization Phase):
   
   • When the voltage-gated sodium channels open, sodium ions rush into the cell. This occurs because sodium is moving down both its concentration gradient (from high to low concentration) and its electrical gradient (since the inside of the cell is negatively charged).
   • The influx of sodium makes the inside of the cell more positive, causing the membrane potential to rise rapidly, which is known as depolarization.
   • This depolarization event is self-amplifying due to a positive feedback loop: as more sodium enters, the membrane potential depolarizes further, which opens more voltage-gated sodium channels, allowing even more sodium to enter the cell.
6. Peak of Action Potential:
   
   • The depolarization continues until the membrane potential reaches approximately +30 mV. This is the peak of the action potential.
7. Repolarization Phase:
   
   • At the peak of the action potential, two key events occur:
     
     • Voltage-gated potassium channels begin to open (though slowly).
     • Voltage-gated sodium channels close, which prevents more sodium from entering the cell.
   • Potassium ions, which are in higher concentration inside the cell, begin to move out, driven by both the concentration gradient and the electrical gradient. Since potassium is positively charged, its efflux makes the inside of the cell more negative again, which is known as repolarization.
   • The membrane potential drops as potassium ions leave, helping return the membrane potential towards its resting state.

This entire sequence of events, from depolarization to repolarization, constitutes the action potential, allowing neurons to transmit electrical signals along their axons.

Question 64

What happens during the after-hyperpolarization phase of an action potential, and how does tetrodotoxin affect the action potential?

Answer 64

• After-Hyperpolarization (AHP) Phase:
  
  • After an action potential has occurred, the membrane potential often drops below the resting membrane potential of -70 mV. This is due to the voltage-gated potassium channels still being open as the action potential ends.
  • The membrane potential becomes more negative than the resting state, creating what is called an after-hyperpolarization event. This event occurs because potassium continues to flow out of the cell, making the inside more negative.
  • This hyperpolarization is temporary, and the membrane potential will eventually return to the resting value of around -70 mV once all voltage-gated potassium channels close and voltage-gated sodium channels transition from their inactive to closed states.
• Restoration to Resting Membrane Potential:
  
  • The restoration to the resting membrane potential happens when the voltage-gated potassium channels fully close, and the sodium channels are no longer in the inactive state but have closed. This allows the cell to return to its resting state and be ready for the next action potential.
• Tetrodotoxin and its Effect on Action Potentials:
  
  • Tetrodotoxin is a potent neurotoxin found in pufferfish. It is dangerous because it blocks voltage-gated sodium channels, preventing sodium ions from entering the neuron.
  • If sodium channels are blocked, it will stop the rapid membrane depolarization needed for an action potential. Without sodium entry, the depolarization phase cannot occur, and as a result, the neuron cannot generate an action potential. This is why consuming improperly prepared pufferfish (which contains tetrodotoxin) can be fatal, as it can lead to paralysis and death due to the failure of neural communication.
• All-or-Nothing Nature of Action Potentials:
  
  • Action potentials are described as all-or-nothing events. This means that once the membrane potential reaches the threshold value (around -55 mV), the action potential will always fire with the same peak amplitude (around +30 mV), regardless of the strength of the stimulus that triggered it.
  • If the stimulus is subthreshold (not strong enough to reach the threshold potential), no action potential will be generated.
  • The frequency of action potentials can vary with stronger stimuli, but the amplitude (height of the action potential) remains the same. The more intense the stimulus, the higher the frequency of action potentials, not the height of the individual action potentials themselves.

Question 65

How does changing the membrane permeability to sodium or potassium affect the membrane potential, and what happens during hyperkalemia and hypokalemia?

Answer 65

1. Effect of Increasing Membrane Permeability to Sodium:
   
   • When the membrane becomes more permeable to sodium, sodium ions will flow into the cell due to both the concentration gradient (high sodium outside, low sodium inside) and the electrical gradient (the interior of the cell is negatively charged).
   • As sodium enters the cell, the membrane potential becomes more positive, leading to depolarization.
   • During an action potential, the increase in sodium permeability (via opening of voltage-gated sodium channels) allows sodium to rush into the cell, raising the membrane potential and initiating the depolarizing phase of the action potential.
   • The greater the sodium permeability, the more depolarized the membrane will become, potentially making the membrane potential rise above the typical resting value of -70 mV.
2. Effect of Increasing Membrane Permeability to Potassium:
   
   • If the membrane becomes more permeable to potassium, potassium ions will move out of the cell due to the concentration gradient (more potassium inside the cell than outside).
   • Even though potassium is electrically attracted to the interior of the cell (due to the negative charge inside), the chemical gradient (which favors potassium leaving the cell) is stronger, so potassium moves out.
   • As potassium leaves the cell, the membrane potential becomes more negative, resulting in hyperpolarization. This causes the membrane potential to drop below the resting membrane potential of -70 mV.
   • This after-hyperpolarization event is seen at the end of an action potential, as some potassium channels remain open even after the action potential has ended, continuing to allow potassium to leave the cell and temporarily making the membrane potential more negative than usual.
3. Hyperkalemia (High Potassium Levels):
   
   • In hyperkalemia, there is an excess of potassium in the extracellular fluid. This reduces the concentration gradient for potassium, making it less likely for potassium to leave the cell.
   • As a result, potassium will remain in the cell, and the membrane potential will not become as negative as usual, leading to depolarization.
   • Neurons in a depolarized state are closer to threshold, meaning they are more likely to fire action potentials, which can lead to increased excitability of neurons and potentially cause problems like arrhythmias.
4. Hypokalemia (Low Potassium Levels):
   
   • In hypokalemia, there is a deficit of potassium in the extracellular fluid. This increases the concentration gradient for potassium, making it more likely for potassium to leave the cell.
   • As potassium leaves the neuron, the membrane potential becomes more negative, leading to hyperpolarization.
   • Hyperpolarized cells are further away from threshold, meaning they are less likely to fire action potentials. This can lead to decreased excitability of neurons and could contribute to muscle weakness, fatigue, or even arrhythmias.

In summary:

• Increased sodium permeability leads to depolarization.
• Increased potassium permeability leads to hyperpolarization.
• Hyperkalemia causes depolarization and increased excitability.
• Hypokalemia causes hyperpolarization and decreased excitability.

Question 66

Which condition, hypokalemia or hyperkalemia, is more dangerous, and why? How does hyperkalemia affect the heart?

Answer 66

• Hyperkalemia vs. Hypokalemia:
  
  • Hyperkalemia (high potassium levels in the blood) is generally more dangerous than hypokalemia (low potassium levels). The reason is that in hyperkalemia, the elevated potassium levels cause the membrane potential of neurons and muscle cells to become closer to threshold. This makes the cells more excitable, leading to more frequent and uncontrolled action potentials.
  • In the heart, hyperkalemia can lead to ventricular fibrillation, where the heart’s contractions become rapid and uncoordinated. This results in inefficient pumping and a lack of blood flow, which is life-threatening.
  • In contrast, hypokalemia causes hyperpolarization of cells, making them less excitable and farther from threshold, leading to decreased action potential firing. This decreases cellular activity but is less immediately dangerous than hyperkalemia.
• Effect of Hyperkalemia on the Heart:
  
  • Hyperkalemia reduces the difference in potassium concentration across the cell membrane, making the inside of the cell less negative and bringing the membrane potential closer to threshold.
  • This can result in sustained depolarization of cardiac cells, preventing the normal rhythm of the heart. In the worst case, this can lead to ventricular fibrillation, where the heart beats in an erratic, ineffective pattern, causing it to stop pumping blood effectively.
  • Ventricular fibrillation is a medical emergency that can lead to death if not treated promptly.

Question 67

What did Otto Loewi’s experiment with the vagus nerve and frog hearts demonstrate about chemical signaling?

Answer 67

Otto Loewi’s experiment, conducted in collaboration with another scientist, played a pivotal role in understanding how nerve signals are transmitted to target cells through chemical means, laying the foundation for the discovery of the chemical synapse. Here’s a breakdown of the experiment and its significance:

1. Experimental Setup:
   
   • Loewi dissected two frog hearts and placed them in a solution that kept the hearts beating.
   • One heart had its vagus nerve intact, while the second heart had its vagus nerve cut off.
   • The hearts were bathed in a common electrolyte solution that allowed them to continue beating.
2. Initial Findings:
   
   • Loewi electrically stimulated the vagus nerve of the first heart (the one with the intact vagus nerve) and observed that the heart rate slowed down as a result.
   • The solution surrounding this heart was then applied to the second heart, which did not have its vagus nerve intact.
   • To Loewi’s surprise, the second heart also slowed down its beating after receiving the solution from the first heart, suggesting that the solution contained something that could influence the heart rate.
3. Key Conclusion:
   
   • The experiment indicated that the vagus nerve releases a chemical substance into the surrounding fluid that is responsible for slowing the heart rate. This was the first evidence of chemical signaling between nerves and target tissues.
   • The substance released was not simply an electrical signal passed from the nerve to the heart, but a chemical agent, which was later identified as acetylcholine.
4. Nobel Prize and the Discovery of the Chemical Synapse:
   
   • Loewi won the Nobel Prize for his discovery, which demonstrated that information in the nervous system is transmitted via chemical signals (neurotransmitters), rather than electrical signals alone.
   • This discovery was groundbreaking because it established the concept of the chemical synapse, the mechanism by which neurons communicate with each other and with other target cells in the body.
5. The Role of Acetylcholine:
   
   • The substance initially described as the “vagus substance” in Loewi’s experiment was later identified as acetylcholine, a neurotransmitter involved in the parasympathetic branch of the autonomic nervous system.
   • Acetylcholine is released by post-ganglionic neurons and acts on target tissues, such as the heart, to regulate functions like heart rate.
6. Impact and Legacy:
   
   • Loewi’s work fundamentally changed our understanding of how signals are transmitted in the nervous system. It showed that neurotransmitters, like acetylcholine, play a crucial role in communication between neurons and their target cells, leading to further exploration of the diverse roles of neurotransmitters in physiological processes.
   • The discovery also emphasized the importance of chemical signaling in the nervous system, which is essential for many bodily functions, including muscle contraction, heart rate regulation, and cognitive processes.

The take-home message is that chemical signaling plays a crucial role in the transmission of information in the nervous system, as demonstrated by Loewi’s experiment with frog hearts.

Question 68

What is the difference between the absolute refractory period and the relative refractory period during an action potential?

Answer 68

The absolute refractory period and the relative refractory period are two distinct phases that occur after the generation of an action potential, during which the neuron’s ability to fire another action potential is impacted. Here’s how they differ:

1. Absolute Refractory Period:
   
   • This is the period during which no new action potential can be generated, regardless of the strength of the stimulus.
   • It occurs from the threshold to the falling phase of the action potential.
   • The voltage-gated sodium channels are either already open or inactivated, meaning they cannot be recruited to open again until they return to the closed state.
   • No matter how strong the stimulus, a second action potential cannot be fired during this period.
2. Relative Refractory Period:
   
   • This period follows the absolute refractory period and is characterized by the possibility of firing a second action potential, but only if the stimulus is stronger than normal.
   • During this time, some voltage-gated sodium channels have closed, while others are still inactivated.
   • A stronger stimulus is required to overcome the increased threshold to recruit the remaining available sodium channels and generate an action potential.
   • Unlike the absolute refractory period, a second action potential is possible but requires a greater-than-normal stimulus to trigger it.

Key Points:

• Absolute Refractory Period: No new action potential can be fired, regardless of stimulus strength.
• Relative Refractory Period: A new action potential is possible but requires a stronger stimulus to overcome the higher threshold.

Important Note: The statement that “a stronger stimulus will lead to the generation of a new action potential” applies only to the relative refractory period, not the absolute one.

Question 69

How does an action potential propagate along an axon after its initiation at the trigger zone?

Answer 69

The propagation of an action potential along an axon begins at the trigger zone and proceeds towards the axon terminals. Here’s how the process works:

1. Trigger Zone:
   
   • The action potential is initiated at the trigger zone because it contains a high density of voltage-gated sodium (Na⁺) and potassium (K⁺) channels. These channels allow the necessary ionic movements to generate and propagate the action potential.
   • The trigger zone is typically located at the axon hillock, the area where the axon connects to the cell body, and it is the first point where the action potential starts.
2. Depolarization (Rising Phase):
   
   • Once the membrane reaches the threshold, voltage-gated sodium channels open, allowing sodium ions (Na⁺) to flow into the cell.
   • This influx of sodium ions causes depolarization, making the inside of the neuron more positive.
3. Local Current Flow:
   
   • The high concentration of sodium ions that enters the cell at the initial point (the trigger zone) causes a local electrical current inside the neuron.
   • This current depolarizes neighboring regions of the axon membrane, leading to the opening of voltage-gated sodium channels in those areas as well. This creates a wave-like propagation of the action potential along the axon.
4. Repolarization (Falling Phase):
   
   • After sodium channels open and the membrane depolarizes, voltage-gated potassium channels open, allowing potassium (K⁺) ions to exit the cell.
   • This results in repolarization, where the inside of the neuron returns to a more negative state.
5. Propagation:
   
   • The action potential propagates in one direction (from the trigger zone towards the axon terminals) because the previous segment of the membrane is in the absolute refractory period and cannot be re-stimulated immediately.
   • As the sodium influx occurs in one segment, the local current moves to the adjacent segment, depolarizing it and causing it to reach threshold, where it generates its own action potential.
6. Role of Sodium and Potassium:
   
   • Sodium channels are responsible for the depolarization of the membrane, while potassium channels are responsible for repolarization.
   • The continuous movement of ions across the membrane is what allows the action potential to propagate rapidly along the axon.
7. Myelinated vs. Unmyelinated Axons:
   
   • In myelinated axons, the action potential “jumps” between nodes of Ranvier (gaps in the myelin sheath), speeding up transmission in a process called saltatory conduction.
   • In unmyelinated axons, the action potential propagates continuously along the entire length of the axon.

Key Concepts:

• Trigger Zone: Initiates the action potential due to a high density of voltage-gated channels.
• Local Current Flow: Sodium ions flow inward and depolarize adjacent areas, propagating the action potential.
• Refractory Period: Prevents backward propagation, ensuring the action potential moves in one direction.

Question 70

How does depolarizing the axon midway along its length affect action potential propagation?

Answer 70

When an axon is artificially depolarized halfway along its length (using a current-injecting electrode), it can generate two action potentials instead of just one propagating from the trigger zone. Here’s the process in more detail:

1. Normal Action Potential Propagation:
   
   • Under normal circumstances, an action potential is initiated at the trigger zone and propagates in one direction towards the axon terminals.
   • The inactivated voltage-gated sodium channels in the region behind the action potential (closer to the trigger zone) prevent the action potential from moving backward.
2. Artificial Depolarization:
   
   • When a current is injected into the axon at a point halfway along its length, the membrane in that area becomes depolarized.
   • This artificial depolarization activates voltage-gated sodium channels both in the region behind the depolarized area (toward the trigger zone) and ahead of it (toward the axon terminals).
3. Bidirectional Propagation:
   
   • The sodium that enters the cell at the depolarized point moves in both directions.
     
     • Moving towards the trigger zone (backward), the sodium ions encounter closed voltage-gated sodium channels, which can now be opened because the inactivation gates are no longer in effect (as they were at rest during the normal action potential).
     • Moving towards the axon terminals (forward), the sodium ions also activate closed voltage-gated sodium channels, starting an action potential in both directions.
   • This results in two action potentials being generated: one moving towards the trigger zone (retrograde) and the other towards the axon terminals (orthograde).
4. Key Concept:
   
   • Normally, the action potential is unidirectional because the region behind the action potential is in its absolute refractory period, preventing further depolarization in that direction.
   • However, in the experimental setup, artificial depolarization can bypass this restriction, allowing bidirectional action potential propagation.

Key Concepts:

• Unidirectional Propagation: Action potentials normally propagate in one direction due to inactivation of sodium channels.
• Artificial Depolarization: Depolarizing the axon artificially allows for bidirectional action potential propagation because sodium channels are activated in both directions.

Question 71

Answer 71

Question 72

Answer 72

Question 73

What is saltatory conduction and how does it affect action potential velocity?

Answer 73

Saltatory conduction is the process by which an action potential propagates along a myelinated axon in a way that the signal appears to “jump” from one node of Ranvier to the next. This process involves several key features:

1. Myelination: The axon is wrapped by glial cells (oligodendrocytes or Schwann cells), creating an insulating layer of myelin. This prevents the loss of sodium ions through leak channels, which occurs in unmyelinated sections of the axon.
2. Nodes of Ranvier: These are the small, uncovered gaps in the myelin where voltage-gated sodium and potassium channels are concentrated. Action potentials are only generated at these nodes, not along the entire length of the axon.
3. Diffusion of Sodium: When sodium ions enter through the channels at the nodes, they diffuse along the axon to the next node. The insulation provided by the myelin allows the action potential to “skip” over the myelinated segments, making the process much faster than in unmyelinated axons.
4. Increased Velocity: Saltatory conduction significantly increases the velocity of action potential propagation. In unmyelinated axons, the action potential travels slower due to the continuous depolarization of the membrane, while in myelinated axons, the action potential “jumps” from node to node, speeding up the signal transmission.

Key Points:

• Saltatory conduction occurs in myelinated axons, where the signal “jumps” from node to node.
• It increases the velocity of action potential propagation compared to unmyelinated axons.
• Myelination prevents the leakage of sodium, allowing faster signal transmission.

Question 74

What are the key differences between electrical and chemical synapses?

Answer 74

Electrical Synapses:

• In electrical synapses, depolarization events can pass directly from one cell to another through gap junctions.
• Gap junctions consist of connexins, proteins that form channels allowing small molecules, like ions, to move between the interiors of two cells.
• Electrical synapses allow for bidirectional signaling, meaning molecules can move in either direction through the gap junctions.
• These types of synapses are commonly found in cardiac muscle, where they allow rapid electrical signaling necessary for coordinated contraction.

Chemical Synapses:

• In chemical synapses, a signal is transmitted from one neuron to another via the release of neurotransmitters, which cross the synaptic cleft.
• Chemical synapses allow for unidirectional signaling, meaning the signal only moves in one direction from the presynaptic to the postsynaptic cell.
• Chemical synapses are the most widely used in the nervous system for communication between neurons.

Key Differences:

• Electrical synapses: Allow bidirectional flow through gap junctions, used in cardiac muscle.
• Chemical synapses: Use neurotransmitter release and are unidirectional, more common in the nervous system.

Question 75

How does an electrical signal in a presynaptic neuron get converted into an electrical signal in a postsynaptic neuron?

Answer 75

• The presynaptic neuron (neuron 1) has axon terminals that contain vesicles filled with neurotransmitters.
• The synaptic cleft is the small gap (about 10-20 nanometers) between the presynaptic neuron and the postsynaptic neuron (neuron 2). The cleft is filled with interstitial fluid.
• In the presynaptic terminal, the vesicles containing neurotransmitters fuse with the axon membrane, releasing their contents into the synaptic cleft.
• The neurotransmitters cross the synaptic cleft and bind to receptors on the postsynaptic membrane, triggering a response in the postsynaptic neuron.
• This process converts the electrical signal from the presynaptic neuron into a chemical signal in the synaptic cleft and back into an electrical signal in the postsynaptic neuron.

The key takeaway is that synaptic transmission involves the conversion of an electrical signal to a chemical signal (via neurotransmitter release) and then back to an electrical signal in the postsynaptic neuron.

Question 76

How does calcium contribute to neurotransmitter release at a synapse?

Answer 76

• Calcium ions act as a second messenger in neurotransmitter release at the presynaptic terminal.
• When an action potential depolarizes the presynaptic membrane, voltage-gated calcium channels open, allowing calcium ions to flow into the axon terminal.
• SNARE proteins (both in the vesicle membrane and presynaptic membrane) help anchor vesicles close to the terminal membrane.
• A calcium-sensing protein interacts with calcium ions, triggering a conformational change that causes SNARE proteins to pull the vesicle to the terminal membrane.
• This interaction leads to the vesicle fusing with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
• The vesicle may undergo either kiss-and-run fusion (vesicle briefly fuses and then detaches) or full fusion, after which the membrane is recycled to prevent the terminal from expanding.

In summary, calcium ions trigger the vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synapse.

Question 77

What happens to neurotransmitters after they are released into the synaptic cleft?

Answer 77

After neurotransmitter release into the synaptic cleft:

1. Binding to receptors: The neurotransmitter may bind to receptors on the postsynaptic membrane, triggering a response.
2. Diffusion away: Some neurotransmitters diffuse away from the synapse into the interstitial fluid or the bloodstream.
3. Enzymatic breakdown: In some synapses, enzymes break down neurotransmitters to stop signaling, such as acetylcholine being broken down by acetylcholinesterase.
4. Reuptake: Some neurotransmitters are reabsorbed by the presynaptic terminal through reuptake transporters for recycling, like dopamine being taken back by dopamine transporters.
   
   • Example: Cocaine blocks dopamine reuptake, leading to prolonged effects of the neurotransmitter.

The breakdown or reuptake of neurotransmitters is important for terminating signals to maintain the sensitivity of the signaling pathways.

Question 78

Answer 78

Question 79

Answer 79

Question 80

Answer 80

Question 81

Answer 81

Question 82

Answer 82

Question 83

Answer 83

Question 84

Answer 84

Question 85

Answer 85

Question 86

Answer 86

Question 87

Answer 87

Question 88

Answer 88

Question 89

What do pump proteins do, and how does a mutation in calcium pump proteins affect muscle function?

Answer 89

Pump proteins, such as the sodium-potassium ATPase pump, consume ATP to move ions against their concentration gradient—from areas of low concentration to higher concentration. This active transport is essential for maintaining cellular function.

One example is the calcium pump proteins, which play a crucial role in muscle function:

1. Muscle Contraction Initiation:
   
   • Acetylcholine (ACh) is released from a motor neuron.
   • ACh binds to nicotinic acetylcholine receptors on the muscle membrane, triggering an action potential in the muscle.
   • The action potential spreads into the transverse tubules (T-tubules), leading to the release of calcium from the sarcoplasmic reticulum (SR).
   • Calcium binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin filaments, allowing myosin heads to bind to actin, leading to contraction.
2. Role of Calcium Pumps in Relaxation:
   
   • After contraction, calcium pumps consume ATP to actively transport calcium back into the smooth endoplasmic reticulum (sarcoplasmic reticulum, SR).
   • This removal of calcium from the cytoplasm stops myosin-actin interaction, allowing the muscle to relax.
3. Effect of a Mutation in Calcium Pumps:
   
   • If calcium pumps are non-functional or mutated, calcium remains in the cytoplasm, preventing muscle relaxation.
   • This results in continuous muscle contraction, which can lead to muscle paralysis.

Thus, calcium pump proteins are essential for both muscle contraction (by allowing calcium release) and muscle relaxation (by clearing calcium from the cytoplasm).

Question 90

How does an action potential in the T-tubule trigger calcium release from the sarcoplasmic reticulum?

Answer 90

The process of calcium release from the sarcoplasmic reticulum (SR) in response to an action potential involves voltage-sensitive proteins in the T-tubule membrane. This sequence is crucial for muscle contraction:

1. Voltage Sensing by DHP Receptor:
   
   • The T-tubule membrane contains a voltage-sensitive protein called dihydropyridine receptor (DHP or DHPR).
   • DHP detects voltage changes when an action potential reaches the T-tubule.
   • DHP undergoes a conformational (shape) change in response to this voltage shift.
2. Activation of Ryanodine Receptors (RyR):
   
   • DHP is physically linked to ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR) membrane.
   • When DHP changes shape, it mechanically pulls open RyR calcium channels on the SR.
3. Calcium Release into the Cytoplasm:
   
   • Opening of RyR increases calcium permeability in the SR membrane.
   • Calcium flows down its concentration gradient from the SR into the muscle cytoplasm.
4. Initiation of Contraction:
   
   • The released calcium binds to troponin, leading to a shift in tropomyosin, exposing myosin-binding sites on actin.
   • This allows myosin heads to engage actin filaments, leading to sarcomere contraction and muscle shortening.

Thus, the DHP receptor detects voltage changes, triggers the opening of RyR, and allows calcium release, initiating muscle contraction.

Question 91

What is a motor unit, and how do motor neurons control skeletal muscle contraction?

Answer 91

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Skeletal muscle contraction occurs only when a motor neuron sends a signal.

1. Motor Neuron Structure and Function:
   
   • Motor neurons have cell bodies in the ventral horn of the spinal cord or brainstem.
   • They send large-diameter axons over long distances to reach muscle fibers.
   • Why large diameter?
     
     • Larger axon diameter reduces electrical resistance, allowing faster signal conduction.
     • This works alongside myelination, which prevents sodium leakage and speeds up action potentials.
2. Motor Units and Muscle Fiber Innervation:
   
   • A motor unit includes a single motor neuron and all the muscle fibers it innervates.
   • Each motor neuron branches and connects to multiple muscle fibers spread throughout the muscle tissue.
   • However, each muscle fiber is innervated by only one motor neuron—it does not receive input from multiple neurons.
3. Motor Unit Activation and Contraction:
   
   • When a motor neuron fires an action potential, all muscle fibers in its motor unit contract simultaneously.
   • Different motor neurons control different subsets of muscle fibers, allowing for precise control of movement.

Thus, motor units are the fundamental functional units of skeletal muscle control, allowing coordinated muscle contractions.

Question 92

How do motor units contribute to varying muscle tension, and what is the role of the neuromuscular junction in muscle activation?

Answer 92

• Motor Unit Recruitment and Muscle Tension:
  
  • A motor unit is one motor neuron and the multiple muscle fibers it controls.
  • Motor unit recruitment determines the force generated by a muscle:
    
    • Light loads (e.g., lifting a child’s bike) → Fewer motor units are recruited.
    • Heavy loads (e.g., lifting an adult bike) → More motor units are recruited.
  • The central nervous system (CNS) regulates the number of motor units needed based on task demands.
• Neuromuscular Junction (NMJ) and Signal Transmission:
  
  • The NMJ is the chemical synapse between a motor neuron and a skeletal muscle fiber.
  • Structure of NMJ:
    
    • The axon terminals of the motor neuron extend over multiple muscle fibers.
    • The muscle membrane is highly folded, increasing nicotinic acetylcholine receptor (nAChR) density.
    • More receptors + large neurotransmitter release → Efficient signaling.
• Action Potential Transmission at the NMJ:
  
  • Acetylcholine (ACh) is released from the motor neuron.
  • ACh binds to nicotinic ACh receptors on the muscle membrane, triggering an end-plate potential (EPP).
  • EPP always reaches threshold, ensuring a 1:1 ratio of motor neuron action potentials to muscle fiber action potentials.
  • No inhibitory signals (IPSPs) exist at the NMJ—only excitation occurs.
• Key Concept:
  
  • Every motor neuron action potential leads to a muscle fiber action potential, ensuring rapid and reliable muscle contraction.

Question 93

What happens when acetylcholine is released at the neuromuscular junction, and how does curare affect muscle function?

Answer 93

• Acetylcholine at the Neuromuscular Junction:
  
  • Acetylcholine (ACh) is released from a motor neuron at the neuromuscular junction.
  • ACh binds to and activates nicotinic acetylcholine receptors on the motor end plate of the muscle.
  • This activation leads to the generation of an end-plate potential, which triggers a muscle action potential.
  • The action potential spreads through the muscle fiber, leading to muscle contraction.
  • This process is crucial for voluntary movement and normal muscle function.
• Effects of Curare (A Nicotinic Acetylcholine Receptor Antagonist):
  
  • Curare is a toxin traditionally used in hunting (dipped onto arrows).
  • It blocks nicotinic acetylcholine receptors, preventing ACh from binding.
  • This inhibits the generation of an end-plate potential, blocking muscle contraction.
  • The main lethal effect is paralysis of the diaphragm, leading to respiratory failure.
  • Used clinically to paralyze eye muscles for surgery and to relax sphincters for medical procedures.
• Muscle Growth and Hypertrophy:
  
  • When muscles grow due to training (e.g., weightlifting), they do not increase in number but rather increase in size.
  • Growth occurs by adding more myofibrils within existing muscle fibers, increasing their ability to generate force.
  • While neuromuscular junctions refine for more precise movement, new muscle fibers do not form.
• Introduction to Cardiovascular Physiology:
  
  • The cardiovascular system consists of three main components:
    
    • Blood (fluid being transported)
    • Heart (pump generating pressure)
    • Blood vessels (pipes transporting blood)
  • Regulated by the endocrine system, nervous system, and kidneys, which play a crucial role in long-term blood pressure regulation.

Question 94

What are the three components of blood after centrifugation, and what is the significance of hematocrit?

Answer 94

When a blood sample is centrifuged at high speed, it separates into three distinct layers:

1. Plasma (55% of blood volume)
   
   • The topmost layer, mostly salty water, contains nutrients, hormones, gases, and waste products.
   • It serves as the medium for transporting substances throughout the body.
2. Buffy Coat (Less than 1%)
   
   • A thin, whitish-gray layer between the plasma and red blood cells.
   • Contains white blood cells (WBCs) for immune defense and platelets for blood clotting.
3. Hematocrit (42-45%)
   
   • The bottom red layer consists of red blood cells (erythrocytes).
   • The hematocrit refers to the percentage of blood volume occupied by red blood cells.
   • Red blood cells contain hemoglobin, which binds to oxygen (98.5%) and carbon dioxide for transport.

Significance of Hematocrit:

• Oxygen Transport: Hematocrit is crucial for determining oxygen-carrying capacity.
• Athletic Performance: Higher hematocrit levels mean more hemoglobin, improving oxygen delivery to muscles.
• Blood Doping Risks:
  
  • Athletes have historically used erythropoietin (EPO) to artificially increase red blood cell production.
  • Excessive hematocrit can make the blood too viscous, leading to circulation issues and even heart attacks.

Gender Differences in Hematocrit:

• Males typically have a higher hematocrit due to testosterone, which stimulates erythropoietin (EPO) production.
• Menstruation can lower hematocrit in females due to periodic blood loss.

Thus, hematocrit is a key measure of blood’s oxygen-carrying capacity and an indicator of overall health.

Question 95

How does blood circulate through the heart and lungs, and what is the difference between oxygen-poor and oxygen-rich blood?

Answer 95

The cardiovascular system can be divided into two halves: the right and left sides of the heart. Each side plays a critical role in the circulation of blood.

• Anatomical Convention: When viewing images of the heart, anatomical convention assumes you are looking down at a patient or cadaver who is looking up at you. Therefore, the right side of the heart (right atrium and right ventricle) is on the left side of the image, and the left side of the heart (left atrium and left ventricle) is on the right side.
• Right Side of the Heart:
  
  • Blood enters the right atrium from the body, which contains deoxygenated blood (blood that has delivered oxygen to tissues and picked up carbon dioxide).
  • The blood moves from the right atrium into the right ventricle.
  • From the right ventricle, the blood is pumped into the pulmonary artery, which carries it to the lungs for oxygenation.
  • In the lungs, blood exchanges gases: it picks up oxygen and releases carbon dioxide. The blood is now oxygenated, although it is not fully oxygenated (around 70-75% saturated with oxygen).
• Left Side of the Heart:
  
  • The oxygenated blood from the lungs enters the left atrium.
  • The blood moves from the left atrium into the left ventricle, which is responsible for pumping oxygen-rich blood into the aorta.
  • The aorta carries oxygenated blood into the systemic circuit, delivering oxygen to tissues and picking up carbon dioxide.
  • The blood in the systemic circuit is fully oxygenated (around 98% saturated with oxygen), enabling the body to perform various metabolic processes.

Key Terms:

• Deoxygenated Blood: This term is commonly used to describe blood that is low in oxygen but still contains some oxygen (around 70-75% saturated). It is more accurately referred to as oxygen-poor.
• Oxygenated Blood: Blood that has passed through the lungs and is rich in oxygen, typically around 98% saturated.

This overview highlights the cyclical nature of blood flow, with deoxygenated blood being pumped from the right side of the heart to the lungs for oxygenation, and oxygenated blood being pumped from the left side to the body for distribution.

Question 96

What are the key functions and pressure dynamics involved in the cardiovascular system, particularly in blood circulation and the role of arteries, veins, and the heart?

Answer 96

The cardiovascular system is crucial for moving blood throughout the body, and it operates based on pressure gradients and the function of various blood vessels. Here’s a detailed breakdown of key concepts:

1. Pulmonary Circulation and Systemic Circulation:
   
   • Pulmonary Circulation: Blood leaves the right ventricle and flows to the pulmonary artery, which takes it to the lungs for oxygenation. The blood in the pulmonary artery is oxygen-poor, having just delivered oxygen to tissues and picked up carbon dioxide.
   • Systemic Circulation: Blood leaves the left ventricle and moves directly to the aorta, which then distributes oxygenated blood to the entire body. The blood in the aorta is oxygen-rich, having been oxygenated in the lungs.
2. Arteries and Veins:
   
   • Arteries: Arteries carry blood away from the heart. This includes the pulmonary artery (carrying oxygen-poor blood to the lungs) and the aorta (carrying oxygen-rich blood to the body). A common misconception is that all arteries carry oxygenated blood, but this is only true for most arteries except the pulmonary artery.
   • Veins: Veins carry blood towards the heart. The venules and veins are responsible for returning blood to the heart after it has circulated through the body or lungs. Veins operate under low pressure compared to arteries.
3. Capillaries:
   
   • Capillaries are the smallest blood vessels where gas exchange occurs. In the lungs, oxygen enters the blood from the air spaces, and in the systemic circuit, oxygen is delivered to tissues (e.g., muscles) and carbon dioxide is picked up. Capillaries have extremely thin walls, allowing for the exchange of gases between blood and tissues.
4. Pressure Dynamics in the Vasculature:
   
   • Blood pressure is high in the arteries, particularly in the aorta, where the heart pumps blood at high pressure to supply the systemic circuit.
   • Blood pressure is low in the veins, especially as blood returns to the heart. The pressure difference between arteries and veins is crucial for maintaining blood flow.
   • Blood flows from areas of high pressure to low pressure, which is fundamental for circulation. Resistance to blood flow causes pressure to drop as blood moves from arteries to veins.
5. Heart’s Pressure Regulation:
   
   • The heart faces the challenge of maintaining proper pressure gradients:
     
     • During diastole (when the heart relaxes), the pressure inside the heart must be lower than the pressure in the veins to allow the heart to fill with blood.
     • During systole (when the heart contracts), the pressure inside the ventricles must exceed the pressure in the arteries to push the blood into the systemic and pulmonary circuits.
   • This pressure gradient between the arteries and veins is essential for blood to flow properly throughout the body, ensuring tissues receive oxygen and nutrients while waste products (like carbon dioxide) are removed.

Key Takeaways:

• Arteries move blood away from the heart (not necessarily oxygenated blood).
• Veins return blood to the heart (under low pressure).
• Capillaries are where gas exchange occurs.
• Blood flow is driven by pressure differences from high (arteries) to low (veins), and the heart must manage these pressures effectively for circulation.

Question 97

What are the key factors that influence blood flow in the circulatory system, and how are they described by Poiseuille’s law?

Answer 97

Blood flow in the circulatory system is influenced by several factors that can either promote or impede movement. These factors can be understood through Poiseuille’s law, which describes the relationship between pressure, resistance, and flow in a tube-like structure, such as blood vessels. Here’s a detailed breakdown of the key factors:

1. Pressure Difference (Pressure Gradient):
   
   • Blood flow is promoted when there is a greater pressure difference between two points in the circulatory system. The greater the pressure gradient, the faster and more efficiently blood can flow.
   • This pressure difference is crucial for maintaining the flow of blood, particularly from the heart to the arteries. The heart’s contraction generates the high pressure needed to push blood into the arteries, overcoming the resistance within the vessels.
2. Resistance:
   
   • Resistance impedes blood flow, and it can be influenced by several factors:
     
     • Length of the vessel: Longer vessels require a higher pressure difference to move the blood through, increasing resistance.
     • Viscosity of the fluid (blood): Thicker fluids (higher viscosity) are harder to move, increasing resistance. For instance, thicker substances like milkshakes require larger straws to move through, compared to thinner liquids like cocktails.
     • Radius of the vessel: The radius of blood vessels has the most significant impact on resistance. According to Poiseuille’s law, resistance is inversely proportional to the radius to the fourth power. This means that a small change in the radius of a vessel can cause a large change in resistance. For example, doubling the radius of a vessel reduces resistance by a factor of 16.
3. Regulation of Blood Flow:
   
   • The body regulates blood flow primarily by adjusting the diameter of arterioles (small arteries). By dilating or constricting arterioles, the body can increase or decrease blood flow to specific tissues. This regulation occurs on a moment-to-moment basis through mechanisms like nitric oxide signaling, which causes the relaxation of smooth muscle in the vessel walls, increasing the diameter and reducing resistance.
   • Changes in the length of vessels or the viscosity of blood are not typically regulated for short-term adjustments. While dehydration can make blood thicker, the body generally doesn’t rely on viscosity changes to regulate blood flow. Similarly, we don’t adjust the length of vessels to manage circulation.
4. Poiseuille’s Law Formula:
   
   • Poiseuille’s law describes the relationship between the pressure difference, resistance, and flow in a tube:Flow=8×Length×Viscosityπ×Pressuredifference×Radius4Flow=π×Pressuredifference×Radius48×Length×Viscosity\text{Flow} = \frac{\pi \times \text{Pressure difference} \times \text{Radius}^4}{8 \times \text{Length} \times \text{Viscosity}}
   • This formula emphasizes that:
     • A greater pressure difference promotes blood flow.
     • Longer tubes or higher viscosity increases resistance.
     • Larger vessel radius reduces resistance dramatically.

Key Takeaways:

• Blood flow is promoted by a high pressure gradient and reduced by resistance.
• Resistance is influenced by vessel length, viscosity, and most significantly, the radius of the vessel.
• The body regulates blood flow mainly by changing the diameter of arterioles, not by altering viscosity or vessel length.

Question 98

How is blood flow regulated, and what is the structure and function of the heart’s membranes and cardiac muscle cells?

Answer 98

• Regulation of Blood Flow: Blood flow is regulated by vasoconstriction and vasodilation. Vasoconstriction refers to the narrowing of blood vessels, which increases resistance to blood flow, while vasodilation involves the widening of blood vessels, which decreases resistance. These processes help maintain adequate blood flow and pressure in the circulatory system.
• Poiseuille’s Law: According to Poiseuille’s law, the smaller the radius of a blood vessel, the higher the resistance to blood flow. This law helps explain the importance of vessel size in determining how easily blood can flow through the cardiovascular system. Smaller vessels will increase resistance, requiring more force to pump blood through them.
• Heart Membranes:
  
  • The heart is surrounded by two important membranes: the outer pericardium and the inner epicardium.
  • There is a fluid between these two membranes, which is crucial for reducing friction. The heart changes shape over 100 times a minute during activities like exercise, and the lubricating fluid prevents the heart from rubbing against surrounding structures in the thoracic cavity. This is important because friction between the heart and other structures can cause intense pain, especially when the membrane layers fail.
• Dissection and Myocardium Exposure:
  
  • In a dissection focused on the heart, the membrane layers are torn away using forceps. Once these layers are removed, the myocardium is exposed.
  • The myocardium is made up of cardiac muscle cells, which are specialized for contraction and relaxation of the heart. These cells are branched and surround the heart’s chambers.
• Cardiac Muscle Cells:
  
  • Cardiac muscle cells are joined by intercalated discs, which appear as small lines between cells in tissue samples.
  • These discs contain gap junctions, which allow ions to flow directly between cells, enabling the heart muscle to contract in a highly coordinated manner. This synchronization ensures that the heart beats as a unit, with all the muscle cells contracting nearly simultaneously.
  • Due to the presence of gap junctions, cardiac muscle cells function as a functional syncytium, meaning they work together as a single unit.
• Fatigue Resistance:
  
  • Unlike skeletal muscle, which can hold tension for a while before tiring (fatigue), cardiac muscle does not have periods of rest.
  • Cardiac muscle cells are continuously active, either in a state of contraction or relaxation, without the time for recovery. This constant activity contributes to the heart’s ability to pump blood without tiring, making cardiac muscle highly fatigue-resistant.
• Heart Chambers and Other Cells:
  
  • While cardiac muscle cells are the primary cell type in the heart, the heart’s chambers themselves contain few cells, much like the vasculature (blood vessels), which mainly consists of endothelial cells and smooth muscle. These cells play roles in the structure and function of the heart but are not involved in contraction.

Question 99

What are the heart valves, their structure, function, and how do they regulate blood flow?

Answer 99

• Heart Valves:
  
  • The heart valves are crucial structures that ensure one-way blood flow through the heart. They are located in various places within the heart to regulate the movement of blood between the chambers and into the arteries. The valves open and close based on pressure differences, directing blood flow in the correct direction and preventing backflow.
• Intravenous Septum:
  
  • The intravenous septum is a thick muscular wall that separates the two ventricles of the heart.
  • In a healthy heart, there are no holes in this wall, which ensures that blood cannot pass directly from one ventricle to the other.
  • However, in certain physiological or pathological states, holes in the septum may form, leading to a mixing of blood that should have different oxygen contents (e.g., oxygen-rich blood and oxygen-poor blood), which can impair heart function.
• Atrioventricular (AV) Valves:
  
  • The AV valves are located between the atria and ventricles.
  • When open, they allow blood to flow from the atria into the ventricles.
  • These valves close when the ventricles contract, preventing blood from flowing backward into the atria. This occurs because the ventricular pressure exceeds the pressure in the atria.
  • The AV valves behave based on pressure differences:
    
    • Open when the ventricular pressure is lower than the atrial pressure.
    • Closed when the ventricular pressure is higher than the atrial pressure, preventing backflow.
• Left and Right AV Valves:
  
  • There are two AV valves:
    
    • The left AV valve has two flaps (also called the bicuspid valve or mitral valve).
    • The right AV valve has three flaps (called the tricuspid valve).
  • The difference in the number of flaps is related to the different pressures in the left and right ventricles:
    
    • The left ventricle has to generate a much higher pressure because it pumps blood into the aorta, sending blood throughout the entire body.
    • The right ventricle, however, pumps blood only to the lungs, so it doesn’t need to generate as high a pressure.
    • Having two flaps on the left side helps minimize leakage and facilitates the higher pressures needed in the left ventricle, which is essential for systemic circulation.
• Semilunar Valves:
  
  • Semilunar valves are found between the ventricles and the arteries (the aorta and pulmonary artery).
  • There are two types of semilunar valves:
    
    • Aortic semilunar valve (between the left ventricle and the aorta).
    • Pulmonary semilunar valve (between the right ventricle and the pulmonary artery).
  • These valves open during ventricular contraction, when the ventricular pressure exceeds the arterial pressure, allowing blood to be ejected into the arteries.
  • They close when the heart is filling and relaxing, preventing backflow of blood into the ventricles.
• Valve Function and Pressure Differences:
  
  • All heart valves function according to pressure differences:
    
    • AV valves (atrioventricular) open when ventricular pressure is less than atrial pressure.
    • Semilunar valves open when ventricular pressure exceeds arterial pressure.
  • This mechanism ensures proper direction and control of blood flow, allowing the heart to function efficiently during both contraction (systole) and relaxation (diastole).
• Valves and Blood Flow Regulation:
  
  • The valves of the heart are essential for preventing backflow of blood and ensuring that blood flows in a single direction through the heart’s chambers and into the arteries.
  • These valves open and close in response to pressure changes during the cardiac cycle, ensuring the heart pumps blood effectively and maintains proper circulation.

Question 100

How do the heart valves function during the cardiac cycle?

Answer 100

• Ventricular Contraction (Systole):
  
  • During ventricular contraction, the goal is to build a high pressure in the ventricles so that blood can be ejected into the arteries.
  • When the ventricular pressure exceeds arterial pressure, the semilunar valves (aortic and pulmonary) open to allow blood to flow from the ventricles into the aorta and pulmonary artery.
  • The AV valves (atrioventricular valves) close at this time to prevent backflow of blood into the atria.
  • The semilunar valves only open during this phase because they are dependent on the ventricular pressure being higher than arterial pressure.
• Ventricular Relaxation (Diastole):
  
  • During ventricular relaxation, the goal is to lower the pressure in the ventricles so they can fill with blood.
  • The AV valves open during this phase, allowing blood to flow from the atria into the ventricles.
  • The semilunar valves close to prevent the blood from flowing back into the ventricles from the arteries.
  • Ventricular relaxation is when the heart fills, and during this phase, the ventricles have a lower pressure than the arterial pressure, which is why the semilunar valves are closed.
• Valves and Pressure Differences:
  
  • There is never a time when all four heart valves (AV and semilunar) are open simultaneously, because the ventricular pressure can only either be higher or lower than arterial pressure or atrial pressure at any given time.
  • The AV valves are closed during ventricular contraction to prevent backflow into the atria. They open only when the ventricles relax and are filling with blood.
  • The semilunar valves open during ventricular contraction, allowing blood to be ejected into the arteries. They close during ventricular relaxation to prevent blood from flowing backward into the ventricles.
• Key Concept:
  
  • During ventricular contraction, the semilunar valves are open and the AV valves are closed.
  • During ventricular relaxation, the AV valves are open and the semilunar valves are closed.
• Quiz Question Explanation:
  
  • A common question asks which event is inconsistent with the semilunar valves being open. The answer is ventricular relaxation, because during this phase, the ventricular pressure is lower than the arterial pressure, and thus the semilunar valves close. If the ventricles were relaxing, the semilunar valves would not be open.

Question 101

How does the heart have its own circulatory system, and how does blood flow through it?

Answer 101

• Heart’s Own Circulatory System:
  
  • The heart has its own circulatory system, which is responsible for supplying the heart muscle (myocardium) with oxygen-rich blood. This system ensures that the heart cells receive the oxygen they need to function efficiently, as the heart is constantly active and requires a continuous supply of oxygen.
• Blood Flow Through the Coronary Arteries:
  
  • Oxygen-rich blood from the aorta flows into the left and right coronary arteries, which are the main vessels that supply blood to the heart itself.
  • These coronary arteries branch into smaller arteries, arterioles, and eventually into capillary beds that surround the heart. This branching structure is similar to the organization of blood vessels in the systemic circuit (the body’s main circulatory system).
  • The coronary arteries and their branches are specifically designed to supply oxygenated blood to the heart muscle. In a physiology course, while it’s not necessary to memorize the exact names of all the branches of the coronary arteries, understanding their role is important.
• Gas Exchange in the Heart’s Capillaries:
  
  • In the capillary beds surrounding the heart, gas exchange occurs. Oxygen is delivered to the heart muscle cells, and carbon dioxide (a waste product of cellular respiration) is picked up from the heart muscle cells.
  • After the exchange, the blood becomes deoxygenated (oxygen-poor) and is returned to the heart.
• Return of Deoxygenated Blood:
  
  • The deoxygenated blood from the heart muscle returns via a vein called the coronary sinus.
  • The coronary sinus empties into the right atrium of the heart, completing the circuit of blood circulation through the coronary system.
• Path to the Pulmonary Circuit:
  
  • From the right atrium, blood flows into the right ventricle and then enters the pulmonary circuit, where it will be oxygenated in the lungs.
• Coronary Sinus and Other Vessels Returning Blood to the Heart:
  
  • The coronary sinus returns blood specifically from the heart’s circulatory system to the right atrium.
  • Similarly, the superior vena cava returns deoxygenated blood from the upper body (e.g., head, neck) to the right side of the heart, while the inferior vena cava returns blood from the lower body to the right atrium.
  • All three of these vessels—superior vena cava, inferior vena cava, and coronary sinus—return deoxygenated blood to the heart.

Question 102

Answer 102

Question 103

Answer 103

Question 104

Answer 104

Question 105

Answer 105

Question 106

Answer 106

Question 107

Answer 107

Question 108

Answer 108

Question 109

Answer 109

Question 111

Answer 111

Question 112

Answer 112

Question 113

Answer 113

Question 114

Answer 114

Question 115

Answer 115

Question 116

Sympathetic vs. Parasympathetic Nervous Systems

Answer 116

Key Takeaways (Reinforced)

• Sympathetic:
  
  • Think “stress response.”
  • Short preganglionic, long postganglionic.
  • Thoracolumbar (T1-L2) origin.
  • Norepinephrine (mostly) at target organs.
  • Ganglia close to spine.
• Parasympathetic:
  
  • Think “relaxation.”
  • Long preganglionic, short postganglionic.
  • Craniosacral origin (CN III, VII, IX, X; S2-S4).
  • Acetylcholine at target organs.
  • Ganglia near target organs.
• Vagus Nerve (CN X):
  
  • Major parasympathetic nerve.
  • Controls many internal organs.
  • Originates in the medulla oblongata.
• Sympathetic Chain: runs along the entire length of the spinal cord, even though the preganglionic neurons only exit the thoracic and lumbar regions.

Question 117

Adrenal Medulla and Sympathetic Activation

Answer 117

Question 118

Neurotransmitters and Receptors in the Autonomic Nervous System

Answer 118

Explanation of Receptor Types:

Nicotinic Receptors (N):
These are ionotropic receptors, meaning they are ligand-gated ion channels.
When ACh binds, they open, allowing ions to flow, leading to depolarization and excitation.
They are found in the autonomic ganglia (where preganglionic neurons synapse with postganglionic neurons) and at the neuromuscular junction in skeletal muscle.
Muscarinic Receptors (M):
These are metabotropic receptors, meaning they are G protein-coupled receptors.
When ACh binds, they trigger intracellular signaling cascades.
They are found on the effector organs of the parasympathetic nervous system (e.g., heart, smooth muscle, glands) and also sweat glands and some blood vessels innervated by the sympathetic nervous system.
There are multiple subtypes of Muscarinic receptors. They can be excitatory or inhibitory.
Adrenergic Receptors (α and β):
These are also metabotropic receptors, activated by norepinephrine (NE) and epinephrine.
They are found on the effector organs of the sympathetic nervous system.
There are multiple sub types of alpha and beta receptors. They also can be excitatory or inhibitory.
Key Points:

Acetylcholine (ACh): The neurotransmitter used by all preganglionic neurons (both parasympathetic and sympathetic) and by postganglionic parasympathetic neurons.
Norepinephrine (NE): The primary neurotransmitter used by most postganglionic sympathetic neurons.
Ganglia: These are clusters of neuron cell bodies where preganglionic neurons synapse with postganglionic neurons.
Effector Organs: These are the target tissues (e.g., heart, smooth muscle, glands) that respond to autonomic nervous system signals.

Question 119

Receptors in PNS

Answer 119

Acetylcholine Receptors (AChRs):

• Location:
  
  • As you correctly stated, AChRs are found in both the somatic nervous system (at the neuromuscular junction) and the parasympathetic nervous system (at both pre- and postganglionic synapses).
  • This means ACh is a very important neurotransmitter in many areas of the nervous system.
• Differential Responsiveness:
  
  • This highlights the existence of different AChR subtypes, each with unique sensitivities to various compounds.
  • Agonists: These are substances that bind to a receptor and activate it, mimicking the action of the natural neurotransmitter (ACh in this case).
  • Antagonists: These are substances that bind to a receptor but block its activation, preventing the natural neurotransmitter from having its effect.

Nicotinic AChRs (nAChRs):

• Location:
  
  • Predominantly found in the somatic nervous system (at the neuromuscular junction) and at the autonomic ganglia (both sympathetic and parasympathetic).
• Response:
  
  • They are named “nicotinic” because they are activated by nicotine, a compound found in tobacco products. This is why nicotine can have effects on both muscle function and autonomic activity.

Muscarinic AChRs (mAChRs):

• Location:
  
  • Primarily found in the autonomic nervous system, specifically on the effector organs of the parasympathetic nervous system.
• Response:
  
  • They are named “muscarinic” because they are activated by muscarine, a toxin found in certain mushrooms. This is why mushroom poisoning can have profound effects on parasympathetic function.
  • Atropine: You correctly stated that atropine is a competitive antagonist of mAChRs. This means it blocks the effects of ACh at muscarinic receptors. This explains why atropine is used to treat certain conditions involving excessive parasympathetic activity.

Adrenergic Receptors:

• Activation:
  
  • These receptors are activated by catecholamines, which include epinephrine (adrenaline) and norepinephrine (noradrenaline).
• Subtypes:
  
  • The existence of numerous subtypes (α1, α2, β1, β2, etc.) allows for diverse and finely tuned responses in different target tissues.
  • The specific subtype expressed by a target cell determines how that cell will respond to catecholamines.
  • Target cell response. For example, Beta 1 receptors increase heart rate, and Beta 2 receptors cause bronchodilation.

In essence:

• The PNS utilizes a variety of receptors to ensure precise control over its target tissues.
• The existence of receptor subtypes and the availability of agonists and antagonists allow for pharmacological manipulation of PNS function.
• The autonomic nervous system uses acetylcholine at the preganglionic neuron, and then uses acetylcholine in the parasympathetic post ganglionic neuron, and norepinephrine in the sympathetic post ganglionic neuron. The sweat glands are a exception to the sympathetic rule, and use acetylcholine.

Question 120

Absolute Refractory Period:

Answer 120

Absolute Refractory Period:

• Key Concept:
  
  • During this period, no matter how strong the stimulus, a second action potential cannot be generated.
• Mechanism:
  
  • The voltage-gated sodium channels are either already open (during depolarization) or in an inactivated state.
  • The inactivation gate, a part of the sodium channel, blocks the channel pore, preventing sodium ions from entering.
  • To remove this inactivation gate and allow the channels to return to their closed (resting) state, the membrane must repolarize.
  • Essentially, the sodium channels are “busy” and unable to respond to another stimulus.

Relative Refractory Period:

• Key Concept:
  
  • During this period, a second action potential can be generated, but it requires a stronger-than-normal stimulus.
• Mechanism:
  
  • Some voltage-gated sodium channels have returned to their resting state, making them available to open.
  • However, some potassium channels are still open, causing ongoing repolarization or even hyperpolarization.
  • Therefore, a larger stimulus is needed to overcome the potassium efflux and depolarize the membrane to threshold.
  • The stimulus must be large enough to overcome the increased potassium permeability, and the resulting hyperpolarization.

Action Potential Propagation:

• Key Concept:
  
  • Action potentials travel along the axon without losing strength (amplitude).
• Mechanism:
  
  • Action potentials are regenerated at each point along the axon.
  • The influx of sodium ions at one point creates a local current that depolarizes the adjacent region of the membrane.
  • This triggers a new action potential in that region, and the process repeats.
  • This is why the action potential at the axon terminal is identical to the one that started at the trigger zone.
  • This is very important for long distance communication within the nervous system.

Generation of Action Potentials:

• For an action potential to occur, the membrane potential must reach threshold.
• The threshold potential is the minimum level of depolarization required to open the voltage-gated sodium channels.
• Once threshold is reached, a rapid influx of sodium ions occurs, leading to the rising phase of the action potential.

In summary:

• The refractory periods ensure unidirectional propagation of action potentials and limit the frequency of nerve impulses.
• Action potential propagation is a self-regenerating process that allows for long-distance signaling.
• The action potential is an all or nothing event. Once the threshold is reached it will occur.

Question 121

What neurotransmitters are released within the somatic vs the autonomic PNS and to which receptors do they bind?

Answer 121

Question 122

A 12 year old girl enters the emergency room in anaphylactic shock after ingesting peanuts, a substance she is very allergic to. By the time you have assessed her, her throat has begun to close. What nervous system is activated and how would you go about treating her? What would you NOT want to do in this case and why?

Answer 122

Question 123

During muscle contraction, does the I band of a sarcomere shorten or stay the same length? The H zone? The A band?

Answer 123

Question 124

Where is calcium that initiates muscle contraction released from and how is it released? Also, how does that calcium enable sarcomeres to shorten, and how is it removed from the sarcomeres during relaxation?

Answer 124

Question 125

True/False: Increased stores of Ca2+ in the … will cause increased contractile force of a given muscle. Why/Why Not?

Answer 125

Question 126

Answer 126

B

Question 127

What is the absolute refractory period? Where are action potential triggered from and where do they go towards? What are the ions associated with this and what direction do they flow?

Answer 127

Question 128

Hematocrit refers to the concentration of [.] in the blood. The hematocrit value is typically higher in [men/women].
Applied Practice:
Aerobic Exercise (i.e. running from a bear) involves activation of the […] nervous system. This will cause an [increase/decrease] in hematocrit, caused by the action of […]. which stimulates erythrocyte production in the […].
During aerobic exercise, other than increase hematocrit, what is another way the body can meet its increased oxygen demand?

Answer 128

Question 129

Answer 129

B

Question 130

Action Potential Conduction:

Answer 130

1. Local Current Flow:
   
   • An action potential is initiated at the trigger zone (often the axon hillock).
   • This creates a local current, which flows towards the axon terminal.
   • This local current depolarizes the adjacent region of the axon membrane (point B).
2. Depolarization and Sodium Influx (Point B):
   
   • The depolarization at point B opens voltage-gated sodium (Na+) channels.
   • Sodium ions (Na+) rush into the cell, further depolarizing the membrane.
   • This triggers a positive feedback loop, where more Na+ channels open, leading to a rapid rise in membrane potential.
3. Repolarization (Point A):
   
   • The missing word is “become”
   • Voltage-gated Na+ channels become inactivated.
   • Voltage-gated potassium (K+) channels open.
   • Potassium ions (K+) flow out of the cytoplasm, causing the membrane to repolarize (become negative again).
4. Refractory Period and Charge Flow:
   
   • The region of the membrane that has just undergone an action potential (point A) enters the absolute refractory period.
   • The positive charge from the newly depolarized region (point B) flows both backward (towards point A) and forward (towards point C).
5. Unidirectional Propagation:
   
   • The backward flow of positive charge has no effect on point A because the Na+ channels are inactivated during the absolute refractory period.
   • Therefore, action potentials cannot move backward.
   • The forward flow of positive charge depolarizes point C, triggering a new action potential.
   • This process continues down the axon, ensuring unidirectional propagation.

Key Concepts:

• Unidirectional Propagation: The refractory period is crucial for ensuring that action potentials travel in only one direction along the axon.
• Local Currents: Action potentials propagate by generating local currents that depolarize adjacent regions of the membrane.
• Voltage-Gated Channels: Voltage-gated Na+ and K+ channels play essential roles in the depolarization and repolarization phases of the action potential.
• Absolute Refractory Period: The period during which a second action potential cannot be generated, regardless of stimulus strength.
• Inactivated Sodium Channels: The inactivation of Na+ channels is the key factor that prevents backward propagation of action potentials.

Question 131

Autonomic Nervous System Receptor Locations

Answer 131

Question 132

action potential conduction along an axon

Answer 132

• Action Potential Propagation: Local currents and ion movement drive the process.
• Local Current at Point B:
  
  • Wave of positive charge moves inside the axon.
  • Depolarizes membrane at Point B.
  • Key point: Local current → depolarization at Point B.
• Voltage-Gated Sodium (Na+) Channels at Point B:
  
  • Open due to depolarization.
  • Sodium (Na+) rushes in, making the inside more positive.
  • Positive feedback loop: More Na+ in → more depolarization → more channels open.
  • Key point: Na+ channels open → sodium influx → positive feedback.
• Propagation to Point C:
  
  • Local depolarization at Point B opens Na+ channels at Point C.
  • New action potential is generated at Point C.
  • Key point: Local depolarization → new action potential at Point C.
• Point A (Previous Action Potential Site):
  
  • Voltage-gated Na+ channels inactivated.
  • Voltage-gated potassium (K+) channels open → K+ flows out → repolarization.
  • Key point: Point A → Na+ channels inactivated → K+ channels open → repolarization.
• Refractory Period at Point A:
  
  • Prevents immediate reactivation.
  • Ensures action potential travels in one direction toward the axon terminal.
  • Key point: Point A → refractory period → unidirectional propagation.
• Summary:
  
  • Local current → depolarization at Point B.
  • Na+ channels open → sodium influx → positive feedback.
  • New action potential at Point C.
  • Point A → Na+ channels inactivated → K+ channels open → repolarization.
  • Point A → refractory period → unidirectional propagation.

Question 133

Cardiac Cycle Timeline: How Heart Valves Function

Answer 133

1️⃣ Ventricular Relaxation (Diastole) - Heart Filling

🕒 Start: Low ventricular pressure → Blood needs to enter the ventricles

🔹 AV Valves (Mitral & Tricuspid): OPEN → Blood flows from atria to ventricles

🔹 Semilunar Valves (Aortic & Pulmonary): CLOSED → Prevents blood from flowing back into ventricles

2️⃣ Ventricles Begin to Contract - Pressure Increases

🕒 Transition Phase: Ventricular pressure builds, but is still lower than arterial pressure

🔹 AV Valves: Close to prevent backflow into atria (“Lub” sound)

🔹 Semilunar Valves: Still closed (Not enough pressure to open them yet)

3️⃣ Ventricular Contraction (Systole) - Blood Ejection

🕒 Peak Contraction: High ventricular pressure → Blood must be pushed into arteries

🔹 AV Valves: CLOSED

🔹 Semilunar Valves: OPEN → Blood is ejected into aorta & pulmonary artery

4️⃣ Ventricles Relax - Pressure Drops

🕒 End of Systole: Ventricular pressure falls below arterial pressure

🔹 Semilunar Valves: CLOSE to prevent backflow (“Dub” sound)

🔹 AV Valves: Still closed at first, but preparing to open for the next cycle

5️⃣ Cycle Restarts - Ventricular Filling (Diastole)

🕒 Low Ventricular Pressure: Blood needs to enter ventricles again

🔹 AV Valves: Reopen → Blood flows from atria into ventricles

🔹 Semilunar Valves: Remain closed

Key Concept Summary:

✔ During contraction (systole): Semilunar valves open, AV valves close

✔ During relaxation (diastole): AV valves open, Semilunar valves close

Question 134

Wiggers Diagram Phases & Valve Function

Answer 134

1️⃣ Atrial Systole (Late Diastole) - “Atrial Kick”

🕒 Event: Atria contract to push final blood into ventricles

🔹 AV Valves (Mitral & Tricuspid): OPEN

🔹 Semilunar Valves (Aortic & Pulmonary): CLOSED

📈 ECG: P wave (atrial depolarization)

🔊 Heart Sound: None (silent phase)

2️⃣ Isovolumetric Contraction - Ventricular Pressure Rises

🕒 Event: Ventricles start contracting, but no blood is ejected yet

🔹 AV Valves: CLOSE (“Lub” sound - S1)

🔹 Semilunar Valves: CLOSED (not enough pressure to open yet)

📈 ECG: QRS complex (ventricular depolarization)

📊 Pressure: Ventricular pressure rapidly rises

3️⃣ Ventricular Ejection - Blood Leaves the Heart

🕒 Event: Ventricles fully contract, pushing blood into arteries

🔹 AV Valves: CLOSED

🔹 Semilunar Valves: OPEN → Blood flows into the aorta & pulmonary artery

📈 ECG: ST segment (ventricular contraction continues)

📊 Pressure: Ventricular pressure > Arterial pressure → Ejection occurs

4️⃣ Isovolumetric Relaxation - Ventricles Relax

🕒 Event: Ventricles stop contracting, pressure drops, but no blood enters yet

🔹 AV Valves: CLOSED

🔹 Semilunar Valves: CLOSE (“Dub” sound - S2)

📈 ECG: T wave (ventricular repolarization)

📊 Pressure: Ventricular pressure drops below arterial pressure

5️⃣ Ventricular Filling - Blood Refills Ventricles

🕒 Event: Low ventricular pressure allows passive blood flow

🔹 AV Valves: OPEN → Blood flows from atria into ventricles

🔹 Semilunar Valves: CLOSED

📈 ECG: No specific wave (silent filling phase)

📊 Pressure: Atrial pressure > Ventricular pressure → Passive filling begins

Key Takeaways in Wiggers Terms:

✔ S1 (“Lub”) = AV valves closing at the start of systole

✔ S2 (“Dub”) = Semilunar valves closing at the start of diastole

✔ Blood ejection only occurs when semilunar valves are open

✔ Ventricular filling happens when AV valves are open