Chapter 4: The Nervous System Flashcards
Neurons
Transmit electrical impulses and then translates those electrical impulses into chemical signals.
Cell body of a neuron
The cell body (soma) of a neuron contains the cells nucleus, The endoplasmic reticulum, and ribosomes.
Dendrites of a neuron
Appendages emanating from the soma of the cell body of a neuron which receive incoming messages from other cells.
Axon hillock of a neuron
The information received from the dendrites is transmitted through the cell body before it reaches the axon hillock, which integrates the incoming signals.
Action potential of a neuron
The transmission of electrical impulses down the axon.
Axon of a neuron
The axon is a long appendage that terminates in close proximity to a target structure (muscle, gland, another neuron).
Myelin of a neuron
Insulation of most mammalian nerve fibers. Fatty membrane to prevent signal loss or crossing of signals. The myelin sheath maintains the electrical signal within one neuron. Myelin also increases the speed of conduction in the axon.
Production of myelin
Myelin is produced by OLIGODENDROCYTES in the central nervous system and SCHWANN cells in the peripheral nervous system.
Dendro- -dendron mean “TREE”
Nodes of Ranvier (rawn-vee-ay)
Small breaks in the myelin sheath at certain intervals along the axon. Critical for rapid signal conduction.
Nerve terminal (synaptic bouton or knob)
Located at the end of the axon, the structure is enlarged and flattened to maximize transmission of the signal to the next neuron and ensure proper release of neurotransmitters.
Neurotransmitters
Chemicals that transmit information between neurons.
Synaptic cleft
A small space into which the terminal portion of the axon releases neurotransmitters which bind into the dendrites of the adjacent neuron (the postsynaptic neuron). Neurotransmitters released from the axon terminal traverse the synaptic cleft and bind to receptors on the post synaptic neuron.
Synapse
Together, the nerve terminal, synaptic cleft, and postsynaptic membrane are known as the synapse.
Nerve
Multiple neurons bundled together in the peripheral nervous system.
Three kinds of nerves
Sensory, motor, or mixed
Refers to the type or types of information they carry. Mixed nerves carry both sensory and motor information.
Tracts of neurons
In the central nervous system, axons may be bundled together to form tracts. Unlike nerves, TRACTS ONLY CARRY ONE TYPE OF INFORMATION. The cell bodies of neurons in the same tract are grouped into NUCLEI.
Nerve tracts are bundles of nerve fibers that connect the nuclei of the central nervous system
When it is said that “tracts of neurons only carry one kind of information,” it means that a specific bundle of neurons within the nervous system is dedicated to transmitting a particular type of sensory data (like touch, pain, vision) or motor command (like muscle movement), rather than carrying a mix of different information types; essentially, each neural tract has a specialized function based on the information it carries.
Multiple sclerosis (MS)
A common demyelinating disorder. The myelin of the brain and spinal cord is selectively targeted. The body mounts an immune response against its own myelin, leading to the destruction of its insulating substance called myelin (demyelination).
Because so many different kinds of neurons are demyelinated, patient who have MS experience a wide variety of symptoms, including weakness, lack of balance, vision, problems, and incontinence.
Other cells in the nervous system
Astrocytes: structural support, wound repair, ion regulation
Ependymal cells: form the blood brain barrier
Microglia: mediate immune response in CNS.
Oligodendrocytes (produces myelin in CNS), and Schwann cell (produces myelin PNS).
Glial cell, or neuroglia
Glial cells play both structural and supportive roles. Neurons must be supported by and myelinated by other cells. Most abundant in central nervous system. Oligodendrocytes (myelinate axons of the CNS) and Schwann cells (myelinate axons of the PNS) are considered glial cells.
Astrocytes
Nourish neurons and form and regulate the blood brain barrier.
Ependymal cells
Line the ventricles (each of the four connected fluid-filled cavities in the center of the brain) of the brain that produce cerebrospinal fluid, which physically supports the brain and services a shock of absorber.
Microglia
Phagocytic cells that ingest and breakdown waste products and pathogens in the central nervous system (CNS). Microglia are the primary immune cells of the central nervous system. Macrophages resident in the CNS.
Oligodendrocytes (CNS) and Schwann cells (PNS)
Produce myelin around axons.
Oligodendrocytes are brain cells that produce myelin in the CNS.
Schwann cells are glial cells that support and protect axons of the PNS by creating myelin sheath.
Action potentials
An all or nothing message used to relay electrical impulses down the axon to the synaptic bouton. Ultimately cause the release of neurotransmitters into the synaptic cleft.
Resting membrane potential
Net electric potential different that exist across the cell membrane, created by a movement of charged molecules across the membrane. The inside of the neuron is negative relative to the outside. The two most important ions involved in generating them and maintaining the resting potential of potassium (K+) and sodium (Na+).
Potassium leak channels
Facilitate the OUTWARD movement of potassium across the cell membrane. As potassium continually leaks out of the cell, the cell loses a small amount of positive charge, leaving behind a small amount of negative charge and making the outside of the cell slightly positively charged.
Equilibrium potential of potassium
As potassium leaves the cell, negative charges buildup inside the cell. This eventually causes an equilibrium where potassium is both leaving and entering the cell at the same rate. This is known as the equilibrium potential of potassium.
Sodium leak channels
There is a driving force pushing sodium INTO the cell across sodium leak channels, this causes a buildup of electrical potential.
Resting membrane potential
The balance of the net effect of sodium and potassium equilibrium potentials. Neither Ion is ever able to established its own equilibrium, so both ions continue leaking across the cell membrane.
Na+/K+ ATPase
Enzyme that continually pump sodium and potassium to where they started: potassium into the cell and sodium out of the cell, to maintain the respective gradient. More ATP spent on this enzyme to maintain these gradients than for any other single purpose.
Maintenance of resting membrane potential
Two kinds of input a neuron can receive
Exititory input (excites a neuron to pass on a signal) and inhibitory (work to prevent a neuron from firing) input.
This distinction truly comes at the level of the neurotransmitter receptors.
Excitatory input
Excitatory input causes depolarization and that makes the neuron more likely to fire an action potential. Raises the membrane potential from its resting potential.
Inhibitory input
Causes hyperpolarization that makes the neuron less likely to fire an action potential. Lowers the membrane potential from its resting potential.
GABA (gamma-aminobutyric acid) is a great example of inhibitory input of a neurotransmitter. GABA is a neurotransmitter.
Threshold value
If the axon hillock receives enough excitatory input to be polarized to the threshold value, an action potential will be triggered.
Summation (nerve stimulus)
The additive effect of multiple signals. A post synaptic neuron may receive information from several different presynaptic neurons, some of which are excitatory and some of which are inhibitory.
The process by which neurons combine electrical impulses from multiple sources to determine whether or not to generate an action potential.
Two types of summation
Temporal summation and spatial summation
Temporal summation
Multiple signals are integrated during a relatively short period of time. Example being a number of small excitatory signals firing at nearly the same moment could bring a postsynaptic cell to threshold, enabling an action potential.
Spatial summation
The additive effects are based on the number and location of the incoming signals. For example, a large number of inhibitory signals firing directly on the soma will cause a more profound hyperpolarization of the axon hillock then the depolarization caused by a few excitatory signals firing on the dendrites over a neuron.
Membrane potential versus time during an action potential
Key concept regarding sodium and nerves
Sodium wants to go into the cell because the cell is more negative inside, an electrical gradient, and has a lower concentration of sodium inside, a chemical gradient.
Impulse propagation
For a signal to be conveyed to another neuron, the action potential must travel down the axon and initiate neurotransmitter release.
Tetrodotoxin (TTX)
Toxin found in puffer fish. Blocks voltage gated sodium channels, blocking neuronal transmission. This can cause death because the phrenic nerves innervating the diaphragm can no longer depolarize, leading to paralysis of the muscle and cessation of breathing.
TTX impacts all neurons in the body effectively paralyzing the body including respiratory functions making respiratory failure the primary cause of TTX poisoning.
Action potentials within the same type of neuron
Action potentials within the same type of neuron have the same potential difference during depolarization. Increased intensity of a stimulus does not result in an increased potential difference of the action potential, but rather an increased frequency of firing.
Saltatory conduction
Given that myelin is an extraordinarily good insulator, it prevents the dissipation of the electrical signal. The insulation is so effective that the membrane is only permeable to ion movements at the nodes of Ranvier. Thus, the signal hops from node to node. This is called saltatory conduction.
Saltatory conduction can be recalled by thinking of the Spanish verb SALTAR, which means to jump.
The synapse
Pre-synaptic neuron
The neuron proceeding the synaptic cleft
Postsynaptic neuron
The neuron after the synaptic cleft
Effector (Neural synapse)
If the postsynaptic cell is a gland or a muscle, rather than another neuron, it is known as an effector.
Three main mechanisms of removing neurotransmitters from the synaptic cleft
Enzymatic reactions, reuptake carriers, and diffusion.
Example of enzymatic removal of a neurotransmitter from the synaptic cleft
Acetylcholesterase will breakdown acetylcholine via enzymatic action.
Reuptake carriers
Neurotransmitter being brought back into the presynaptic neuron. Example being re-uptake of serotonin (5-HT) and norepinephrine (NE).
Diffusion of neurotransmitters
Where the neurotransmitter simply diffuse out of the synaptic cleft. For example nitric oxide, a gaseous signaling molecule, will simply diffuse out of the synaptic cleft.
Major divisions of the nervous system
Homeostasis
the ability of living organisms to maintain a stable internal environment in response to external changes
Functions of the nervous system
Sensation and perception
Motor function
Cognition (thinking) and problem-solving
Executive function and planning
Language, comprehension and creation
Memory
Emotion and emotional expression
Balancing coordination
Regulation of endocrine organs
Regulation of the heart rate, breathing rate, vascular resistance, temperature, and exocrine glands
Three kinds of nerve cell in the nervous system
Sensory neurons, motor neurons, and interneurons
Sensory neurons
AFFERENT neurons. Transmit sensory information from sensory receptors to the spinal cord and brain.
(AFFERENT neurons Ascend toward the brain, EFFERENT neurons Exit to spinal cord)
Motor neurons
EFFERENT neurons. Transmit motor information from the brain and spinal cord to muscles and glands.
(EFFERENT neurons Exit to spinal cord, AFFERENT neurons Ascend toward the brain)
Interneurons
Found between the other neurons and are the most numerous of the three types. Interneurons are located predominantly in the brain and spinal cord and are often linked to reflexive behavior.
Where does processing of stimuli and response generation occur?
Processing of stimuli response generation may happen at the level of the spinal cord, or may require input from the brainstem or cerebral cortex.
Example: when a reflex hammer hits the patellar tendon, the sensory information goes to the spinal cord where a motor signal is sent to the quadriceps muscle. No input of the brain is required.
What are the Supraspinal circuits?
The Supraspinal circuits are neuronal networks in the brain and brainstem that process stimuli and control complex motor tasks and other functions such as motor function, pain relief, cognitive function, affective functions (suffering, anxiety, fear, depression)
Supra prefix means “above”. Supraspinal means above the spine…..
Example: locomotion
Two primary components of the nervous system
The central nervous system (CNS)
The peripheral nervous system (PNS)
The central nervous system (CNS)
Composed of the brain and spinal cord. Both the brain and the spinal cord consist of white matter and gray matter.
White matter of the brain
Consist of axons encased in myelin sheaths. Lies deeper than the gray matter. White matter is considered “white” because of the pretense of fatty substance called myelin.
A good way to remember the location of white matter is that longer nephrons need to be myelinated to transmit signals along distances. The nervous systems begins extending the distance of the signals deeper in the brain as it goes into the spinal cord. Conversely, grey matter (unmyelenated) is in the periphery of the brain where the neurons are short and in direct connection with each other and thus need no myelin.
Gray matter of the brain
Consists of unmyelinated Cell bodies in dendrites. Composes the periphery of the brain.
Grey matter functions effectively despite lacking extensive myelination because it primarily consists of neuron cell bodies and short, unmyelinated axons, which allows for local information processing and complex integration of signals within a specific brain region, rather than rapid long-distance transmission like myelinated white matter does; essentially, grey matter is where information is received, processed, and modified before being sent further through the myelinated axons in white matter.
The spinal cord and it’s four regions
Part of the central nervous system, that extends downward from the brainstem and convey divided into four regions:
Cervical, thoracic, lumbar, and sacral
Consist of white and gray matter.
White matter of the spinal cord (location)
Lies on the outside of the spinal cord
Gray matter of the spinal cord (location)
Lies on the inside of the spinal cord. The inside of the spinal cord is composed of grey matter because this is where the cell bodies of neurons are located, which are the primary components of grey matter, allowing for the processing and integration of neural signals within the spinal cord itself; essentially, it’s the “working center” of the spinal cord where information is received and relayed to other parts of the nervous system.
Dorsal root ganglia
Cell body of the sensory neurons that bring information in from the periphery and end on the DORSAL side of the spinal cord.
Pathway of information in a sensory neuron in the spinal cord
The sensory neuron brings information in from the periphery and enter on the dorsal, or back, side of the spinal cord. This information from the sensory neuron runs through the dorsal root ganglia, as the name suggests is in the dorsal side. Motor neurons exit the spinal cord, ventrally, or front side, of the body.
Peripheral nervous system (PNS)
Connect the central nervous system to the rest of the body, and can itself be subdivided into the somatic and autonomic nervous systems. Made up of nerve tissue and fibers outside the brain and spinal cord. 31 pairs of spinal nerves and 10 of the 12 pairs of cranial nerves.
Subdivision of the peripheral nervous system
Somatic nervous system and autonomic nervous system (ANS)
Somatic nervous system
Consist of sensory and motor neurons distributed throughout the skin, joints, and muscles.
Sensory neurons transmit information through afferent (Ascend the spinal cord to CNS) fibers.
Motor impulses travel along efferent (Exit the spinal cord to body) fibers.
The somatic system primarily uses acetylcholine as a neurotransmitter (cholinergic)
Autonomic nervous system (ANS)
Generally regulates heartbeat, respiration, digestion, and glandular secretions, regulate body temperature. The autonomic nervous system manages the involuntary muscles associated with many internal organs and glands. Automatic, or independent of conscious control.
Peripheral component of the autonomic nervous system
PRE AND POSTGANGLIONIC
The peripheral component of the autonomic nervous system contains two neurons. Two neurons work in series to transmit messages from the spinal cord. The first neuron is known as the PREGANGLIONIC neuron, the second is the POSTGANGLIONIC neuron. The soma of the preganglionic neuron is in the central nervous system, and its axon travels to a ganglion in the peripheral nervous system.
The first neuron in the ANS is the pre and the second is the post.
All post ganglionic parasympathetic autonomic are cholinergenic. Pre ganglionic sympathetic are cholinergenic. Postganglionic sympathetic are norandrogenic (except for sweat gland which are cholinergenic)
Autonomic nervous system subdivisions
Sympathetic nervous system and parasympathetic nervous system. Usually act in opposition to each other, or are ANTAGONISTIC.
Example: the sympathetic nervous system works to accelerate the heart rate and inhibit digestion, while the parasympathetic nervous system decelerates heart rate and promote digestion.
Sympathetic: fight or flight
Parasympathetic: rest and digest
Parasympathetic nervous system role
The main role of the parasympathetic nervous system is to conserve energy. Associated with resting and sleeping states and act to reduce heart rate and constrict the bronchi, manages digestion by increasing peristalsis and exocrine secretions.
ACETYLCHOLINE IS THE NEUROTRANSMITTER RESPONSIBLE FOR PARASYMPATHETIC RESPONSES IN THE BODY AND IS RELEASED BY BOTH PREGANGLIONIC AND POSTGANGLIONIC NERVE NEURONS.
Vagus nerve (cranial nerve x)
Responsible for much of the parasympathetic innervation of the thoracic and abdominal cavity.
Sympathetic nervous system roles.
Activated by stress whether it’s a mild stress or emergency. Closely associated with rage and fear, also known as fight or flight reactions.
Recall that the sympathetic nervous system primarily uses norepinephrine as a neurotransmitter (known as being norandrogenic).
Increases heart rate, redistributes blood to muscles of locomotion, increases blood glucose concentration, relaxes the bronchi, decreases digestion and peristalsis, dilates the eyes to maximize light intake, releases epinephrine into the bloodstream.
Neurotransmitter of sympathetic nervous system (pre and post ganglion neurons)
Acetylcholine in preganglionic neurons (cholinergenic)
Norepinephrine In postganglionic neurons (norandrogenic or adrenergic)
Neurotransmitter of parasympathetic nervous system
Acetylcholine
Reflex arcs
Neural pathway that controls an automatic response to a stimulus, without involving the brain.
Handled by interneurons after a signal from the sensory neurons.
Two types of reflex arcs
Monosynaptic and polysynaptic
Monosynaptic reflex arc
There is a single synapse between the sensory neuron that receives the stimulus and the motor on that responds to it.
The classic example is the knee-jerk reflex. When the patellar tendon is stretched, information travels up the sensory (afferent presynaptic) neuron to the spinal cord, where it interfaces with the motor (efferent postsynaptic) neuron that causes contraction of the quadriceps muscle.
Polysynaptic reflex arc
There is at least one interneuron between the sensory and motor neurons. A real example is the withdrawal effect.
You step on a nail and are stimulated to flex to get your foot from the nail. This is a monosynaptic reflex, similar to the knee-jerk reflex, except to maintain balance the other foot must be planted firmly on the ground. For this to occur, the motor neuron that controls the quadriceps muscle in the opposite leg must be stimulated, extending it.
Nerves versus tracts
Nerves are in groups of neurons in the peripheral nervous system.
Tracts are groups of nerves in the central nervous system.
What is the phrenic nerve?
The phrenic nerve controls the diaphragm.
