Chapter 11- Fundamentals of nervous system and nervous tissue Flashcards
Main functions of the nervous system (3)
These functions can overlap or co-occur
- Sensory input
- Integration
- Motor output (motor response)
Sensory input function
Monitors changes that occur inside the body and outside the body. We have many receptors, usually for one specific sensation
Integration function
Processing and integration of input information- the nervous system decides what response to make. The brain decides what the input means
Motor response function
Nervous system activates effector organs to cause a response. The brain or spinal cord sends a message based on the input.
Neuroglia (glial cells)
Provide support and maintenance to neurons
Neurons
Nerve cells that can respond to stimuli and transmit electrical signals. They can change membrane potential in a fraction of a second, which creates a message.
Neurons are the functional unit of the nervous system
What organs make up the central nervous system?
Brain and spinal cord
Central nervous system function
Function- is responsible for interpreting sensory input and deciding motor output
What is the peripheral nervous system composed of?
Composed of bundles of nerves coming from the brain/spinal cord
Peripheral nervous system function
Function- spinal nerves and cranial nerves link the rest of the body to the central nervous system. If a nerve is severed, that part of the body no longer exists to your brain
2 subdivisions of the peripheral nervous system
- Afferent division
2. Efferent division
Afferent division (PNS)
Carries impulses from the body to the central nervous system. Impulses allow the CNS to interpret information and send out a response.
Impulses from this division arrive at the brain
Efferent division (PNS)
Carries impulses from the CNS to the effector organs. Impulses activate muscle or glands to carry out motor response.
Impulses from this division exit the brain
Types of neuroglia (glial cells) in the CNS (4)
- Astrocytes
- Microglial cells
- Ependymal cells
- Oligodendrocytes
Types of neuroglia (glial cells) in the PNS (2)
- Satellite cells
2. Schwann cells
Astrocytes
Most abundant and versatile of the neuroglia
Star shaped, with projections connecting to and wrapping around neurons, synaptic nerve endings, and surrounding blood capillaries. Found in the CNS.
Major functions of the astrocytes (4)
- Provide a nutrient supply for neuron cells
- Makes the sure the neuron is growing in the correct direction
- Allows migration of young neurons
- “Clean up” outside neuron cells
What do astrocytes clean up outside of neuron cells?
Some substances could interfere with the function of the neurons if they’re freely floating around, such as leaked K+ ions, neurotransmitter
Microglial cells
Have long and thin processes that touch neurons, found in the CNS.
Microglial cells functions (2)
- Contact nearby neuron cells to monitor health - microglial cells pick up on neurons not doing what it’s supposed to do
- Phagocytize injured neurons
Why must microglial cells phagocytize injured neurons?
Microglial cells migrate toward injured neurons, where they transform into a macrophage and phagocytize the neuron (these neurons can’t send messages correctly, so they must be cleaned up).
Importance- the immune system has very limited access to the central nervous system
Ependymal cells
Function- lines central cavities of the central nervous system to circulate cerebrospinal fluid (CSF) within cavities. Most cells have cilia (to move and circulate CSF). Found in the CNS.
Oligodendrocytes
Associated with thicker nerve fibers in the CNS, extensions of the cells wrap around each fiber to produce a myelin sheath
Oligodendrocytes function
Create an insulating covering for individual neurons of the CNS
Importance- allows for fast and efficient transmission of electrical impulses
Satellite cells
Support and protect neuron cells in the PNS-cleanup duty, helps neurons get oxygen and nutrients.
Functionally similar to astrocytes, found in the PNS.
Schwann cells
Surround nerve fibers of PNS to form myelin sheaths, functionally similar to oligodendrocytes
Other than transmitting electrical impulses, what 3 characteristics do neurons have?
- Longevity- neurons last a lifetime. The nervous system will not be replaced over time, in contrast to other organ systems
- Amitotic- neurons do not reproduce
- Metabolism- neurons are very metabolically active. A portion of oxygen and nutrients dedicated to the nervous system
What 2 general structures do neurons have?
- Cell body
2. Processes
Cell body of neurons
Portion of cell containing the nucleus. Most are found in the CNS, protected by bone
Cell body function
Function- plasma membrane can receive information from surrounding neurons
Nuclei
Name for clusters of cell bodies in the CNS, however, ganglia is typically used as an umbrella term for all cell bodies.
Ganglia
Name for clusters of cell bodies in the PNS, but is typically used as an umbrella term for all cell bodies
Processes of neurons
Arm like extensions from the cell body of all neurons. There are 2 types.
Types of processes of neurons (2)
- Axons
2. Dendrites
Dendrites
Main receptive region of the neuron. A single neuron can have dozens of dendrites
Dendrites function
Function- provide increased surface area for incoming signals to be received, convey incoming messages toward the cell body. You want neurons to be highly sensitive to messages.
Axon
Single nerve fiber coming from a cell body. The axon is the conducting region of the neuron. Axons branch at the end to form terminal branches and form axon terminals.
Axon function
Function- generates and transmits nerve impulses away from the cell body
Tracts
Bundles of axons in the CNS- axon bundles save space. However, nerve is typically used as an umbrella term
Nerves
Bundles of axons in the PNS- axon bundles save space. However, nerve is typically used as an umbrella term for the CNS as well.
Axon terminals
Axons branch at the end to form terminal branches and form axon terminals.
The axon terminal is the portion of the neuron that releases the neurotransmitter, which will be received by the dendrites
Axon terminal function
Function- neurotransmitter released at the axon terminal to pass along the impulse to the next neuron
Myelin sheath function (2)
- Protects and electrically insulates long and/or large nerve fibers
- Increases speed at which these impulses are transmitted
Myelin sheath
Fatty covering, usually associated with specific types of fibers, like very long axons. Not usually found with short or thin fibers. Not all axons are myelinated.
Myelin sheath gaps
There are multiple Schwann cells on the axon in the PNS, but do not make contact with one another
How is myelination in the PNS accomplished?
Accomplished by Schwann cells, which wrap themselves around the axon multiple times to create layers of insulation
How is myelination in the CNS accomplished?
Accomplished by oligodendrocytes- a single oligodendrocyte can cover 60+ axons with branching processes. The oligodendrocyte processes wrap around the axon in layers, covering a much larger portion than a schwann cell.
A single oligodendrocyte has multiple processes and can therefore insulate multiple neurons
How are neurons grouped?
According to the direction in which nerve impulses travel relative to the central nervous system. Afferent or efferent
Afferent neurons
Sensory neurons- afferent neurons transmit signals from the body to the CNS.
Receptive endings can function as actual sensory structures, or they are associated with larger sensory receptors (other cell types).
A larger sensory receptor can receive more information and send it to the brain.
Where are the cell bodies of afferent neurons found?
Cell bodies are found outside the CNS
Efferent neurons
Motor neurons- efferent neurons transmit motor responses from the CNS to the body.
The cell bodies found inside the CNS.
Impulses travel to effector organs (muscles and glands).
Interneuron
Lie between sensory and motor neurons. Most neurons in the body are interneurons.
Mostly confined only to the CNS, make up 99% of all neurons in the body
Interneuron function
Function- pass signals through CNS pathways where integration and interpretation occurs
What is the resting membrane potential value?
-70 mV
Why do cells have a resting membrane potential?
So the inside of the cell is more negatively charged than the outside. All cells have a resting membrane potential, but neurons can change theirs faster than other cells.
Resting membrane potential importance in neurons
Neurons are able to change their resting membrane potential faster than other cell types- neuron communication occurs when the membrane potential changes. Without this quality- the nervous system loses its function. The CNS would not know what was going on. There must be a membrane potential and it must be able to change quickly.
Voltage
The measure of the potential energy by separate electrical charges (measured in V or mV). The greater the difference in charge between two points (or charges), the higher the voltage.
What creates voltage in the human body?
In the human body, difference in charge on either side of the cell’s plasma membrane creates voltage (also called a potential). Each side of the membrane is considered 2 points.
Current
Flow of electrical charge from one point to another, can be used to do work
What creates current in the human body?
In the human body, currents reflect movement of ions across a membrane along the axon
Resistance
Hindrance of charge flow from substances through which charge must travel
What creates resistance in the human body?
In the human body, plasma membranes provide resistance. Sodium is stopped from entering the cell entirely.
Insulator
A substance with high resistance (the myelin sheath is an example).
Conductor
A substance with low resistance
What types of signals can be produced by changes in resting membrane potential? (2)
- Graded potential
2. Action potential
Graded potential
Signals that have variable (graded) strength. They vary directly with stimulus strength, strong stimulus= strong graded potential. Usually, they are incoming signals over short distances. Can be hyperpolarizing or depolarizing.
Action potential
A very brief reversal of membrane potential (from -70 mV to +30 mV). They do occur long distance and only occur in axons. Strength does not vary- action potentials are all or nothing.
A change in membrane potential can be produced by (2)
- Anything that alters ion concentrations on two sides of the membrane- this would decrease the difference between points of charge.
- Anything that changes membrane permeability to any ion. Only permeability changes are important for transferring information. Ions can cross in greater amounts or prevent them from moving entirely.
Ion channels
Selective proteins in the plasma membrane allow passage of ions into/out of the cell.
Types of ion channel proteins (2)
- Leakage (non-gated) channels
2. Gated proteins: chemically, voltage, or mechanically gated.
Leakage (non-gated) protein channels
Always open, allow free flow of ions. Function depends on the specific protein.
Gated proteins
Part of protein forms a gate that must be opened before ions can move. When the channels open, ions will move across the membrane
Chemically gated channel
Only open when a certain chemical (neurotransmitter) binds to a protein
Voltage gated channel
Open and close in response to changing membrane potentials
Mechanically gated channel
Open in response to physical deformation of the receptor, usually associated with sensory receptors for touch and pressure.
What determines the direction of ion movement?
Electrochemical gradients
Components of electrochemical gradients (2)
- Concentration gradient- ions move from higher to lower concentration
- Electrical gradient- ions move to an area of opposite charge
These components rarely work together
Depolarization
A decrease in membrane potential (charge difference). The inside of the membrane becomes less negative than resting potential (potential (mV) becomes more positive).
Depolarization function
Function- depolarizing the membrane increases the probability of producing a nerve impulse, results in excitation of a neuron.
Hyperpolarization
An increase in membrane potential (greater difference in charge). The inside of the membrane becomes more negative than resting potential (potential (mV) becomes more negative).
Types of graded potentials (2)
- Receptor potential
2. Postsynaptic potential
Receptor potential
Produced when sensory receptor is excited by its specific stimulus- the sensory receptor is responding directly to its stimulus
Postsynaptic potential
Produced when the stimulus is released by another neurotransmitter
Why do graded potentials only occur over short distances?
Charge is lost quickly due to leaky channels- current dies off quickly as well
Function of graded potentials
Important function- graded potentials are absolutely necessary to initiate an action potential at the axon. No graded potential= no function in neuron
Which cells do action potentials occur in?
Neurons and muscle cells only
Action potentials importance
Important because this is the actual message sent to or from the nervous system
How are action potentials generated?
APs are generated from graded potentials, originating at the trigger point. Change in membrane potential from graded potential causes voltage gated channels to open- generation of an AP involves opening of voltage-gated ion channels in the membrane in response to changing membrane potential
Trigger point
The beginning of the axon coming from the cell body where action potentials originate
2 gates of a sodium channel
- Activation gate- voltage sensitive, opens at depolarization. If the voltage of the membrane changes, the gate opens
- Inactivation gate- blocks the ion channel once it is open
Process of generating an action potential (4 steps)
- All voltage gated channels are closed at the resting state (-70 mV)
- Depolarization- voltage gated sodium channels open
- Repolarization- sodium channels are inactivated and potassium channels open
- Hyperpolarization- excess potassium leaves the cell
What molecules does the sodium potassium pump move out and in?
3 Na+ out, 2 K+ in
When can a nerve fiber send out another action potential after sending the first?
When the sodium and potassium concentrations outside and inside the cell have been re-established.
Threshold point
Depolarization must reach a “threshold point” to generate an AP. The threshold value= -55 mV.
What is the purpose of a threshold point?
Not all depolarizing events will result in an AP- sometimes, incoming graded potentials are too weak. A threshold point weeds out weak signals that shouldn’t be integrated
How does an action potential travel the length of an axon?
The influx of sodium causes a local current. This change causes depolarization of adjacent areas of the membrane, and Na+ channels open in these new areas as well. Na+ channels nearer the AP origin are inactivated, so no new AP can be generated here (must be unidirectional).
Why are action potentials unidirectional?
Action potentials are unidirectional- sending information in multiple directions will likely result in the nervous system either not receiving the message or taking a very long time to receive the message. An AP always propagates away from the origin.
How does the nervous system discriminate between a strong stimulus and a weak stimulus?
This depends on how frequently nerve impulses are generated. For strong stimuli- impulses are sent more frequently in a given period of time. Your brain recognizes that this is something it should pay attention to. For weak stimuli- impulses are sent less frequently in a given period of time
Refractory period
A period of time in which a second AP cannot be generated at an axon. Occurs when Na+ voltage gated channels are open
Types of refractory periods (2)
- Absolute refractory period
2. Relative refractory period
Absolute refractory period
Begins when Na+ gated channels open and continues until Na+ channels reset to their original state. During this time, another AP cannot be generated in the area, no matter how strong the stimulus is.
Importance of the absolute refractory period (2)
- Ensures each AP is a separate, all or none event
2. Enforces one way transmission of the AP
Relative refractory period
Follows the absolute refractory period, occurs after depolarization.
Stimuli that are relatively weak cannot stimulate an AP, but an exceptionally strong stimulus can- hyperpolarization causes mV to be more negative, so you need a stronger stimulus to reach threshold
Which 2 factors does conduction speed depend on?
- Axon diameter. A larger axon= faster conduction
2. Degree of myelination- more myelination= faster conduction
How do myelin sheaths influence conduction speed (2)?
- Continuous conduction
2. Saltatory conduction
Continuous conduction
Propagation in unmyelinated fibers- voltage gated ion channels are adjacent. Occurs slowly- taking very small steps
Saltatory conduction
Propagation in myelinated fibers. Voltage gated ion channels found in myelin sheath gaps
AP can only be generated in these gaps. Electrical signals will “jump” from gap to gap, goes much faster
Synapse
Junction between 2 neurons that sends information from one neuron to the next. Signals are transmitted between neurons at synapses
Presynaptic neurons
Conduct impulses toward the synapse
Postsynaptic neurons
Conduct signals away from the synapse
Synaptic cleft
A fluid filled space that separates neurons.
Types of synapses (2)
- Electrical synapses
2. Chemical synapses
Electrical synapses
Transmit signals very quickly. They have gap junctions that connect the cytoplasm of one neuron to another, allowing ions and small molecules to travel directly between them
Where are electrical synapses typically found?
Are more common in embryos than adults, found in areas of the brain associated with stereotypic (repetitive) movements.
Chemical synapses
Allow release and reception of neurotransmitters. Most common synapse type in the body
Synaptic vesicles
Found in chemical synapses. Presynaptic neuron releases neurotransmitter at the axon terminal via synaptic vesicles, and the postsynaptic neuron has a receptor region (dendrite/body cell) that senses the neurotransmitter.
Process of transmission of action potentials from one neuron to another (5 steps)
- Action potential arrives at the axon terminal of the presynaptic neuron
- Voltage gated Ca2+ channels open in response to AP, and calcium flows into the axon terminal of the presynaptic neuron
- Synaptic vesicles fuse with the membrane in response to Ca2+ influx. The neurotransmitter enters the synaptic cleft
- Neurotransmitter crosses cleft, binds to proteins on the postsynaptic neuron
- The binding of the neurotransmitter opens ion channels, resulting in graded potentials
After an action potential is transmitted, how are neurotransmitter effects terminated?
Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. The astrocytes will take up excess neurotransmitter and bring it back to the nerve fiber so it can store it for later. Also, enzymes can break down the neurotransmitter into smaller parts so they can’t interact with synapses. The neurotransmitter could also just float away from the synaptic cleft and therefore not interact with anything
Which aspects of the neurotransmitter can cause the graded potentials to vary in strength?
- Amount of neurotransmitter released
2. How long the neurotransmitter stays in the synaptic cleft
What determines if chemical synapses are excitatory or inhibitory?
Depends mostly on how membrane potential is affected
Excitatory postsynaptic potential (EPSP)
If enough neurotransmitter is bound to a postsynaptic neuron, an excitatory postsynaptic potential (EPSP) occurs at the postsynaptic membrane. Depolarization of postsynaptic membrane occurs, and EPSP triggers an action potential at the beginning of the axon. If the current reaching this portion of the axon is at the threshold, an AP is generated
Excitatory synapses
More likely to generate an AP, depolarizing in nature. Must be strong enough to reach threshold to trigger an AP
Inhibitory postsynaptic potentials (IPSP)
Inhibitory synapses reduce ability to generate an AP by hyperpolarizing the postsynaptic membrane
K+ channels or Cl- channels are opened, making the inside of the cell more negative and pushing membrane potential farther from the threshold value. This results in inhibitory postsynaptic potentials (IPSP)
Are EPSPs or IPSPs dominant?
Most neurons will receive both excitatory and inhibitory input from 1000+ other neurons. Whichever signal (EPSP or IPSP) is stronger will have greater influence on the postsynaptic neuron. If EPSP is predominant, an AP is generated.
Types of summation (2)
- Temporal summation
2. Spatial summation
Temporal summation
One or more presynaptic neurons transmit impulses in rapid fire order
Spatial summation
Postsynaptic neuron stimulated by a large number of terminals on multiple dendrites at the same time
Neurotransmitters
Chemical signals produced in the cell body that move into the axon. Most neurons produce at least 2 types- neurons can release one or more neurotransmitters simultaneously
Anterograde movement
Anterograde movement= away from cell body. This is how neurotransmitters move into the axon
What effects do neurotransmitters have (2)?
- Functional effects- can be excitatory, inhibitory, or can do either depending on the receptor type they bind
- Action effects- can be direct or indirect
Direct neurotransmitters
cause opening of ion channels directly
Indirect neurotransmitters
Have longer lasting effects, act through second messenger molecules. They can increase likelihood that a direct neurotransmitter can bind
Acetylcholine
Released at neuromuscular junctions (junction between nerve and muscle cells), formed from acetic acid and choline. Function- mostly stimulates skeletal muscle tissue and neurons in the autonomic nervous system (ANS).
Acetylcholinesterase (AChE)
The enzyme that degrades acetylcholine. Acetylcholine is split into choline and acetic acid so it can’t interact with the muscle fiber. Do not want unwanted or prolonged muscle contraction, so it must be degraded very quickly
Which neurotransmitters are considered biogenic amines? (5)
- Dopamine
- Norepinephrine
- Epinephrine
- Serotonin
- Histamine
What are biogenic amines synthesized from?
Various amino acids
Epinephrine
Biogenic amine. Adrenaline- helps with mobilizing the body for action, inducing the fight or flight response. Take a while to break down- people don’t “come down” from a situation for a few hours.
Histamine
Biogenic amine. Neuromodulator, involved with immune system function.
Biogenic amine function
Mostly involved in emotional behavior and regulates the biological clock.
Imbalances in biogenic amine neurotransmitters can result in (2)
- Schizophrenia or hallucinations. Hallucinations can result from drug use, schizophrenia occurs when an individual’s brain reacts differently to serotonin or dopamine.
- Low amounts of dopamine and serotonin are linked to depression and anxiety disorders
Amino acids neurotransmitters (4)
- Glutamate (excitatory)- acts in conjunction with substance P
- Aspartate (excitatory)
- Glycine (inhibitory)
- Gamma-aminobutyric acid (GABA) (inhibitory)
Peptides
Strings of amino acids that take on a broad spectrum of molecules, all of which having various effects. 2 types.
Peptide neurotransmitters (2)
- Substance P
2. Endorphins
Substance P
Peptide, mediator for pain signaling
Endorphins
Peptide. Reduce pain perception (opiates).
They are released during high stress or traumatic situations.
Endorphins take a while to break down- people don’t “come down” from a situation for a few hours.
Purines (2)
One type of nitrogen containing base that make up DNA and RNA. Includes ATP and adenosine.
ATP
Purine. More primitive neurotransmitter found in CNS and PNS, can produce fast and slow responses
Adenosine
Purine. Inhibitor in the brain. Caffeine blocks adenosine receptors- prevents inhibition and therefore makes you more alert.
Gases and lipids neurotransmitters
Different because they are not stored, they are produced by a neuron on demand. They are lipid soluble or small, so they pass through the membrane and interact with receptors inside the cell, not outside
Gases and lipids (3)
- Nitric oxide
- Carbon monoxide
- Hydrogen sulfide
Nitric oxide function
Participates in formation of new memories by increasing synapse strength in the brain (forming a synapse)
Endocannabinoids
Neurotransmitters that act on the same receptors as THC. Made on demand, not stored. Similar functions to THC- controls appetite, suppresses nausea, neuronal development, learning and memory
Types of neurotransmitter receptors (2)
- Channel linked receptors
2. G-coupled protein receptors
In neurons, a sodium influx causes
Depolarization
In neurons, a chloride influx causes
Hyperpolarization
Channel linked receptors
These receptors mediate fast synaptic transmission. Includes ligand gated ion channels, where a ligand binds to a channel linked receptor protein. Action is immediate, but brief
What happens when a ligand binds to a receptor protein?
When ligand binds to the channel linked receptor protein- the protein changes shape and the channel opens
G-protein coupled receptors
Contains transmembrane protein complexes. Response is indirect on the postsynaptic neuron and is complex and longer than the effect of a channel linked receptor.
Effects tend to include widespread metabolic changes in the postsynaptic cell
General process of a G-protein coupled receptor (3 steps)
- Neurotransmitter binds to the receptor
- G-protein is activated
- G-protein activates adenylate cyclase- second messengers are produced
Second messengers purpose
Second messengers can activate certain genes to produce proteins, can open/close ion channels, etc.
