Nervous System I Anatomy and Physiology | Tutorial Notes Nervous System I LEARNING OBJECTIVES After study of this chapter, the student should be able to: 1. Describe the general functions of the nervous system 2. Classify the general divisions of the nervous system 3. Identify the two types of cells that comprise the nervous system 4. Describe the parts of a neuron and indicate the function of each part. 5. Compare and contrast myelination in the PNS and the CNS 6. Distinguish between the sources of gray matter and white matter 7. Identify the 3 structural types of neurons 8. Identify the 3 functional types of neurons 9. Identify the 4 types of neuroglia in the CNS and indicate the function of each type. 10. Identify the 2 types of neuroglia in the PNS and indicate the function of each type. 11. Explain the major events that occur during synaptic transmission from a presynaptic neurons to a postsynaptic cell. 12. Define “membrane potential” and indicate what factors cause a membrane potential. Explain which cells have a membrane potential. 13. Define “resting membrane potential” and indicate which cells have a resting membrane potential. 14. Define “polarized”, “depolarized”, and “hyperpolarized” 15. Compare and contrast graded potentials with action potentials. 16. Describe the events leading to the generation of an action potential. 17. Explain the 3 phases of an action potential. Name the event that causes each phase. 18. Explain how an action potential is propagated along an axon. 19. Describe what is meant by “all-or-none” response 20. Explain how the strength of a stimulus affects the strength and frequency of action potentials. 1
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Nervous System I
Anatomy and Physiology | Tutorial Notes
Nervous System I
LEARNING OBJECTIVES
After study of this chapter, the student should be able to:
1. Describe the general functions of the nervous system
2. Classify the general divisions of the nervous system
3. Identify the two types of cells that comprise the nervous system
4. Describe the parts of a neuron and indicate the function of each part.
5. Compare and contrast myelination in the PNS and the CNS
6. Distinguish between the sources of gray matter and white matter
7. Identify the 3 structural types of neurons
8. Identify the 3 functional types of neurons
9. Identify the 4 types of neuroglia in the CNS and indicate the function of each type.
10. Identify the 2 types of neuroglia in the PNS and indicate the function of each type.
11. Explain the major events that occur during synaptic transmission from a presynaptic neurons to a postsynaptic cell.
12. Define “membrane potential” and indicate what factors cause a membrane potential. Explain which cells have a membrane potential.
13. Define “resting membrane potential” and indicate which cells have a resting membrane potential.
14. Define “polarized”, “depolarized”, and “hyperpolarized”
15. Compare and contrast graded potentials with action potentials.
16. Describe the events leading to the generation of an action potential.
17. Explain the 3 phases of an action potential. Name the event that causes each phase.
18. Explain how an action potential is propagated along an axon.
19. Describe what is meant by “all-or-none” response
20. Explain how the strength of a stimulus affects the strength and frequency of action potentials.
21. Compare and contrast impulse conduction in myelinated and unmyelinated neurons.
22. Explain 2 ways excitatory postsynaptic potentials (EPSPs) exert their effect on a postsynaptic cell.
23. Explain 2 ways inhibitory postsynaptic potentials (IPSPs) exert their effect on a postsynaptic cell.
24. Describe how EPSPs and IPSPs summate and indicate where summation occurs on the postsynaptic neuron.
23. Explain 2 ways postsynaptic cells are prevented from being continuously stimulated.
24. Indicate the function of acetylcholinesterase and monoamine oxidase
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25. Compare and contrast convergent pathways and divergent pathways with regards to neuronal pools.
TUTORIAL OUTLINE
I. Functions of the Nervous System
A. Maintains homeostasis
B. Receives sensory input
C. Initiates motor output
D. Integrates information into meaningful messages
E. Higher Cognitive Activity: critical thinking, judgment, memory, problem solving, etc.
II. Divisions of the Nervous System
A. Central Nervous System
1. Brain
2. Spinal Cord
B. Peripheral Nervous System
1. 12 pairs of cranial nerves
2. 31 pairs of spinal nerves
III. Neurons of the Peripheral Nervous System (PNS)
A. Sensory (Afferent) – transmits information from sensory receptors in the PNS towards the CNS.
B. Motor (Efferent) – transmits information from the CNS towards effectors (muscles & glands) in the PNS
IV. Divisions of the Peripheral Nervous System
A. Somatic Nervous System – under voluntary control.
1. Skeletal muscles
B. Autonomic Nervous System – under involuntary control
1. Smooth Muscles
2. Cardiac Muscles
3. Glands
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V. Cells of the Nervous System
A. Neurons – transmit impulses
B. Neuroglia – provide functional and structural support
VI. Parts of a Neuron: 1. Dendrites 2. Cell Body 3. Axon
A. Dendrites – receive input from other cells or from environment.
1. Dendrites transmit input towards the cell body.
2. Dendritic Spines – tiny processes that serve as contact points.
Neurons can adjust their sensitivity by adding or removing dendritic spines.
3. Cells may have no dendrites or thousands of dendrites.
B. Cell Body (Soma or Perikaryon)
1. Contains organelles similar to most cells: cytoplasm, nucleus, mitochondria, lysosomes, Golgi Apparatus, etc.
Makes cells very leaky to potassium, but not to sodium.
Even as Na+/K+ ATPases pump potassium into the cell, intracellular potassium continues to leak back out of the cell making the inside even more negative.
3. Intracellular Proteins and DNA
Negatively Charged proteins and DNA contribute to the membrane potential making the inside of a neuron more negative.
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XIII. Resting Membrane Potential (RMP)
A. RMP = membrane potential of an excitable cell at rest.
B. Neuron cells and muscle cells are excitable.
C. RMP of a neuron = -70mV (inside the cell)
D. Changes in the Membrane Potential
1. Depolarization – inside of the cell becomes less negative than RMP.
2. Hyperpolarization – inside of the cell becomes more negative than RMP.
XIV. Non-Gated and Gated Channels
A. Non-gated “Leak” channels.
1. Non-gated channels are always open, allowing specific ions to freely move through thecell membrane.
2. Cells have many potassium leak channels, making them very leaky to potassium.
B. Gated Channels – open/close in response to a stimulus.
1. Mechanically gated channel
a. Opens in response to mechanical deformation of the cell membrane.
b. Examples include: touch, pressure, vibrations, hearing, etc.
2. Ligand (chemical) gated channel.
a. Opens when a chemical (ligand) binds to a receptor.
b. Ligands may be hormones, neurotransmitters, drugs, toxins, etc.
3. Voltage-gated channels
a. Open/Close in response to changes in the membrane potential.
b. Voltage-Gated Sodium Channels
o open when the membrane potential = -55mV
o -55mV is called the threshold potential
c. Voltage-Gated Potassium Channels
o open when the membrane potential approaches +30mV
4. Other-gated Channels
Channels may open when stimulated by a photon (light receptor) or other stimuli.
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XV. Local (Graded) Potentials
A. Typically, stimuli affect membrane potential by opening gated ion channels.
B. If the membrane potential becomes more negative, the membrane is hyperpolarized
C. If the membrane potential becomes less negative, the membrane is depolarized
D. Local potentials are Graded: greater stimulus = greater depolarization.
E. Threshold Potential = -55mV
1. If graded potentials depolarize the cell to threshold (-55mV), then an action potential occurs.
2. When a neuron depolarizes to – 55mV, Voltage-gated Sodium channels (Vg Na+) on the axon open, resulting in an action potential.
Vg Na+ channels are especially prominent on the Axon Hillock, making it sensitive to the threshold potential (hence….trigger zone).
3. Subthreshold Potentials = any stimulus that does not reach threshold will not initiatean action potential. Eg. – 65mV would be a subthreshold potential.
F. Summation – graded potentials have the ability to summate (add together)
1. Temporal Summation – occurs when a dendrite is stimulated at a high frequency.
2. Spatial Summation – occurs when multiple dendrites are stimulated at once.
XVI. Action Potential
A. Occurs along the axon, beginning at the axon hillock (trigger zone).
B. 3 Phases of an Action Potential
1. Depolarization
Voltage-Gated Na+ channels open
Sodium diffuses into the cell
Cell depolarizes to +30mV
2. Repolarization
Voltage-Gated K+ channels open, and Voltage-gated Na+ channels begin to close
Potassium diffuses out of the cell
Cell repolarizes to -90mV, overshooting RMP.
3. Hyperpolarization
Vg K+ channels are slow to close, so the membrane potential hyperpolarizing to -90 mV.
Na+/K+ ATPases (pumps) reestablish sodium and potassium concentrations.
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3 Phases of an Action Potential
C. Propagation of the Action Potential
1. At threshold (-55mV), Vg Na+ channels at the Axon Hillock open, depolarizing theregion to +30mV.
2. Na+ diffusing into the cell diffuses to adjacent region, depolarizing it to thresholdthus, generating another action potential.
3. The second action potential causes neighboring Vg Na+ channels to open, againdepolarizing the adjacent region. Again, Na+ diffuses to adjacent region, causing it todepolarize to threshold.
4. Sequence of events causes a series of action potentials to occur sequentially along theentire axon without decreasing in amplitude.
→
+30mV → (-55 mV) -70mV RMP
→ threshold
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D. All-or-none Response
1. Action potentials exhibit an All-or-None Response. That is, a neuron fires fully andcompletely, or not at all.
2. Any stimulus beyond threshold stimulus does not affect the strength of the actionpotential.
3. A greater intensity stimulus produces a higher frequency of action potentials, not astronger action potential.
E. Refractory Period - Brief period during an impulse when an axon becomes unresponsive to additional stimuli.
1. Absolute Refractory Period
No new action potential can be generated, no matter how strong the stimulus.
Lasts about 1 millisecond
2. Relative Refractory Period
Stronger than normal (threshold) stimulus may generate another action potential.
Occurs as the membrane reestablishes resting membrane potential
About 1 – 3 milliseconds.
3. Refractory Period limits number of action potentials that may be generated at a time.
Theoretical limit for humans is about 700 impulses per second.
More commonly human neurons are limited to just over 100 impulses per second.
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XVI. Impulse Conduction (Unmyelinated Vs. Myelinated Axons).
A. Unmyelinated Axons
1. Unmyelinated axons generate action potentials across the length of the entire axon.
2. Impulses are slow (around 1 mile per hours)
B. Myelinated Axons
1. Myelin sheath prevents action potentials wherever the axon is myelinated.
2. Action Potentials occur only at the nodes of Ranvier
3. The nerve impulse passes through the myelinated portion as an electrical current,which is much faster (impulse appears to jump from node to node)
4. This is called Saltatory Conduction
(Action Potential → Electrical Current → Action Potential)
5. Impulse travels at a much higher speed (about 280 miles per hour)
C. Example of myelinated and unmyelinated axons: Cutting your hand with a knife.
1. The original “sharp” pain reaches the brain via myelinated axons.
2. The deep throbbing pain after the cut reaches the brain via unmyelinatedaxons.
D. Diameter also increases the speed of nerve impulses. Larger axons = faster impulses.
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XVII. Synaptic Transmission
A. Neurotransmitters released from the presynaptic neuron bind to receptors on thepostsynaptic neuron.
B. Neurotransmitters may either be excitatory or inhibitory
1. Excitatory Postsynaptic Potential (EPSP)
EPSPs open Na+ or Ca2+ channels
Positively charge ions diffuse into the cell, depolarizing the membrane.
2. Inhibitory Postsynaptic Potential (IPSP)
IPSPs open K+ or Cl- channels
When K+ channels open, positively charged ions diffuse out of the cell, hyperpolarizing the membrane.
When Cl- channels open, negatively charged ions diffuse into the cell, hyperpolarizing the membrane.
C. Activation of the Postsynaptic cell depends on the integrated sum of EPSPs and IPSPs.
1. If sum of EPSPs and IPSPs depolarize the cell to – 55 mV (threshold) then an actionpotential results.
2. If sum of EPSPs and IPSPs do not depolarize the cell to – 55mV (subthreshold) then noaction potential occurs.
3. Summation of EPSPs and IPSPs occurs at the Axon Hillock
2. Norepinephrine – stimulates the sympathetic response “fight-or-flight”
3. Dopamine – Creates a sense of well-being in the CNS
4. Serotonin – Leads to sleepiness
5. Glutamate – primary excitatory neurotransmitter in the CNS
6. GABA – primary inhibitory neurotransmitter in the CNS
7. Substance P – pain perception
8. Endorphins (and enkephalins) – reduce pain by inhibiting substance P secretion
9. Nitric Oxide – vasodilation
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B. Types of Neurotransmitters
1. Monoamines – modified amino acids
Norepinephrine
Dopamine
Serotonin
2. Unmodified amino acid
GABA
Glutamate
3. Peptides (small chains of amino acids)
Substance P
Enkephalins
4. Gasses – nitric oxide
*Monoamines and Amino Acid neurotransmitters are synthesized locally in the synaptic knob. Peptide neurotransmitters are synthesized on ribosomes in the cell body, then transported to the axon terminal.
C. Impact of drugs on neurotransmitters
1. Cocaine –
prevents the reuptake of dopamine in the CNS
Dopamine remains in the synapse stimulating postsynaptic cells
Excess dopamine leads to a sense of euphoria
2. Nicotine
Nicotine binds to ACh receptors on dopaminergic neurons in the CNS
By activating ACh receptors, nicotine causes an excess release of Dopamine.
Excess dopamine leads to a sense of pleasure.
3. Viagra
Viagra prevents the breakdown of nitric oxide
Excess nitric oxide leads to vasodilation of erectile tissue in the penis.
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XIX. Neuronal Pools
A. Neuronal pool – group of functionally similar interneurons in the CNS.
B. Convergence pool – Axons from different areas converge onto one area onto CNS.
1. Convergence gathers information from multiple senses allowing the CNS to collect,process, and make sense of the environment.
C. Divergence pool – axons from one or a few neurons diverge, synapsing onto more and more neurons.
1. Divergence can be used to amplify a stimulus from a minor source.
2. Divergence can send one signal to multiple parts of the CNS….
Example: olfactory signals are sent to the 1. Cerebral cortex where sense of smell is perceived and 2. Limbic system, where the sense of smell triggers a childhood memory.