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.

2. Chromatophilic Substance (Nissl bodies): mostly rough Endoplasmic Reticulum.

Ribosomes on the Rough ER = site of protein synthesis.

3. Nucleus: contains…

Nucleolus – synthesizes ribosomes

Chromatin – DNA + packaging proteins. Encodes the genetic information for protein synthesis

4. Neurofibrils (bundles of neurofilaments)

Forms cytoskeleton of the neuron

Extends into and supports the axon.

Play a role in axonal transport of proteins from the cell body to the axon terminal.

C. Axon

1. Transmits impulses away from the cell body.

Neurons have at most one axon.

2. Axon Hillock (trigger zone)

cone-shaped thickening where the axon meets the cell body

Action potential (electrical impulse) is initiated at the axon hillock.

3. Collaterals – branches of the axon

4. Axon Terminal – distal end of the axon.

5. Synaptic Knob – enlarged end of the axon terminal.

Contains secretory vesicles with neurotransmitters.

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6. Synaptic Cleft – small space between synaptic knob and postsynaptic cell

7. Axoplasm – cytoplasm within the axon of a neuron

Axoplasmic transport – transports proteins, organelles, and other substances from the cell body to the axon.

VII. Myelination of axons

A. Myelin sheath

1. The myelin sheath

Myelin is a lipid rich coating covering the axon of many neurons.

The myelin sheath increases the speed of a nerve impulse.

Nodes of Ranvier – narrow gaps in the myelin sheath.

B. Myelination in the Peripheral Nervous System (PNS)

1. Schwann Cells form the myelin sheath in the PNS

Schwann cell membranes wind and wrap around axons forming a thick layer of insulation. = myelin.

Neurilemma – cytoplasm and nucleus of Schwann cells is pushed outward forming an outer layer, called the neurilemmal.

Several Schwann Cells myelinate each axon.

Schwann cells are separated by a small gap, called the node of Ranvier

Schwann Cells also enclose, but do not wrap around unmyelinated axons.

Remak Bundle – bundle of unmyelinated axons, bundled by Schwann Cells.

C. Myelination in the Central Nervous System (CNS)

1. Oligodendrocytes form the myelin sheath in the CNS.

The Cell Body sits in between axons, giving off cell processes that myelinate multiple axons.

White Matter – groups of myelinated axons within the CNS. The Myelin sheath imparts a whitish appearance.

Gray Matter – groups of unmyelinated axons and cell bodies within the CNS. Unmyelinated tissue imparts a grayish appearance.

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VIII. Structural Classification of Neurons

A. Multipolar Neurons

a. Contain many dendrites but only one axon

b. Most neurons of the brain and spinal cord (CNS) are multipolar

c. Motor neurons are multipolar

B. Bipolar Neurons

a. Contains two processes: one dendrite and one axon.

b. Found in some special senses: eyes, nose, ears

C. Pseudounipolar (Unipolar) Neurons

a. One process attached to the cell body divides into two branches that act as a singleaxon.

b. Peripheral process – contains dendrites near the peripheral end

c. Central process – enters the brain or the spinal cord.

d. Most unipolar neurons are found in ganglia (eg. Dorsal root ganglia).

IX. Functional Classification of Neurons

A. Sensory (afferent) neurons

a. conduct impulses from the periphery towards the brain or spinal cord.

b. sensory neurons detect changes in the outside world or in the internal environment.

c. most sensory neurons are unipolar

B. Motor (efferent) neurons

a. conduct impulses from the CNS towards effectors (muscles or glands) in the PNS

b. Somatic motor – skeletal muscles, under voluntary control

c. Autonomic motor – smooth muscle, cardiac muscle, and glands – under involuntary control.

d. motor neurons are multipolar

C. Interneurons (association fibers)

a. lie completely within the brain or spinal cord

b. form links with other neurons

c. relay information from one part of the CNS to another part of the CNS

d. may relay incoming sensory information to the appropriate region for processing

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X. Neuroglia

A. Neuroglia within the CNS

1. Astrocytes “star cell”

a. Found between blood vessels and neurons

b. Astrocytes participate in the Blood-Brain-Barrier. They do not form the BBB,but transfer nutrients from the bloodstream to the neuron.

c. Astrocytes regulate ion concentrations, strengthen synapses, and prevent thespread of infection by depositing connective tissue (glial scars)

2. Ependymal Cells

a. Cuboid (or columnar) epithelium often ciliated.

b. Line ventricles of the brain and central canal of the spinal cord.

c. Regulate the composition of cerebrospinal fluid (CSF)

3. Microglia

a. Small cells that are normally inactive.

b. When activated by infection or inflammation, microglia enlarge and becomemacrophages.

c. Microglia phagocytize bacterial cells and cell debris.

4. Oligodendrocytes

a. Myelinate axons in the CNS

b. Oligodendrocytes form rows in between neurons and form processes thatmyelinate multiple axons.

B. Neuroglia in the PNS

1. Satellite Cells

a. Surround bodies within ganglia

b. Provide metabolic and structural support.

2. Schwann Cells

a. Forms myelin sheath in the PNS

b. Several Schwann Cells myelinate one axon

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C. Properties of Neuroglia

1. Neuroglia outnumber neurons 10 to 1.

2. Many diseases of the nervous system originate from neuroglia, not neurons.

Multiple-Sclerosis

o Autoimmune disorder – immune system attacks myelin sheath in the CNS.

o During repair, neuroglia deposit Connective Tissue in place of myelin forming several Scars (Scleroses)

o Muscles innervated by scarred motor neurons stop contracting and eventually atrophy.

Most Primary Brain Tumors are due to neuroglia that divide too often.

Huntington Disease

o Microglia release a toxin that damages neurons.

o Causes uncontrollable movements and cognitive impairment.

D. Neuroglia and Axonal Regeneration

1. Damage to neuron’s cell body usually kills the neuron, because mature neurons do notdivide. But a damaged peripheral axon may regenerate.

2. After damage distal portion of axon and its myelin sheath degenerate.

3. Macrophages remove the fragments of myelin and other debris.

4. Nearby neuroglia secrete growth factors that guide developing sprouts from the cellbody into a tube formed by the remaining Schwann Cells.

5. Schwann cells along the regenerating axon form new myelin.

6. The developing axon often ends up at the wrong place, so full function often does notreturn.

XI. The Synapse

A. Presynaptic Neuron – sends the signal

1. Synaptic Knob – contains secretory vesicles with neurotransmitters

2. Synaptic Cleft – gap between neurons

3. Neurotransmitters diffuse across the synaptic cleft and bind to receptors onpostsynaptic cell

B. Postsynaptic Neuron – receives the signal

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C. Synaptic Transmission

1. Action potential (electrical signal) travels along presynaptic neuron to axon terminal.

2. Calcium channels on synaptic knob open, Calcium diffuses into synaptic knob.

3. The Presynaptic Neuron releases neurotransmitters into the synaptic cleft viaexocytosis.

4. Neurotransmitters diffuse across synaptic cleft and bind to receptors on postsynapticcell.

5. Depending on the receptor and the neurotransmitter, the neurotransmitter may eitherexcite the postsynaptic cell or inhibit the postsynaptic cell.

By Nrets [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

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XII. Membrane Potential

A. Cell membranes are polarized (electrically charged).

B. Inside of cell is negatively charged compared to the outside of the cell.

C. Membrane Potential – refers to the electrical charge across the cell membrane.

D. All Cells have a membrane potential.

E. Membrane potential is maintained by three factors…

1. Na+/K+ ATPase (pumps)

Transports 3 Na+ out of the Cell, but only 2 K+ into the cell.

As positive charges leave the cell, inside of cell becomes negatively charged.

Maintains

o high Extracellular Sodium Concentration &

o high Intracellular Potassium Concentration

https://upload.wikimedia.org/wikipedia/en/4/46/Ion_channel_activity_before_during_and_after_polarization.jpg

2. Potassium Leak (non-gated) channels

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

XVIII. Neurotransmitters

A. Neurotransmitters

1. Acetylcholine – stimulates skeletal muscle contractions.

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.

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