Nerve physiology
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Nerve physiology First things first… Brain and cranial nerves lab practical See questions 2 and 3 on study guide 5 The nervous system: What does it do? Sensory perception of stimuli Integration Motor output Muscles or glands How is it organized? Central nervous system (CNS)

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Nerve physiology

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Nerve physiology

First things first…

  • Brain and cranial nerves lab practical

    • See questions 2 and 3 on study guide 5

The nervous system:What does it do?

  • Sensory perception of stimuli

  • Integration

  • Motor output

    • Muscles or glands

How is it organized?

  • Central nervous system (CNS)

    • Brain and spinal cord

    • Integrating/command center

  • Peripheral nervous system (PNS)

    • Nerves extending from brain/spinal cord

      • Links body parts to CNS

    • Spinal nerves: messages to and from spinal cord

    • Cranial nerves: messages to and from brain

    • Split into subdivisions

What are the PNS subdivisions?

  • Sensory (afferent) division

    • Information from sensory receptors to CNS

    • Somatic afferent fibers: from skin, skeletal muscle, joints

    • Visceral afferent fibers: from viscera

  • Motor (efferent) division

    • From CNS to effector organs, muscles, glands

    • Divided into two main parts

What are the PNS motor subdivisions?

  • Somatic (voluntary) nervous system

    • CNS to skeletal muscles

  • Autonomic (involuntary) nervous system

    • CNS regulates smooth muscles

    • Two subdivisions

      • Sympathetic nervous system: fight or flight

      • Parasympathetic nervous system: feed or breed

What types of cells are found in the nervous system?

  • Neurons: excitable cells

  • Neuroglia: supporting cells (AKA glial cells)

    • Ten times more common than neurons

    • Four in CNS

    • Two in PNS

Astrocytes: most abundant

Support/brace neurons

exchange with capillaries

guide migrating young neurons

Clean up K+, neurotransmitters

Microglia: functions as clean-up

Substitute for immune system

What glial cells are in the CNS?

What glial cells are in the CNS?

  • Ependymal cells:

    • Line central cavities of brain, spinal cord

    • Form permeable barrier for CSF

    • Produce CSF

  • Oligodendrocytes:

    • Form myelin sheaths

What glial cells are in the PNS?

  • Satellite cells: surround neuron somas

    • Function unknown

  • Schwann cells: form myelin sheaths

    • Essential for PNS nerve cell regeneration

Why do PNS neurons regenerate?

  • Myelin sheaths form regeneration tube

    • Direct new axon into place

    • CNS neurons don’t regenerate

What about neurons?

  • Long-living

  • Amitotic

    • Except olfactory and hippocampus (memory) neurons

  • V. high metabolic rate

  • Bundles of arm-like processes

    • Tracts in CNS

    • Nerves in PNS

What are a neuron’s parts?

  • Cell parts

    • Soma: all organelles but centrioles

      • Nissle bodies (rough ER)

      • Nuclei = cluster of cell bodies in skull/cord

      • Ganglia = cluster of cell bodies in PNS

What are a neuron’s parts?

  • Dendrites

  • Axon

    • Axon hillock

    • Axon collaterals (rare, right angle)

    • Terminal branches

    • Synaptic knob, axonal terminals

    • Axoplasm

    • Axolemma

Myelin sheaths

What are myelin sheaths?

  • Protein-lipid filled cytoplasm of Schwann cells

    • Neurilemma: outermost part w/nucleus and cytoplasm

    • Myelin sheath: inner layers of PM

  • Protects/insulates axon(never dendrites)

    • Allow for rapid transmission of action potential

What are myelin sheaths?

  • Nodes of Ranvier: gaps between adjacent Schwann cells

  • Oligodendrocytes serve same purpose in CNS

    • White matter: areas of myelinated (primary fiber tracts)

    • Gray matter: nerve cell bodies (unmyelinated)

Classify by function or structure


Multipolar neurons

Most common (99%)

Three or more processes

Many dendrites, some no axon

Bipolar neurons

Retina, olfactory mucosa


One process; divides into proximal and distal branches (both are considered axons)

What kinds of neurons are there?

What kinds of neurons are there?

  • Function

    • Sensory (afferent) neurons

      • Conduct toward CNS from skin, internal organs

      • Usually unipolar; soma located outside CNS

      • More on sensory receptors in special senses lecture

    • Motor (efferent) neurons

      • Conduct away from CNS; multipolar

      • Cell bodies in CNS

    • Interneurons (association neurons)

      • Between sensory and motor neurons; multipolar

      • Usually entirely in CNS; 99% of all your neurons

Nerve physiology:Action potentials

What does it mean when a neuron “fires”?

  • Firing = excitability = action potential = nerve impulse

  • Recall resting potential of all cells

    • High K+ in; high Na+ out

    • Cell is polarized

    • Cell overall neg. charge inside due to molecules like proteins, RNA, DNA

      • Charge measured in millivolts

      • Potential = difference in charge across PM

      • Current = flow of charge (ions) from one point to another

What lets ions move across the PM?

  • Membrane ion channels (proteins)

    • Passive (leakage): always open

    • Active (gated): usually either opened or closed depending on type of gate

      • Chemically-gated: ligand-gated

        • E.g. ACh ion gate

      • Voltage-gated: open/close in response to change in potential

What causes resting potential in the first place?

  • Membrane permeability

    • K+ permeable, but not Na+ permeable

      • Creates membrane potential

      • K+ leave cell but Na+ can’t enter

        • Result: overall neg. charge inside cell

    • Na+/K+ pump maintains but does not create resting potential

      • Always a lot of K+ leaking out and a little Na+ leaking in

What is depolarization?

  • Reduction in membrane potential

    • Less difference between in- and outside of cell

    • i.e cells becomes less negative (-70 mV to -50 mV)

    • Cell can also temporarily become positive

    • Excitatory event

  • Hyperpolarization

    • Cell becomes more negative than normal

    • e.g. -70 mV to -90 mV

    • Inhibitory event

What are local potentials?

  • Short-lived, local changes in membrane potential

  • Can depolarize or hyperpolarize cell

  • Ligand-regulated

  • Graded = magnitude varies w/strength of stimulus

    • Stronger stimulus = greater voltage change, longer travel of current

    • Caused when ion gates open due to stimulus

What happens during an action potential?

  • Follow on graph

  • Sodium ions arrive at axon hillock

    • Depolarizes membrane

  • Threshold reached (-55 mV)

What happens during an action potential?

  • Voltage-regulated Na+ (fast) gates open

    • Slow voltage-regulated K+ gates also open

    • Depolarization begins

  • Propagation of signal

What happens during an action potential?

5. Na+ gates close (inactivate) above 0 mV

- voltage peaks around 35 mV

- fully depolarized

6. At voltage peak, K+ gates are finally fully open

- repolarization begins at K+ flows out

  • How is this different from resting potential?

What happens during an action potential?

7. K+ gates closer more slowly than Na+ gates

- result: more K+ out than Na+ in

- overshoot = hyperpolarization

What happens after an action potential?

  • Refractory period: few millisecs

    • Time during which can’t stimulate neuron a second time

    • Happens until recovery of resting potential

  • Two stages

    • Absolute refractory period

      • No new action potential possible

    • Relative refractory period

      • Can trigger new action potential if stimulus is very strong

How do action potentials travel down the axon?

  • Nerve signal = traveling wave of excitation produced by action potentials

  • Unmyelinated sheaths

    • Slower transmission

    • Action potential must open all gates between hillock and synaptic knob

      • Called continuous conduction

How do action potentials travel down the axon?

  • Myelinated sheaths

    • Many times faster transmission

    • Action potential skips from one node of Ranvier to the next

      • Called saltatory conduction


What else influences speed of action potential?

  • Axon diameter

    • The larger the diameter, the faster the speed of transmission

    • Less resistance to current flow with larger diameter

Slower transduction

Faster transduction

What happens if myelination is lost?

  • Multiple sclerosis

    • Autoimmune disease

    • Usually young adults

    • Blindness, problems controlling muscles

      • Ultimately paralysis

    • Immune system attacks myelin sheaths and nerve fibers

      • Scar tissue (scleroses) replaces some damaged cells

      • Other now unmyelinated axons sprout Na+ channels

        • Accounts for sporadic nature of disease?


What happens when the nerve signal reaches the synaptic knob?

  • First some terminology

    • Synapse: junction between two neurons

      • Use neurotransmitters

        • Allows for integration/evaluation of information

    • Presynaptic neuron

      • Can synapse with next neurons dendrites, soma or axon

    • Postsynaptic neuron

    • Synaptic cleft

What are neurotransmitters?

  • Chemicals which cross synaptic cleft

    • Communicate with postsynaptic neuron

  • Over 100 known neurotransmitters

    • ACh, serotonin, glutamate, aspartate, glycine, GABA, NE, dopamine, histamine

  • Excitatory or inhibitory

How do other neurotransmitters work?

  • ACh and some others are ionotropic

    • Alters membrane potential

  • Rest are metabotropic

    • Use secondary messenger (e.g. cyclic AMP) to alter postsynaptic cell metabolism

    • Neurotransmitter activates cAMP production

    • For example…


How does a nerve signal stop?

  • Neurotransmitters usually bind for only about 1 msec

    • Then detaches, then reattaches, then detaches…

  • If no new neurotransmitter available, stimulus stops

    • This can happen one of three ways

      • Diffusion

      • Destruction (e.g. AChE)


      • Reuptake

        • Cocaine


        • SSRIs


How do SSRIs work?

How do neurons integrate multiple signals?

  • Like a democracy: count the votes!

  • Mechanisms neurons use to process, store and retrieve information

  • Postsynaptic potentials

    • Excitatory postsynaptic potential (EPSP)

      • Na+ flows in an cancels some of neg. charge

      • Glutamate, aspartate

    • Inhibitory postsynaptic potential (IPSP)

      • Increases neg. charge

      • Neurotransmitter opens Cl- gates into cell

      • Glycine, GABA

    • ACh, NE can be either EPSPs or IPSPs

How do neurons integrate multiple signals?

  • Summation: adding up postsynaptic potentials

    • Sum determines if fire or not

    • Need about 30 EPSPs to reach threshold

  • Temporal summation: new EPSPs arrive before decay of previous EPSP

    • Summation exceeds threshold

  • Spatial summation: several different synapses all emit EPSPs

    • Enough Na+ enters to reach threshold

What are neuronal circuits?

  • Pathways among neurons

  • Diverging circuits

    • Large scale muscle contraction

  • Converging circuits

    • Good for incoming sensory information to converge in one part of brain

  • Reverberating circuit

    • Promotes inhalation (when reverberation stops, you exhale)

  • Parallel after-charge circuit

    • Seeing light bulb image after closing eyes

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