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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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first things first
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
The nervous system:What does it do?
  • Sensory perception of stimuli
  • Integration
  • Motor output
    • Muscles or glands
how is it organized
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
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
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
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
what glial cells are in the cns
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 cns9
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
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
Why do PNS neurons regenerate?
  • Myelin sheaths form regeneration tube
    • Direct new axon into place
    • CNS neurons don’t regenerate
what about neurons
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
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 parts14
What are a neuron’s parts?
  • Dendrites
  • Axon
    • Axon hillock
    • Axon collaterals (rare, right angle)
    • Terminal branches
    • Synaptic knob, axonal terminals
    • Axoplasm
    • Axolemma
what are 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 sheaths18
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)
what kinds of neurons are there
Classify by function or structure

Structure

Multipolar neurons

Most common (99%)

Three or more processes

Many dendrites, some no axon

Bipolar neurons

Retina, olfactory mucosa

Unipolar

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

What kinds of neurons are there?
what kinds of neurons are there21
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
what does it mean when a neuron fires
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
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
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
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
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
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 potential29
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 potential30
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 potential31
What happens during an action potential?

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

- result: more K+ out than Na+ in

- overshoot = hyperpolarization

http://www.blackwellpublishing.com/matthews/channel.html

what happens after an action potential
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
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 axon34
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
      • http://www.blackwellpublishing.com/matthews/actionp.html
what else influences speed of action potential
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
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
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
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
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…
slide41
Also: http://www.blackwellpublishing.com/matthews/neurotrans.html
how does a nerve signal stop
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)
        • http://www.microvet.arizona.edu/Courses/MIC420/lecture_notes/clostridia/clostridia_neurotox/movie/botulinum_movie.html
      • Reuptake
        • Cocaine
          • http://www.wnet.org/closetohome/animation/coca-anim2-main.html
        • SSRIs
          • http://www.paxil.com/flash/depression.swf
how do neurons integrate multiple signals
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 signals45
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
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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