Propagation of impulse Moving the action potential down the axon

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Propagation of impulseMoving the action
potential down the axon
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Nerve conduction velocities

• uvelocity
• Kconstant
• ddiameter
• True for non-myelinated neurons

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Myelin sheath

• Cat motor neuron

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• Myelinated neuron velocity is dependent upon size
  (linear)
• However, related to inter-node distance, not
  diameter
• Inter-node distance greater in large axons
• Non-myelinated neuron velocity related to square
  root of diameter

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Synapses

• Electrical
• Gap junctions
• fast
• Chemical
• Neurotransmitters
• slow

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Basic Structure of a Chemical Synapse

• Synaptic cleft approximately 20nm across
• Mucopolysaccharide glue
• Signal passes from pre-synaptic to post-synaptic
  cell
• neurotransmitter

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Neuromuscular junction
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Several types of ion channel
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Chemically gated ion channel
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Variety of synapse structure
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Neurotransmitters and Synaptic Transmission

• Neurons communicate across synapses using
  neurotransmitters
• Released from presynaptic membrane
• Binds to receptor on post synaptic membrane
• Excitatory transmitters
• Cause depolarization (EPSP)
• Inhibitory transmitter
• Cause hyperpolarization (IPSP)

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Ionotropic and metabotropic synapses
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Neurotransmitters

• Must be released by pre-synaptic cell
• Must elicit effect on post-synaptic cell
• Agonists should work the same way
• Must be removed after action
• Can be small molecules or large peptides

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Acetylcholine gated ion channel

• Pore formed by 5 homologous subunits
• Ringed by M-2 a-helices forming gate
• Acetylcholine binding changes gate conformation

Nicotinic Ach receptor
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Acetylcholine effect at neuromuscular junction

• Calcium ions in presynaptic cell cause vesicle
  transport

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Deactivation of Acetylcholine

• Enzyme. Acetylcholinesterase
• Raid
• Sarin
• VX

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Ionotropic and metabotropic synapses
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• Binding of nor-epinephrine to difference receptor
  subtypes

• Arterial wall
• Lung smooth muscle
• Smooth muscle/cardiac muscle

Metabotropic synapses
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Deactivation of nor-epinephrine
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Integration of input
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• Axon Hillock
• Determines sensitivity to stimulation
• High number of ion channels

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Effect of ion channel density on membrane
threshold

• To generate an AP, a certain number of sodium
  ions must cross membrane
• Determined by number of open sodium ion channels
• Ion channels have varying voltage sensitivities
  (normal distribution)

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Effect of ion channel density on membrane
threshold, part deux

• Cell One (Blue)
• 1,000 channels per mm2
• Must generate enough potential to open 1/10th
• Cell Two (Red)
• 10,000 channels per mm2
• Must generate enough potential to open 1/100th

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• Axon Hillock
• Determines sensitivity to stimulation
• High number of ion channels

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• Spatial summation
• Excitatory input

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Integration of excitatory and inhibitory inputs

• Spatial summation
• Excitatory and Inhibitory input

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Temporal Summation
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• Temporal summation
• Allows integration of signals in time

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Impulse frequency is proportional to level of
post-synaptic potential (depolarization)
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• Nerves define stimulus strength by the number of
  impulses generated
• Increased pre-synaptic input elevates
  post-synaptic potential at hillock
• Leads to elevated rate of AP generation down axon
• Can be modulated for accommodation