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Physiology 3120 Lecture Notes - Axon Hillock, Saltatory Conduction, Schwann Cell

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Tom Stavraky

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Human Physiology
Friday, October 2, 2009
Propagation of Action Potentials
Generally, begins at axon hillock, and flow orthodromically down the axon to the nerve terminus
Current flows from positive to negative
During AP, current carried by positive ions
Non-myelinated neurons
Diagram on EC-65 shows cathodal electrode (i.e. attracts positive charges)
Regions adjacent to original AP become depolarized as the positivity inside the cell spreads in
both directions
Causes the action potential to occur in adjacent regions
In vivo, action potential only travels in one direction because of refractory periods
Action potentials stays all-or-none all along the axon
Myelinated neurons
Insulated by myelin which is produced by. . .
Schwann cells (peripheral)
Oligodendrocytes (CNS)
Myelin increases resistance across membrane by about 5,000 times, so ions won’t move across
membrane where there is myelin
Voltage-gated channels located at nodes of Ranvier only
Use saltatory conduction
Depolarization jumps between nodes of Ranvier
Much faster. . .conduction velocity increases:
oWith the square root of diameter in a non-myelinated axon
oDirectly with the diameter of a myelinated axon
Conserves energy
Susceptibility of myelinated fibres (multiple sclerosis)
Neuromuscular junction
Alpha motor nerves meet up with skeletal muscle at the NMJ
Have voltage-gated calcium channels
Open upon depolarization, and calcium flows into the nerve terminal
Synaptic vesicles at NMJ contain ONLY acetylcholine (ACh)
End plate of muscle cell is thrown into synaptic folds to increase surface area
Space between end plate & nerve terminal = synaptic trough
At the top of the folds, have ACh receptors, which are associated with non-specific cation
channels (nAChR)
Voltage-gated channels located at bottom of trough
Basement membrane contains acetylcholinesterase (breaks down ACh)
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