PHY2011 Lecture Notes - Lecture 8: Saltatory Conduction, Myelin
20 Jun 2018
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Week 3 L2 (T2L3) Propagating the action potential
Refractory periods
Refractory periods restrict the ability of a neuron to produce an second AP after the first one
Absolute refractory period = 1-2mSec
oPeriod after an AP, where no matter how strong of a stimulus is provided, no
response can occur
oCan be thought of as the threshold going from -55mV to infinity
oAnd it takes 1-2mSec to go below +30mV (outside absolute refractory period)
oTherefore, an AP can be generated, but really strong stimulus required.
It takes 8-9mSec for the threshold to return to normal (-55mV) and a normal amount of
stimulus is required
During the end 6-7mSec, it can still respond, but needs a gradually less amount of stimulus,
as threshold returns to normal. = Relative refractory period
Relaying the information -propagating the AP from the initiation site, down the neuron
Two methods: one fast but inefficient, one slow, but efficient
Electrotonic flow:
oAP tells one point on a neuron when input has occurred
oInformation transmitted down neuron by passive current flow (like copper wires,
but ion movement, not electron)
oInefficient as neurons aren’t insulated, therefore the current will leak out
oRate depends on size of axon (larger have less leakage, smaller have faster leakage)
oFlows both left and right
Regeneration of action potentials
oAs current flows, it depolarises adjacent points, causing voltage to reach -55mV,
therefore reaching the threshold for Na+ channels to open, causing another AP,
which depolarises the next adjacent points
oDoesn’t leak, but slightly slower
oMoves as a wave, not as stages, the next AP is starting before the previous finishes
oCan only flow in one direction because of refractory period of channels -adv
Speeding up information flow
Every time an AP is produced, the flow of info is slowed
find more resources at oneclass.com
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Document Summary
Week 3 l2 (t2l3) propagating the action potential. Refractory periods restrict the ability of a neuron to produce an second ap after the first one. It takes 8-9msec for the threshold to return to normal (-55mv) and a normal amount of stimulus is required. During the end 6-7msec, it can still respond, but needs a gradually less amount of stimulus, as threshold returns to normal. Relaying the information -propagating the ap from the initiation site, down the neuron. Two methods: one fast but inefficient, one slow, but efficient. Every time an ap is produced, the flow of info is slowed. Reducing number of times required to make an ap speeds up the flow of info. Increasing axon diameter reduces loss of current, therefore reducing number of times aps occur. Electronic flow used to push current till next ap is required diameter of neuron effects speed b/c of leaks. Conduction velocity (meters/second) is directly proportional to axon diameter ( m)
