PSL300H1 Lecture Notes - Lecture 3: Saltatory Conduction, Multiple Sclerosis, Exocytosis

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5 Jun 2018
Lecture 3: Neurophysiology 3
Impulse Conduction
When a patch of excitable membrane generates an action potential, this causes an influx of Na+ and reverses the
potential difference across the membrane
The local reversal in potential temporarily goes from negative on the inside to positive on the inside
o -70 mV to +30 mV
This local reversal in potential serves as the source of depolarizing current for adjacent membranes
Na+ channels opened in the adjacent membrane
o If the next membrane depolarizes from -70 mV to -55 mV than the action potential can travel
Therefore, once started, an action potential will propagate from its origin across the rest of the cell
o Continue until the action potential reaches the axon
Might die out before it reaches the end of the membrane
o The current will travel passively from one part of the membrane to the next part of the membrane
Some current might be lost between the transmission
Excitable Cells
Most cells are not ‘excitable’, (they cannot generate action potentials) for the simple reason that they lack voltage-
gated Na+ channels
These cells will however conduct passive currents, but will not generate AP’s
Most cells are not interested in carrying a signal any distance, they don’t have an ‘axon’
An axon is a long extension of the cell body (like a wire) that carries AP’s away to some other location
Therefore, only neurons with long ‘axons’ and muscle cells generate propagating action potentials
The first patch of membrane that can generate an AP is the trigger zone/axon hillock
o Other areas of the cell do not have as many Na+ channels to fire an action potential
o The action potential that is generated will propagate (travel) to the axon terminal
In biological tissue if we put a voltage across a membrane on one location and measure the voltage across the
membrane some distance away it will decrease
The membrane properties shape the form of the signal
We are losing signal as the current travels along the membrane
Myelin helps prevent the loss of signal
o Conduction velocity depends on the constant lambda
o Bigger the lambda the longer the potential difference can be carried without losing its original velocity
o measures how quickly a potential difference disappears (decays to zero) as a function of distance
Thus, the conduction velocity of an AP along an axon depends on the membrane length constant
o is increased by increasing diameter (the largest diameter > less internal resistance > less voltage is lost
across that resistance as the currents travel down the membrane)
o is increased by increasing membrane resistance (the higher the membrane resistance > less current is
leaked out > current if forced down the membrane)
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Document Summary

70 mv to +30 mv: this local reversal in potential serves as the source of depolarizing current for adjacent membranes, na+ channels opened in the adjacent membrane. Ideally, you want to increase as much as possible so that the depolarizing current will spread a great distance. Increasing membrane resistance (i. e. myelination) is the most efficient means of increasing conduction velocity. > reduces the leakage of current out of the membrane. 10 nodes: this type of conduction has a big safety factor, you could poison some of the nodes and the depolarizing current will just skip past that and move on to the next healthy patch of membrane. Node of ranvier: at the node, there is high density of voltage-gated na+ channels (1000-2000/micron2, the density is so high that is squeezes any other channels out, no voltage gated k+ channels no after-hyperpolarization. Unmyelinated axons: the unmyelinated axons do not have this extensive wrapping around the outside.

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