BIOB34H3 Chapter Notes -Hyperkalemia, Multiple Sclerosis, Demyelinating Disease

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8 Apr 2012
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BIOB30 Chapter 8 (266-273)
Neurons: Cellular and Network Properties
1
Action Potentials Are Conducted
Conduction the high speed movement of an action potential along the axon
During an action potential, lost energy is constantly replenished
The depolarization of a section of axon causes positive current to spread
through the cytoplasm in all directions by local current flow
o At the same time, on the outside of the axon membrane, current flows
back toward the depolarized section
o The local current flow in the cytoplasm diminishes over distance as
energy dissipates and would die out if not for the voltage-gated
channels
Axon contains many voltage-gated Na+ channels
o When depolarization reaches these, the open and allow more Na+ to
enter reinforcing the depolarization
o This mechanism initiates the positive feedback loop
Positive feedback loop
o An above threshold graded potential enters the trigger zone
o Depolarization opens voltage-gated Na+ channels
o Na+ enters the axon and the initial segment of axon depolarizes
o Positive charge from the depolarized trigger zone spreads to adjacent
sections of membrane
It is repelled by Na+ that entered the cytoplasm and attracted
by the negative charge of the resting membrane potential
The flow of local current toward the axon terminal begins
conduction of the action potential
o When the membrane distal to the trigger zone depolarizes, its Na+
channels open and Na+ enters
o Positive feedback loop: depolarization of Na+ channels, Na+ enters,
causing more depolarization and opening more Na+ channels in the
adjacent membrane
This has begun
The continuous entry of Na+ ensures the signal wont diminish
as the action potential propagates itself
In graded potentials, Na+ enters only at the point of the
stimulus so they lose strength over distance
o As each segment reaches the peak of action potential, its Na+ channels
inactivate
o During the falling phase, K+ channels are open allowing K+ to leave
the cytoplasm
o Then the K+ channels close and the membrane in that segment of
axon returns to its resting potential
o Positive charge from a depolarized segment of membrane may flow
backward to the trigger zone but it has no effect
That section of axon is in the absolute refractory period so has
inactivated Na+ channels
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