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
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
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
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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