Action Potential

Matt Fraser mof at
Tue Dec 10 22:21:47 EST 1996

On Tue, 10 Dec 1996, Melissa M wrote:

> How does an impulse travel down an axon in action potential

All cells in the body have different concentrations of certain ions inside
them than exist in the outside, extracellular space.  These ions are
charged particles and the separation of charge creates a voltage across
the cell's plasma membrane. Changes in the voltage by fluxes of ions
across the plasma membrane can be one of the signals for changes in the
cell's activity.

A neural impulse traveling down an axon is a traveling wave of ionic
flux. Any flow of ions creates an electric current, causing a
change in voltage across the axon's membrane.  Voltage sensative sodium
channels open up at the leading edge of the wave, causing an influx of
sodium, which changes the voltage across the membrane, causing more
voltage sensative sodium channels to open in neighboring axonal membrane
(downstream).  It's sort of like a fuse being lit, except that there is
another event that occurs after the sodium ion flux that returns the
tailing edge to it's resting state.

The whole thing starts at the neuronal cell body, where different ion
fluxes caused by neurotransmitter stimulated ion channels are summated to
give a change in voltage.  When a certain level of change has been
reached resulting in the attainment of the threshold voltage of the
voltage gated sodium channel, the junction of the axon and the neuronal
cell body "sparks" off, and the traveling wave is on its way.

At the end of the journey, at the synaptic terminal, the voltage sensative
sodium channels are joined by voltage sensative calcium channels, and it
is the influx of calcium that is responsible for causing the
neurotransmitters, packages in little membrane vesicles, to be released
onto the next cell.  That next cell may be another neuron, and the whole
thing could start again at the next cell, and so on ...

Hope that helped.



Matt Fraser                        "Hey, You! Don't help them 
                                       To bury the Light.
mof at               Don't give in
mattf+ at                         Without a Fight."

                                           Pink Floyd

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