How is passive current propagated?
A J Harrington
apence at pantheon.yale.edu
Thu Oct 31 15:51:35 EST 1996
: In article <leipzjn9-301096000943 at rts0107.ppp.wfu.edu> Jeremy Leipzig,
: leipzjn9 at wfu.edu writes:
: >Does anyone know exactly how passive current spreads down a dendrite or
: >myelinated axon. One of my professors says the depolarization moves in
: >successive collisions of repelling cations, in a manner not unlike the
: >propagation of sound waves. Another one says that the electric field
: >created by incoming cations is enough to depolarize adjoining regions,
: >implying that passive current spreads close to the speed of light. I have
: >also heard in intro courses that simple diffusion of the cations is
: >responsible. Which, if any, is the correct explanation?
To answer your question as stated in the subject line, passive current is
NOT propagated at all. I'll explain.
For subthreshold depolarizations, current injected via an electrode into
an axon spreads passively and decays over distance. In this case the axon
is acting exactly like a cable. Problem with this is that it doesn't
travel very far before being attenuated.
Action potential conduction is entirely different, although it starts out
the same way. Either 1 larger current pulse, or several small current
pulses near to each other in time and/or space along the axon, can cause a
cell to depolarize (as above) until it reaches a certain threshold
depolarization voltage. Depolarization causes voltage sensative ion
channels in the membrane to open, and when enough of them open, it causes
an all-or-none event called an action potential, in which many sodium
channels open and cause a much larger depolarization. This depolarization
spreads to nearby sodium channels and causes them to open, thus
propagating the AP down the axon. This is like something that you
mentioned before, but note we're not talking about electrons moving here,
we're talking about *ions* moving, and the movement of ions is
considerably slower. The time it takes for sodium channels to open, for
sodium ions to rush through them to the inside of the cell, causing
depolarization, and then open nearby sodium channels is MUCH slower than
the speed of light.
So, more or less, the first explanation you listed basically explains
subthreshold passive spread of depolarization over a short distance, the
second explanation comes close to explaining action potential propagation,
and the part about passive diffusion is just innacurate.
apence at minerva.cis.yale.edu
Department of Neuroscience
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