rsn_ at _comcast.net
Sat Mar 19 11:54:54 EST 2005
On Sat, 19 Mar 2005 10:49:09 -0500, "Rick Craik"
>"Matthew Kirkcaldie" <m.kirkcaldie-remove at unsw.edu.au> wrote in message
>news:m.kirkcaldie-remove-20D280.19451519032005 at un-2park-reader-02.sydney.pipenetworks.com...
>> In article <1111189232.476510.133610 at o13g2000cwo.googlegroups.com>,
>> "JGCASEY" <jgkjcasey at yahoo.com.au> wrote:
>> > Electrochemical pulses only move along the axon in
>> > one direction as far as I know.
>> Generally yes, but dendrites carry signals in both directions - to the
>> cell body, and from the cell body to the dendrite tips. That's one of
>> the big determinants of which signals have the most influence.
>> Cheers, MK.
> He says "axon", you say "dendrites"?
Electrical signals can travel in both dendrites and in axons in either
direction. It just depends where the signal is initiated. In axons,
the action potential for vertebrate neurons is often initiated in the
axon hillock at the soma. It then travels down the axon away from the
cell body. It can also travel back into the soma as far as there is
electrically excitable membrane. When artificially stimulated at the
distal end of the axon, the action potential can easily travel
backwards to the soma. There do exist cells with multiple spike
initiation sites where the action potential normally travels from
wherever it is initiated in any and all directions, as long as the
membrane is electrically excitable.
Passive (electrotonic) potentials such as synaptic potentials,
receptor potentials, and even the effect of the action potential,
travel down all cell processes -- axon and dendrite alike -- equally
well in either direction. The action potential spreads this way
decrementally down the dendrites and synaptic potentials spread this
way in both direction along dendrites. Note that there are also
synapses terminating on axons and these potentials spread out in both
directions along the axon. If it occurs on axon terminals, it can
spread only back down the axon towards the cell body.
Of course, passive potentials spread with decay, and can only travel a
few space constants before attenuating so severely that they can be
ignored. However, a few space constants can be several hundred
micrometers and a lot of things can happen within this distance!
Think of the brain of a fruit fly. A few hundred micrometers is a big
distance inside that structure.
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