Q: dendrites/gates

r norman rsn_ at _comcast.net
Sun Mar 20 21:00:26 EST 2005

On Mon, 21 Mar 2005 12:27:03 +1100, Matthew Kirkcaldie
<m.kirkcaldie-remove at unsw.edu.au> wrote:

>In article <pk6s31dqj1uk6k2tkkr1cb4lh3qcjts976 at 4ax.com>,
> Kalman Rubinson <kr4 at nyu.edu> wrote:
>> Well, technically, the nodes do slow conduction since that is where
>> the time-consuming task of regenerating the AP occurs.  The
>> transmission through the internodal portions of the axon is much
>> faster.  
>Splitting hairs here, but it's only current which flows through the 
>internodal portions; what the axon is TRANSMITTING is the action 
>potential - since that occurs at the nodes, I don't think you can talk 
>meaningfully about the speed of the internodal regions, since that speed 
>is defined as when the action potential occurs at the next node.
>The only reason I would come up with such a petty nit-pick is that I 
>often have students who think axons somehow transmit current - whereas 
>of course they actually replicate a triggered event which occurred at 
>the initial segment; current is one of the mechanisms by which the 
>replication is achieved.
>I hasten to add that I'm not saying this because I assume you don't know 
>it (which you clearly do) but more for the benefit of anyone reading 
>this thread for edification about neuronal activity!
>      Cheers, MK.

Yes, it is all very tricky.  Axonal "transmission" is, in fact, the
traveling of an action potential down the axon.  Since there is no
membrane excitation or  transmembrane currents, it doesn't make sense
to talk of the action potential in the internodal region.

Still, axons DO in a sense transmit current longitudinally down their
lengths.  It is these current loops extending from one node to the
next that causes the next node to depolarize and hence fire.  The
effect of the myelin is two-fold.  It forces the current loops to
travel farther down the axon and so they carry depolarization much
farther than they would without the myelin.  And it decreases the
current so that, since i = C dV/dt, a smaller capacitance means the
current  causes a much more rapid depolarization and triggering of the
action potential at the next node.

Still, the real "transmission" of the action potential, as you say, is
the successive replication of a completely new action potential at
each node.  The longitudinal flow of current is simply the decremental
spread of activity that is the mechanism underlying the triggering.

If find that it usually takes two or three rounds of courses for
students to really start to get it.  It starts in intro biology and
continues in animal physiology.  By the third time in a specialized
neuro course, it finally starts to really sink in and gel.

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