Roderick Hetzel asked how the myelin sheath facilitates the
transmission of the neural impulse.
Larry Z. Daily replied:
> I'll give this one a shot, and others can correct me if I get it wrong.
> There are two types of conduction that can occur in an axon - straight
> electrical (like in a wire) and the action potential. Of the two, straight
> electrical conduction is faster and not as costly metabolically. However,
> the signal degrades the farther it travels. An action potential is slower
> and more costly, but it doesn't degrade. What happens is that an action
> potential is triggered, the signal travels quickly through a sheathed
> portion of the axon (but degrades). At the next Node of Ranvier another
> action potential is triggered and so on. As I understand it, the nodes of
> Ranvier act something like a booster station in the electric grid.
I agree with this but I'll add a few points. What slows the action
potential in unmyelinated axon is the need for an ionic
exchange--sodium in, followed by potassium out at every point along
the axon. Myelin insulates the axon, and prevents this interchange
except at the nodes of Ranvier, spaced about 1 mm apart (moreover, I
believe there are very few sodium channels under the myelin which are
needed to allow the sodium to move). So the wave of depolarization
roars down the axon between the nodes, and then pauses while the
action potential is generated through ionic exchange. Then the wave of
depolarization takes off again. So the conduction speed is very fast
between nodes, and briefly slows at each node.
Normally, as Larry noted, the depolarization dies out very quickly.
But myelination prevents leakage of current across the membrane, and
reduces capacitance, which soaks up charge. The result is that the
depolarization can spread much farther in a myelinated axon than it
could without the myelin wrapping. I believe if one node of Ranvier is
blocked, the current can spread as far as the next, and still generate
an action potential. But it can't stretch any farther than that.
This mode of conduction (by a wave of depolarization spreading down
the axon, dying out as it goes) is called, I believe, conduction by
cable properties (treats the axon as a hunk of electrical cable). I
have a long-standing problem with it.
Some texts imply that conduction between the nodes is
instantaneous--the action potential "jumps". Others imply or state
that conduction is at the speed of electricity down a wire, i.e. at
the speed of light. I think these ideas are wrong and while the speed
is zippy, it's less than the speed of light. Unfortunatley, except for
a few vague and unsatisfactory comments I've collected about this,
I've been unable to verify this. And it's not for lack of trying. I
routinely check all texts in physiological psychology, neuroscience,
and neurophysiology, to see if any discuss the issue. None do. I've
even bugged famous people about it without success. I seem to recall
an exchange with James Kalat (of _Biological Psychology_), for
example, and I believe his response was that he didn't have a clue.
I just checked his 6th edition (soon to be the 7th), and I see that
he's changed from a speed of light position to stating that the
conduction is by positive ions moving down the axon. No reference,
unfortunately. I'll flatter myself by thinking it was my bugging that
made him do it.
I should also say I've gone back and looked at the original study
reporting saltatory conduction. The graph shows essentially
instantaneous conduction between nodes, with a brief delay at the
node. But that may be because the equipment used wasn't sensitive
enough to record the fast but not that fast time it takes to travel
betwee nodes.
-Stephen
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Stephen Black, Ph.D. tel: (819) 822-9600 ext 2470
Department of Psychology fax: (819) 822-9661
Bishop's University e-mail: [EMAIL PROTECTED]
Lennoxville, QC
J1M 1Z7
Canada Department web page at http://www.ubishops.ca/ccc/div/soc/psy
Check out TIPS listserv for teachers of psychology at:
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