Q: dendrites/gates

Kalman Rubinson kr4 at nyu.edu
Tue Mar 22 19:40:49 EST 2005


On Tue, 22 Mar 2005 17:22:28 -0500, r norman <rsn_ at _comcast.net>
wrote:

>On Tue, 22 Mar 2005 15:17:08 -0500, Kalman Rubinson <kr4 at nyu.edu>
>wrote:
>
>>On Tue, 22 Mar 2005 13:18:19 -0500, Joseph Legris
>><jalegris at xympatico.ca> wrote:
>>
>>>Kalman Rubinson wrote:
>>>> On Mon, 21 Mar 2005 11:06:57 -0500, Joseph Legris
>>>> <jalegris at xympatico.ca> wrote:
>>>> 
>>>> 
>>>>>Kalman Rubinson wrote:
>>>>>
>>>>>>Well, another nit to pick is that the existence of a slightly
>>>>>>decremented potential (such as the AP) at the internodal region
>>>>>>requires neither transmembrane currents or membrane excitation, only a
>>>>>>potential difference (which does exist).
>>>>>>
>>>>>>Kal
>>>>>
>>>>>Yes, but in a conducting medium, potential difference == current flow.
>>>> 
>>>> 
>>>> But not between the inside and outside of an internodal region.
>>>> 
>>>> Kal
>>>> 
>>>
>>>O.K. but your statement above reduces to:
>>>
>>>"the existence of a slightly decremented potential at the internodal 
>>>region requires ... only a potential difference (which does exist).
>>>
>>>What then is the cause of the decrement to the action potential between 
>>>nodes?
>>
>>Longitudinal resistance and charging of the (small) membrane
>>capacitance.  
>>
>
>Are you allowed to nitpick a nit?

It deserves it.

>The membrane capacitance is normally not all that small -- the time
>constant is very large compared to the duration of the action
>potential.  The only way the membrane can generate a potential that
>quickly is for the membrane resistance to drop dramatically during the
>production of the action potential, thus reducing the time constant
>dramatically.
>
>The internodal region in a myelinated axon is different.  The multiple
>layers of membrane dramatically decrease the capacitance (they act as
>capacitors in series).  On the other hand, they dramatically increase
>the membrane resistance (resistors in series). So the time constant
>remains more or less unchanged.  

More or less?  Kandel states that "the action potential, which spreads
quite rapidly along the internode because of the low capacitance of
the myelin sheath, slows down as it crosses the high-capacitance
region of each bare node."  

>Still the degradation of the action
>potential down a passive cable is probably more due to capacitance and
>membrane charging than to longitudinal resistance.

Sure.

>To get out of nitpick mode -- the cause of the decrement to the action
>potential between nodes is the same as the cause of the decrement of
>any potential in a passive cable -- it is the effect of the cable
>properties (a combination of longitudinal and transverse resistance
>and transverse capacitance).  The important point is that the axon is
>not electrically excitable (does not have voltage gated Na and K
>channels) between nodes.

Certainly.  Nit picked clean?

Kal




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