primary afferent depolarization

Doug Knowles knowled at ccsmtp.ccf.org
Tue Jan 26 13:06:52 EST 1993

In article <1993Jan26.123642.11726 at ringer.cs.utsa.edu> senseman at lucy.brainlab.utsa.edu (David M. Senseman) writes:
>From: senseman at lucy.brainlab.utsa.edu (David M. Senseman)
>Subject: Re: primary afferent depolarization
>Date: Tue, 26 Jan 1993 12:36:42 GMT
>In article <9301251254.AA21883 at xray1.cshl.org> anderson at CSHL.ORG (John Anderson) writes:

>>Could someone please explain this statement?  How does presynaptic
>>depolarization reduce the presynaptic spike amplitude?  Seems like it
>>should enhance it.
>PAD (primary afferent depolarization) isn't all that mysterious.
>Why would expect depolarization to ENHANCE transmitter release?
>Assuming that the axonal spike completely invades all the terminals,
>(probably a reasonable assumption for most systems Eva was 
>referring to), then depolarization would DECREASE syanptic release
>simply because the membrane potential of the presynaptic terminal
>would be closer to V    which is going to be somewhere above +100 mv.
>                    Ca
>That means the electrical force driving Ca inward would be reduced 
>so that calcium entry during the terminal spike would be reduced.
>Since transmitter release is dependent on entry of external Ca,
>less transmitter release would occur.

	I don't agree.  The driving force on Ca++ is determined by the
	voltage *during* the spike when the voltage-sensitive Ca channels 
	are opened.  Besides, the driving force on Ca++ is so large that
	a few millivolts depolarization from rest wouldn't make much 

>If you get enough PAD so that the terminal membrane is really
>depolarized, then additional factors could come into to play
>such as channel inactivation.
	This is probably more important--inactivation can occur
	near resting membrane potential.
>David M. Senseman, Ph.D.              | Imagine the Creator as a low 
>(senseman at lonestar.utsa.edu)          | comedian, and at once the world 
>Center for Information Visualization  | becomes explicable.
>University of Texas at San Antonio    |               H.L. Mencken 

W. Douglas Knowles, Ph.D.            E-mail: knowled at ccsmtp.ccf.org
Department of Neurosciences          voice:  (216) 444-3870
Cleveland Clinic Foundation          FAX:    (216) 444-7927
9500 Euclid Avenue                   
Cleveland, OH 44195 USA              other:   hey, you!
quote: "We need not all give our whole time to the currently more glamorous
"molecular biology"; it will doubtless come soon enough."  I.H. Page 1962

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