primary afferent depolarization

Doug Knowles knowled at ccsmtp.ccf.org
Mon Jan 25 11:52:34 EST 1993

In article <9301251254.AA21883 at xray1.cshl.org> anderson at CSHL.ORG (John Anderson) writes:
>From: anderson at CSHL.ORG (John Anderson)
>Subject: primary afferent depolarization
>Date: 25 Jan 93 12:54:59 GMT
>Could someone please explain this statement?  How does presynaptic
>depolarization reduce the presynaptic spike amplitude?  Seems like it
>should enhance it.
>John E. Anderson
>W. M. Keck Structural Biology Laboratory
>Cold Spring Harbor Laboratory

The amount of transmitter released is dependent on the amount of
calcium which enters the presynaptic terminal, which is dependent 
on the amplitude of the action potential in the presynaptic
terminal.  Prior depolarization of the terminal (e.g. by inceased
extracellular K+) reduces the difference between the (depolarized)
resting potential and the peak of the action potential, thus 
reducing the action potential amplitude.  This causes less calcium
entry, less transmitter release, and a smaller postsynaptic 

I'm not sure why the calcium entry is dependent on the amplitude
of the action potential, rather than absolute membrane potential
difference.  Probably because of the time and voltage dependence
of the calcium channels.  I'll have to defer to my more informed
biophysics colleagues  (Bill?).

W. Douglas Knowles, Ph.D.            E-mail: knowled at ccsmtp.ri.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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