Neural Firing

Doug Knowles knowled at ccsmtp.ri.ccf.org
Tue Jan 19 12:52:11 EST 1993

In article <1993Jan19.151055.2097 at newssun.med.miami.edu> dbrown at newssun.med.miami.edu (Daniel Brown) writes:
>Newsgroups: bionet.neuroscience
>Path: bme.ri.ccf.org!usenet.ins.cwru.edu!magnus.acs.ohio-state.edu!zaphod.mps.ohio-state.edu!wupost!news.miami.edu!newssun.med.miami.edu!dbrown
>From: dbrown at newssun.med.miami.edu (Daniel Brown)
>Subject: Neural Firing
>Message-ID: <1993Jan19.151055.2097 at newssun.med.miami.edu>
>Organization: University of Miami, School Of Medicine
>Distribution: bionet
>Date: Tue, 19 Jan 1993 15:10:55 GMT
>Lines: 16
>I have a question concerning the firing of a neuron.  If one were to 
>place anions around a neuron (in vitro, say, so that the negative 
>charge wasn't rapidly dissipate), would this make the neuron fire?
>If these extra anions (also assume that they are unpermeable) were
>put around the axon, they would reduce the potential difference across
>it (by making the extracellular fluid less positive).  By reducing
>the potential difference above the threshold potential difference, the
>neuron would fire.  Does this reasoning sound correct?  It seems to
>depend on the proximity of the anions to the neuron's membrane and 
>the voltage dependence of the Na+ channels (are they dependent on the
>potential difference across the membrane, or are there other influences?).
>Thanks for the replies in advance,

Well, you can't really just add anions to a solution.  Ever see a bottle
of chloride?  No, just sodium chloride, potassium chloride, etc.  But 
because the membrane is selectively permeable to various ions, like 
potassium, you can change thedifference in potential across the membrane by 
adding potassium chloride.This will reduce the tendency of the potassium 
ions in the neuron to diffuse out, down their concentration gradient, 
resulting in a decrease in the membrane potential, a depolarization, which 
is excitatory.  You could look at it as a decrease in the number of positive 
ions leaving the neuron, although the actual number of ions moving is *very* 
small and does not change the concentrations of ions.  

Alternatively, you can pass electrical current through the extracellular 
space, creating a relatively negative region near one electrode.  This will
reduce the transmembrane potential in that region.  If the axon initial 
segment is in this region, the cell will tend to increase its firing.  
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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