[Neuroscience] Re: Neuronal activity in the brain

Matthew Kirkcaldie via neur-sci%40net.bio.net (by m.kirkcaldie from removethis.unsw.edu.au)
Tue Oct 24 16:51:29 EST 2006


In article <ehgcnr$1dm$1 from mailhub227.itcs.purdue.edu>,
 "Fijoy George" <tofijoy from yahoo.co.in> wrote:

> We know that the primary sources of electrical currents in the brain are the 
> pyramidal cells in the cortex. We also see that the pyramidal cells are 
> normally oriented to the cortical surface. Suppose that a particular portion 
> of the cortex (a cortical patch) is activated during a task. Suppose we 
> measure the *current density* normally oriented to the cortical surface at 
> various locations in the patch. Now, will the amplitudes of the current 
> density be maximum at the center of the patch and slowly decline as we move 
> along the cortical surface away from the center?

Who's "we"?  When you talk about "electrical currents in the brain" it 
sounds as if you believe that neurons signal by passing electrical 
current along their axons, like a wire passing current along its length.  
That is completely wrong. When a neuron fires, minuscule currents 
flicker across tiny distances to change the voltage on the membrane, and 
this voltage change gets reproduced, spreading along the membrane at 
high speed. However it only causes tiny currents (picoamps) to flicker 
briefly through tiny sections of membrane, for very short periods, 
followed by a restoration of the resting state. There is NO net current 
flow, in the sense of a quantity of charge moving over distance.

So to answer your question as it is posed, if we were to measure the 
current density normally oriented to the cortical surface,  the 
amplitude would be zero at the centre of the patch, and zero as we move 
away from the centre. Perhaps that gives you some indication of why it's 
difficult to give you a proper answer.

      Cheers, MK.


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