[Neuroscience] Re: Neuronal activity in the brain
(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.
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