[Neuroscience] Re: Neuronal activity in the brain

Aric Agmon via neur-sci%40net.bio.net (by aric.agmon from gmail.com)
Wed Oct 25 12:55:55 EST 2006


Fijoy,

I am posting this because I think you were somewhat shortchanged by the
previous responders.
The answer to your question is "In general, yes".  When a patch of cortex is
activated by a focused bundle of thalamocortical axons, during some sensory
task, there will be a massive opening of synaptic channels in the zones of
termination of these axons (layer 4 and lower 5/upper 6, in primary sensory
areas).  Current will flow ("sink") into the postsynaptic neurons in those
regions, and will dissipate out of the cells along their apical dendrites,
in more superficial locations, thus generating a (small but measurable)
counter-current in the extracellular space, flowing normal to the pial
surface and directed inwards. You can crudely measure this current (or
rather, the voltage change generated by the extracellular resistance to this
current) with scalp EEG electrodes, and more finely with "field potential"
electrodes, and it will be maximal at the center of the activated patch and
gradually decline towards the periphery of this area.

Aric Agmon.


Aric Agmon, Ph.D.
Assoc. Prof.
Dept. of Neurobiology and Anatomy
Sensory Neuroscience Research Center
West Virginia University
Morgantown, WV 26506-9128

Tel. (304) 293-0602
----- Original Message ----- 
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Sent: Wednesday, October 25, 2006 1:01 PM
Subject: Neur-sci Digest, Vol 17, Issue 15


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>   1. Re: Neuronal activity in the brain (Matthew Kirkcaldie)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Wed, 25 Oct 2006 07:51:29 +1000
> From: Matthew Kirkcaldie <m.kirkcaldie from removethis.unsw.edu.au>
> Subject: [Neuroscience] Re: Neuronal activity in the brain
> To: neur-sci from net.bio.net
> Message-ID:
> <m.kirkcaldie-345A02.07512925102006 from news.sydney.pipenetworks.com>
>
> 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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