[Neuroscience] Re: Neuronal activity in the brain

Fijoy George via neur-sci%40net.bio.net (by tofijoy from yahoo.co.in)
Thu Oct 26 10:24:07 EST 2006

Prof. Agmon,

Thank you so much for your throughful answer. In fact, I am a student (in 
Computer Science) currently working on the EEG/MEG neural source imaging 
problem, and I wanted to know how to correctly simulate activation patches 
on the cortical surface.


"Aric Agmon" <aric.agmon from gmail.com> wrote in message 
news:mailman.210.1161799459.23274.neur-sci from net.bio.net...
> 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 ----- 
> From: <neur-sci-request from oat.bio.indiana.edu>
> To: <neur-sci from magpie.bio.indiana.edu>
> 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)
>> ----------------------------------------------------------------------
>> 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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