In article <mailman.210.1161799459.23274.neur-sci from net.bio.net>,
"Aric Agmon" <aric.agmon from gmail.com> wrote:
> 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, with respect, your opinion here seems to have no experimental
basis. For one, you describe the fMRI signal as due to activity driven
by thalamocortical afferents, which is only likely to be true for
primary sensory regions. Secondly, you appear to imply that the
"inactive" cortex won't have intrinsic firing activity, which I doubt
you intended. Thirdly, you imply a link between an fMRI signal and
specific neuronal activity; while this is a common assumption, you
should flag it as such, since as far as I'm aware there has not been a
comprehensive causal chain linking particular types of neuronal activity
with particular changes observed in MRI intensity. The latter is the
reason why I think the original poster's hypothesis is untrue: since we
don't know exactly what aspects of neuronal activity result in fMRI
signal, it would be naive to assume that reduced fMRI intensity at the
edge of a region would correlate nicely with a reduced current density /
activity / firing.
Lastly, you describe current "sinking" into the neuron and "dissipating
out ... along ... apical dendrites" which sounds very unlike my
understanding of membrane currents in neurons. You also appear to blur
the line between current (movement of charge) and voltage (electrical
potential) in your description.
Since you see my attempts to clarify as "shortchanging" the original
poster, could you improve things by giving sources for these ideas?
Specifically, in what sense does neuronal firing involve current sinking
into the neuron (at the soma, I presume) and dissipating out of the
Although I'm not happy with your explanation as it stands, I'm happy to
learn something, or to see it from a different viewpoint.