Cortical localization

G.Coleman at G.Coleman at
Tue Apr 18 18:34:38 EST 1995

In article <3mkki2$8td at> v102nq9f at writes:
> >Laurie J. Lundy-Ekman (lundyekl at PACIFICU.EDU) wrote:
> >: Now that we have a sensorimotor cortex (rather than primary motor and 
> >: somatosensory areas), do we still have secondary cortical areas? Do we 
> >: still call the area posterior to the postcentral gyrus secondary 
> >: somatosensory, or is there a more current term?  Thanks!
>In article <3mgtlo$e0o at cmcl2.NYU.EDU>, rubinsnk at (Kalman Rubinson) writes:
> >1.  Sensorimotor cortex is not a new term and doesn't mean that the 
> >cortex on the two side of the central sulcus are the same.
> >2.  Yes, they are still called secondary.  Until we get something better.
>aren't they called secondary for a reason?
>it is my understanding that primary *means* the first cortical tissue to
>receive the afferents..
>secondary means that's where the primary cells synapse....... no?
>if anyone can confirm this, i would appreciate it.

Primary and secondary areas of somatosensory areas (SI & SII), as well as
additional areas of cortex, are designated  numbers according to the order 
in which they were discovered.  In other words, the *primary* area was 
discovered by electrophysiological mapping first, followed by the second 
somatosensory area.  In the cat additional areas SIII, SIV and SV have 
been described.  Additional areas in primates have been named differently,
but there are a number of additional somatosensory areas in primates also.

There is _NO_ evidence that SI receives "primary" synaptic connections and 
that SII receives only secondary connections in the cat.  Both receive 
their input from the same nucleus of the thalamus (the ventral posterior
nucleus) and process them in parrallel.  The function of multiple separate 
areas in this species, and other non-primates, is unclear.

There has been some controversy over the thalamocortical connections of SI 
and SII in simian primates in recent years.  Pons, Kaas, Garraghty, Mishkin
and I think Friedman (in the US) have argued that SII in primates represents
a higher cortical processing area and that, unlike cats, rabbits etc., 
receives no direct somatosensory information from the thalamus.  Rather, they
believe it receives all its information about discriminative touch from SI.
These findings have been questioned by other researchers, and recent work by
Zhang and Rowe, with others in their lab (University of NSW, Sydney Australia)
have called their findings into question.

In summary, the nomenclature of somatosensory areas of cortex reflects the
order in which they were discovered, and provides no clear indication of 
their relative importance or ordering.  SI and SII appear, in most species,
to process subcortical inputs in parallel.  The role of the additional areas
is as yet largely undiscovered.  SI admittedly appears to be the largest and 
possibly most important area, and is possibly the ONLY somatosensory area in
some marsupial and monotreme species, but is not the first synapse in a 
hierarchical system with SII, SIII and so on.

Just a brief comment on sensorimotor areas.  The precentral and postcentral
gyrus in primates are often referred to as the _sensorimotor_strip_.  They
are interconnected, but their locations and functions are quite distinct.
In contrast, the two areas extensively overlap in a number of species, 
notably in most marsupial and monotreme mammals studied.

Hope this is useful to you - any feedback or further questions would be 
welcome (it gives me something to do when I'm bored).

G. Coleman, School of Physiology and Pharmacology, University of NSW

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