Somatosensory Cortex: Descending Influences on Ascending Systems

In view of its limited interconnections with other than adjacent cerebral tissue, the somatic sensorimotor cortex may be regarded as an almost isolated unit of cerebral tissue — a unit interacting extensively with subcortical structures, but having little direct commerce with other regions of the cerebral mantle. A recent paper by Myers (1967) highlights the almost complete isolation of several cerebral regions, one from the other, and hints that the brain may be organized after the manner of the inflorescence of an umbelliferous plant. However, somatic sensorimotor cortex is a different sort of unit; it straddles two lobes, receives input from adjacent tissue in both lobes, and has a conspicuous one-way internal connection. Part of its output — the pyramidal tract — is the only one known in detail, and hence will occupy a prominent place in this chapter.