Differential activity among wide-field neurons of the cat postcruciate cerebral cortex

Abstract

Extracullular recordings were made of the evoked activity of neurons in sensorimotor cortex of cat following cutaneous stimulation of each limb. Neurons responding only to stimulation of the somatotopically “on focus” limb (CFP) were classed as s and separated from those responding to stimulation of at least one additional limb; the latter “wide-field” neurons were classed as m. The ratio of m to s increased from “sensory” to “motor” recording sites. Analysis was focused on m neurons of the postcruciate recording site—here 80% of the pyramidal tract (PT) cells were m neurons. Both “fast” and “slow” PT cells were identified, the latter residing more superficially and firing later than the former. Despite the wide receptive fields of these cells, a unique pattern of PT activity develops in the cortex that is somatotopically related to the cutaneous site of stimulation. This pattern results from a facilitatory action of s neurons on the PT, m elements, particularly the “slow” PT cells. The PT response patterns following stimulation at any other site are remarkably similar and suggest a “coadunating” function either in thalamus or cortex itself. The non-PT, m elements behave in a similar manner. These results restore a measure of somatotopic organization to the wide-field neurons. However, consideration of the anatomical distribution of PT elements and the physiological data supports the hypothesis that the pyramidal-tract elements originating in the postcruciate cortex of the cat serve primarily in an excitability-modulating capacity.