Functional subsets of neurons in somatosensory thalamus of the cat

Abstract

Functional properties of somatosensory thalamic neurons isolated within a region encompassing nucleus ventralis posterolateralis were studied in chloralose-anesthetized cats. A sample of these neurons responding to electrical stimulation of the contralateral forepaw was reconstructed by pooling data from 46 individual animals on the basis of unit spatial location. Seventy percent of the 640 neurons studied responded only to the contralateral forepaw; they had small receptive fields as determined by natural stimulation, generally on the digits and the dorsum of the paw, and 75% of this group were excited by light touch or deflection of hairs. Thirty percent of the neurons studied responded to the contralateral forepaw and to other limbs as well, their larger receptive fields often being bilateral. Approximately 50% of these wide-field neurons were excited by gentle mechanical stimulation. Some cells of this type were identified as projection neurons by virtue of their antidromic activation from the S I forepaw area of cerebral cortex. The wide-field neurons described here are not adequately explained as vestigial aberrations persisting at the feline stage in an evolutionary trend toward strictly somatotopic thalamic representation. Comparison of the dynamic response properties (latency, spikes per discharge, and frequency-following) of small- and wide-field somatosensory thalamic neurons suggests that they are activated via different input mechanisms and that each has a particular functional role in somesthesis.