Tactile neuron classes within second somatosensory area (SII) of cat cerebral cortex

Abstract

Microelectrode recordings were made in unanesthetized cats from individual neurons in the forelimb projection focus of the second somatosensory area of cortex, SII. The 164 neurons sensitive to tactile stimulation of the glabrous skin of the foot pads were divided into 3 functional classes: 5% responded throughout a steady indentation of the skin lasting 1-1.5 s and were designated slowly adapting neurons; the remaining 95% displayed rapidly adapting responses to steady indentation. The few slowly adapting neurons studied were relatively insensitive to small indentations (< 500 .mu.m) of the footpads but at higher amplitudes their stimulus-response relations were graded, at least up to amplitudes of 1-1.5 mm. For the 2 classes of rapidly adapting or dynamically sensitive tactile neurons, there was little overlap in sensitivity to cutaneous vibration if vibration amplitude did not exceed 25-50 .mu.m. SII neurons in the class sensitive to low-frequency cutaneous vibration respond with phase-locked activity to vibration in the range 10-80 Hz. Quantitative measures suggest that these neurons are able to encode information about vibratory frequency within the 10- to 80-Hz range by means of the pattern of their impulse activity. Among the SII neurons receiving input from the forelimb footpads there was little evidence of bilateral convergence. For 25 neurons sensitive to high-frequency vibration the input was confined in all cases to the contralateral limb as it was for all but one of 34 neurons responsive to low-frequency vibration. Each of the 3 classes of tactile neurons in SII activated from the distal glabrous skin of the limbs derives its input selectively from 1 of the 3 known classes of tactile receptor in that region.sbd.Merkel disk receptors, intradermal encapsulated corpuscles, or Pacinian corpuscles. Although the same 3 classes of neurons were described in SI, there appear to be differences in the proportional representation of the 3 classes within SI and SII. Cortical somatosensory areas I and II are perhaps involved in parallel, but in some respects differential, processing of tactile information from the distal glabrous skin.