INPUTS FROM LOW THRESHOLD MUSCLE AND CUTANEOUS AFFERENTS OF HAND AND FOREARM TO AREAS 3A AND 3B OF BABOONS CEREBRAL-CORTEX

The posterior wall of the central sulcus in the forelimb area of SI was explored with extracellular micro-electrodes in baboons lightly anesthetized with nitrous oxide and sodium thiopentone. The excitory responses of 130 single units to low intensity electrical stimulation of the deep radial (muscle) and the superficial radial (cutaneous) nerves were investigated. Units that responded only to muscle nerve stimulation were located in area 3a but overlapped into area 3b. Units that responded only to cutaneous nerve stimulation were found mainly in area 3b but a number occurred in area 3a. Units that responded to both muscle and nerve stimuli (convergent units) were found throughout area 3a and the rostral part of area 3b. Latency analyses of all 3 response groups revealed a single population of units responding to low threshold muscle nerve stimulation (mean latency 8.5 ms), and both early and late populations responding to low threshold cutaneous nerve stimulation (mean latencies 9.5 and 13.6 ms respectively). A number of the convergent units had very similar latencies for both inputs. Electrical stimulation within area 3a demonstrated a projection from areas 1 and 3b to area 3a; such a pathway may provide a route for excitation of the late skin population which was found mainly in area 3a. In area 3a units commonly responded to light touch, local pressure or deep pressure but only rarely to movement of hairs. A number of the convergent units responded to natural stimulation of cutaneous receptors. The deep radial nerve was cut, and no responses to limb movement were seen. The convergence of inputs from both cutaneous and muscle nerves observed in many units appears to provide and exception to the general rule of submodality specificity in SI. These units possess some of the attributes expected of neurons concerned with recognition of limb position and movement.