Activity of neurons in cortical area 3a during maintenance of steady postures by the monkey

Vallbo has reported 7 a positive correlation between spindle discharge frequency and torque produced during isometric contraction of finger muscles, indicating gamma-efferent control of muscle spindle sensitivity in close relation to voluntary muscle contraction. Area 3a of the cerebral cortex is known to be one projection site of muscle spindle afferents 2. Massive inputs from these receptors, especially from primary endings, have been described a-5, but relatively little is known as to how neuronal discharge in area 3a is modulated by these inputs in the course of normal movement. The present experiment was carried out to determine the extent to which discharge of neurons in area 3a reflects muscle length and/or muscle tension when an animal maintains different postures while supporting different loads. Prior to single cell recording, a monkey (Macaca mulatta) was trained to grasp a movable handle and position it in a correct zone which was signaled by a lamp. The monkey's right wrist and elbow were held fixed in plastic tubes so that the task was performed with flexion-extension movements at the wrist joint. As the monkey gained proficiency in this task, the correct zone was progressively narrowed to about 2 ° of the wrist joint. It was required that the monkey held the handle in the zone for 3.5-6 sec in order to receive a reward of fruit juice. After each reward, the correct zone was shifted randomly to one of 4 zones which demanded that the wrist joint be extended to either about 15 ° or 30 ° or flexed to either 15 ° or 30 ° from a neutral position, and the monkey had to search for the new correct zone with the aid of the white lamp. An external steady load varying from 40 to 100 g was then applied to the handle by a torque motor which opposed either flexion or extension of the wrist. A brief (100 msec) torque pulse was sometimes applied to the handle in order to observe neuronal responses to phasic stretch or brief unloading of muscles. The recording technique for single cortical units during task performance has been described previously 1. Stimulating electrodes were chronically implanted in median, radial and ulnar nerves in the right brachial plexus. Low threshold cortical unitary discharges to the nerve stimulus (at !.3 times threshold for the evoked potential in the cortex) and characteristic neural responses to natural stimulation, namely to