Afferent input to movement-related precentral neurones in conscious monkeys

Publisher Website
Google Scholar
Abstract
Monkeys were trained to perform a stereotyped movement task, and to accept passive manipulation and natural stimulation of the limbs while remaining relaxed and quiet. All training, both of movement and for relaxation, was with food rewards. The effects of natural stimuli on 257 precentral neurons showing consistent modulations in discharge frequency during the performance of the movement task were investigated. Most precentral neurons had small, stable input zones located on the contralateral arm: 197 facilitatory and 17 inhibitory responses were obtained, while the remaining 43 cells were unaffected by the natural stimuli used. The most common natural stimulus capable of influencing precentral neurons was joint movement: 152 cells responded to joint movement, including 98 which only responded to movement at a single joint. Joint movement rather than joint position was the effective stimulus and none of these cells was influenced by palpation of muscles acting at the joint. The next most common natural stimulus capable of influencing precentral neurons was muscle palpation: 35 cells responded to a tap applied to a localized portion of the muscle belly, including 26 cells which also responded to movement of the joint at which the muscle acted. The direction of joint movement which influenced the cell was usually such as to stretch the muscle containing the receptors for the effective afferent input set up by tapping. The natural stimulus which influenced the smallest number of precentral neurons was tactile stimulation of the skin: 27 cells had cutaneous receptive fields, most of which were small (< 5 cm2) and confined to the hand. Included in the total sample were 51 pyramidal tract neurons. The behavior of these was similar to the unidentified neurons examined in the same animals with respect to their afferent input. However, there was a tendency for pyramidal tract neurons to be in receipt of a more convergent input than unidentified neurons in their vicinity. The majority of neurons recorded in close proximity to one another (within 500 .mu.m or less) usually received their afferent input from the same peripheral region, but a significant proportion of such cells received inputs from different and remote peripheral zones. Thus, the afferent input to the precentral motor cortex is not organized to provide independent and spatially segregated projections from particular peripheral sites only to limited and localized radial aggregations of neurons.