The development of motor control in the rhesus monkey: evidence concerning the role of corticomotoneuronal connections

Abstract

Corticospinal fibres terminate in three areas in the spinal grey matter of the rhesus monkey: the nucleus proprius of the dorsal horn, the intermediate zone and directly upon motoneurons, particularly those inervating limb muscles. It was suggested at their earliest discovery that direct corticomotoneuronal (CM) connections might constitute a critical anatomical substrate for the type of fractionated movement exemplified by relatively independent finger movements (RIFM). The present study was undertaken in an attempt to provide further evidence in support of this idea. The evidence is based upon the anatomical finding that few CM connections are present at birth in the monkey, the bulk being formed postnatally to reach an adult density at approximately 8 months of age. There were two experimental approaches: in the first it was argued that if RIFM are dependent upon the presence of CM connections it would be expected that such movements would appear only gradually postnatally, developing in parallel with the development of the connections. In 2 normal monkeys studied from this point of view discrete distal movements did develop only gradually with RIFM reaching an adult level at 7 to 8 months of age. The second experimental appraoch was based upon the fact that pyramidal lesions in newborn animals interrupt corticospinal fibres before CM connections are formed. It was argued that such a situation might provide a better opportunity for other descending pathways to establish the connections necessary for RIFM. Five infant monkeys had bilateral pyramidal tract lesions made from 5 days to 4 weeks after birth and were observed for a period of three years. In the 4 animals in which the lesions were complete, RIFM failed to develop. In the fifth animal, in which the lesions were incomplete, there was development of some degree of RIFM. It was concluded that the development of RIFM is dependent upon fibres passing in the pyramidal tracts and that in infant monkeys, as well as in adults, other descending pathways are unable to form the connections necessary for these movements. The possible contribution to RIFM of the corticospinal fibres arise in the somatosensory cortex and terminate in the dorsal horn was studied by bilateral removal of the arm and leg area of the somatosensory cortex in one animal. Since RIFM were only minimally and transiently altered, it was concluded that such movements are not dependent upon these fibres. The different lines of evidence presented in the study are interpreted as giving further support to the idea that corticomotoneuronal connections constitute an essential anatomical substrate for fractionated movements such as RIFM. Observations were also made of the development of general locomotor activity in normal and pyramidotomized infant monkeys.