PROTRACTED POSTNATAL-DEVELOPMENT OF CORTICOSPINAL PROJECTIONS FROM THE PRIMARY MOTOR CORTEX TO HAND MOTONEURONS IN THE MACAQUE MONKEY

We have studied the development of corticospinal projections from the hand area of the primary motor cortex to the spinal cord using anterog rade transport of WGA-HRP. In the neonate, as in the adult, corticospi nal projections to the intermediate zone at the C8/T1 spinal level wer e clearly present. However, in contrast to the adult, there was only v ery faint and barely visible labelling in the dorso-lateral motor nucl ei which supply the hand muscles. No aberrant projections to other mot or nuclei were seen. By 2.5 months. a ring of dense labelling was pres ent around the dorso-lateral motor nuclei, but labelling was still spa rse in the central region. This labelling was more pronounced at 11 mo nths, but was still not as heavy as in the adult. There was no labelli ng among the ventral motoneurones at any age. The conduction velocity (c.v.) of the fastest corticospinal fibres was determined in each of t he monkeys. There was an age-related increase in c.v. within the spina l cord. At birth, the fastest axons had a c.v. of only 8 m.s(-1). At 1 1 months c.v. was still substantially slower (55 m.s(-1)) than the adu lt value of 73 m.s(-1). In contrast, by 11 months, the axonal c.v. wit hin the brain was close to the adult value, suggesting a rostro-caudal maturation of the corticospinal system. Our results demonstrate that corticospinal projections in the macaque monkey mature gradually over a period of at least 11 months, much longer than previously thought.