AN ELECTROPHYSIOLOGICAL STUDY OF THE POSTNATAL-DEVELOPMENT OF THE CORTICOSPINAL SYSTEM IN THE MACAQUE MONKEY

Postnatal development of the corticospinal system was investigated in 13 macaques using noninvasive transcranial magnetic stimulation (TMS) of the motor cortex and direct electrical stimulation of corticospinal axons in the medullary pyramid and spinal cord. The latency of antidr omic corticospinal volleys evoked from the pyramid and recorded from t he motor cortex decreased dramatically during the first postnatal mont hs. Our data predict that conduction velocity (CV) of the fastest cort icospinal neurons over their cranial course would reach adult values a t similar to 11 months. The CV of corticospinal neurons in the spinal cord increased with age but with a slower time course. In the neonate, the fastest spinal CV was estimated at 7.8 m/sec, similar to 10 times slower than in adults (mean 80.0 m/sec). Our data predict that full m yelination of corticospinal axons in the spinal cord would not occur u ntil similar to 36 months. No short-latency EMG responses were elicite d in arm and hand muscles by TMS until 3 months of age; TMS thresholds were high initially and then fell progressively with age. When correc ted for body size, relative latencies of EMG responses showed an expon ential decrease during the first postnatal months. Our data are consis tent with the hypothesis that fine finger movements are not observed b efore functional CM connections are well established and that rapid ch anges in the physiological properties of the corticospinal system coin cide with the period in which precision grip is known to mature (3-6 m onths). However, corticospinal development continues long after simple measures of dexterity indicate functional maturity, and these changes may contribute to the improved speed and coordination of skilled hand tasks.