EVIDENCE FOR DIRECT CONNECTIONS BETWEEN THE HAND REGION OF THE SUPPLEMENTARY MOTOR AREA AND CERVICAL MOTONEURONS IN THE MACAQUE MONKEY

In primates the corticospinal neurons of the hand representation of th e primary motor cortex (M1) give rise to direct contacts with the cerv ical motoneurons that control distal forelimb muscles. We investigated , at the light-microscopy level, whether corticospinal cells present i n the hand area. of the supplementary motor area (SMA) also establish direct connections with cervical motoneurons, particularly those inner vating hand and finger muscles. The hand representation of the M1 (two monkeys) or SMA (two monkeys) was located using intracortical microst imulation and injected with the anterograde tracer biotinylated dextra n amine to label corticospinal terminals. Forearm muscles acting on th e wrist and hand as well as hand muscles acting on the thumb and index finger, thus including those activated by intracortical stimulation, were injected with the retrograde tracer cholera-toxin B subunit, in o rder to label the motoneurons. A consistent zone of overlap between th e two markers was found in the cervical cord. Close appositions betwee n corticospinal axonal terminals and the somata or dendrites of motone urons were found after injection in the M1, confirming previous observ ations. The new finding is the observation of similar close apposition s after injection in the SMA, suggesting its control of hand movements in parallel with the M1.