Efferents of area 4 in a South American monkey (Saimiri). I. Terminations in the spinal cord.

A single injection of either tritiated proline or a mixture of proline and leucine was made in area 4 of 8 squirrel monkeys. The locus of the injection was systematically varied from medial to lateral among animals. Autoradiographs revealed a strong contralateral lateral corticospinal tract. A sparsely labelled ipsilateral lateral tract was also present in all animals. In 2 animals, a few labelled fibers indicative of an ipsilateral anterior tract were observed; the fibers terminated at lumbar levels. Grain counts over the cervical and lumbar gray showed that area 4 efferents terminated contralaterally in laminae IV--IX with a peak in lamina VII; only sparse input was seen in the vicinity of the large alpha-motoneurons of lamina IX. On the ipsilateral side, the terminals were largely confined to lamina VIII. This pattern was in accordance with that reported in other primates. The terminal fields at sacral and coccygeal levels were radically different in that large numbers of fibers recrossed to the ipsilateral side and ended in laminae V through IX; the functional significance of this strong bilateral termination was discussed.