Corticospinal terminations in two new-world primates: further evidence that corticomotoneuronal connections provide part of the neural substrate for manual dexterity.

Anterograde transport of 2-10% WGA-HRP was used to examine the pattern of termination of efferents from the primary motor cortex to cervical segments of the spinal cord in cebus (Cebus apella) and squirrel (Saimiri sciureus) monkeys. We have compared the pattern of termination in these monkeys because of marked differences in their manipulative abilities. Both primates have pseudo-opposable thumbs; however, only cebus monkeys use independent finger movements to pick up small objects. We found that corticospinal terminations in cervical segments of the cebus monkey are located in three main zones: a dorsolateral region of the intermediate zone, a dorsomedial region of the intermediate zone, and the ventral horn. The projection to the ventral horn in these monkeys is particularly dense at C8-T1 segments, where terminations form a "ring" that encircles the lateral motoneuronal cell group. In contrast, there are only two main zones of terminations in the squirrel monkey: a dorsolateral region of the intermediate zone and a dorsomedial region of the intermediate zone. As others have noted, efferents from the primary motor cortex of squirrel monkeys have, at best, only sparse terminations within the ventral horn. Thus, there are marked differences between cebus and squirrel monkeys in the extent of corticospinal terminations within the ventral horn. These observations provide further support for the concept that monosynaptic projections from the primary motor cortex to motoneurons in the ventral horn provide part of the neural substrate for dexterous movements of the fingers.