TOPOGRAPHIC ORGANIZATION OF CORTICOSPINAL PROJECTIONS FROM THE FRONTAL-LOBE - MOTOR AREAS ON THE LATERAL SURFACE OF THE HEMISPHERE

We examined the topographic organization of corticospinal neurons in the primary motor cortex and in the two premotor areas on the lateral surface of the hemisphere [i.e., the dorsal premotor area (PMd) and the ventral premotor area (PMv)]. In two macaques, we labeled corticospinal neurons that project beyond T7 or S2 by placing crystals of HRP into the dorsolateral funiculus at these segmental levels. In another seven macaques, we labeled corticospinal neurons that project to specific segmental levels of the spinal cord by injecting the fluorescent tracers fast blue and diamidino yellow into the gray matter of the cervical and lumbosacral segments. In one set of experiments (n = 2), we defined the representations of the arm and leg in each cortical motor area by injecting one of the two fluorescent tracers into lower cervical segments (C7-T1) and the other fluorescent tracer into lower lumbosacral segments (L6-S1) of the same animal. In another set of experiments (n = 5), we defined the representations of distal and proximal parts of the forelimb in each cortical motor area by injecting one of the two fluorescent tracers into lower cervical segments (C7-T1) and the other tracer into upper cervical segments (C2-C4) of the same animal. In the primary motor cortex and the PMd, cortical regions that project to lower cervical segments were largely separate from those that project to lower lumbosacral segments. In the PMv, few neurons were labeled after tracer injections into lower cervical segments or lower lumbosacral segments. However, corticospinal neurons were labeled in the PMv after tracer injections into upper cervical segments and after HRP placement in the dorsolateral funiculus at T7. The region of the PMv that projects to upper cervical segments was separate from that which projects below T7. Cortical regions that project to upper and lower cervical segments of the spinal cord overlapped considerably in the primary motor cortex and in the PMd. Despite this overlap, we found that the regions of the primary motor cortex and PMd that project most densely to upper cervical segments were largely separate from those that project most densely to lower cervical segments. Furthermore, we found two separate regions within area 4 that send corticospinal projections primarily to the lower cervical segments. One of these regions was located within the classical ''hand'' area of the primary motor cortex. The other was located at the medial edge of arm representation in the primary motor cortex. These results provide new insights into the pattern of body representation in the primary motor cortex, PMd, and PMv. Our findings support the classic distinction between the ''arm'' and ''leg'' representation in the primary motor cortex. However, the demonstration that two regions in the primary motor cortex project densely to lower cervical segments suggests that the organization of distal representation in the ''arm'' area is more complex than previously thought. Our study provides additional support for the existence of separate arm and leg representations in the PMd and possibly in the PMv. Furthermore, we present evidence that the arm area of the PMd contains separate regions of distal and proximal representation. Thus, our results imply that the PMd is involved in the control of both distal and proximal arm movements. Finally, we found some striking differences in the pattern of corticospinal projections from the PMd and PMv. These differences provide additional support for distinguishing between these two premotor areas.