Terminal organization of the corticospinal projection from the arm/hand region of the rostral primary motor cortex (M1r or old M1) to the cervical enlargement (C5-T1) in rhesus monkey.

High-resolution anterograde tracers and stereology were used to study the terminal organization of the corticospinal projection (CSP) from the rostral portion of the primary motor cortex (M1r) to spinal levels C5-T1. Most of this projection (90%) terminated contralaterally within laminae V-IX, with the densest distribution in lamina VII. Moderate bouton numbers occurred in laminae VI, VIII, and IX with few in lamina V. Within lamina VII, labeling occurred over the distal-related dorsolateral subsectors and proximal-related ventromedial subsectors. Within motoneuron lamina IX, most terminations occurred in the proximal-related dorsomedial quadrant, followed by the distal-related dorsolateral quadrant. Segmentally, the contralateral lamina VII CSP gradually declined from C5-T1 but was consistently distributed at C5-C7 in lamina IX. The ipsilateral CSP ended in axial-related lamina VIII and adjacent ventromedial region of lamina VII. These findings demonstrate the M1r CSP influences distal and proximal/axial-related spinal targets. Thus, the M1r CSP represents a transitional CSP, positioned between the caudal M1 (M1c) CSP, which is 98% contralateral and optimally organized to mediate distal upper extremity movements (Morecraft et al., 2013), and dorsolateral premotor (LPMCd) CSP being 79% contralateral and optimally organized to mediate proximal/axial movements (Morecraft et al., 2019). This distal to proximal CSP gradient corresponds to the clinical deficits accompanying caudal to rostral motor cortex injury. The lamina IX CSP is considered in the light of anatomical and neurophysiological evidence which suggests M1c gives rise to the major proportion of the cortico-motoneuronal (CM) projection, while there is a limited M1r CM projection.