Quantitative inter-segmental and inter-laminar comparison of corticospinal projections from the forelimb area of the primary motor cortex of macaque monkeys

Corticospinal projections from the forelimb area of the primary motor cortex to the C2–Th2 spinal cord segments were quantitatively analyzed using the high resolution anterograde tracer, biotinylated dextran amine (BDA), in rhesus monkeys (n=5). The majority of descending axons were located in the contralateral dorsolateral funiculus (DLF) (85–98%), but a minor portion was observed in the ipsilateral DLF (1–12%) and ventromedial funiculus (VMF) (1–7%). In the gray matter, axon collaterals and terminal buttons were found mainly in the contralateral laminae VI–VII and IX and ipsilateral lamina VIII. The majority of projections to the contralateral gray matter originated from the contralateral DLF, but a minority originated from the ipsilateral DLF. Axons from the ipsilateral DLF were not found to project collaterals on the ipsilateral side, but directly entered the contralateral side after crossing the midline. On the other hand, projections to the ipsilateral lamina VIII were from the ipsilateral VMF, and commissural axons were from the contralateral DLF. Terminal buttons in the motoneuron pool in the contralateral lamina IX were found mainly at the C7–Th1 spinal cord segments, whereas the projections to the contralateral laminae VI–VII, ipsilateral lamina VIII, and commissural axons were also found in more rostral segments, abundantly at the C4–C8 segments, 1–3 segments rostral to the motoneuronal projections. These results suggest that cortical control of contralateral forelimb motoneurons accompanies regulation of interneuronal systems in the contralateral laminae VI–VII and the ipsilateral lamina VIII located a few segments rostral to the motoneurons.

Research Highlights▶Primate M1 controls spinal motoneurons and rostrally located interneurons in parallel. ▶Commissural corticospinal axons are abundant in C4–C8 segments. ▶Uncrossed corticospinal fibers mostly cross the midline at the spinal level.