Some corticospinal neurons give rise to axons which terminate directly upon motoneurons, thereby establishing corticomotoneuronal connections. The location of corticomotoneuronal synapses upon motoneurons innervating intrinsic muscles of the hand in the monkey was demonstrated by the use of intra-axonal and intracellular horseradish peroxidase (HRP). After a number of corticospinal axons originating from the “hand” area of the precentral gyrus had been injected with HRP in the lateral funiculus at C7-C8, a number of nearby identified intrinsic hand muscle motoneurons were also injected. Connections between corticomotoneuronal fibres and motoneurons were reconstructed from longitudinal parasagittal sections treated by the cobalt-enhanced diaminobenzidine method.
Corticospinal axons in the lateral funiculus gave rise to main collaterals which provided an extensive arborization in lamina IX, where it was predominantly longitudinal, and in the adjacent intermediate zone. En passant and single or clustered groups of terminal boutons arose from preterminal branches of these arbors. Seven light-microscopically identified corticomotoneuronal synapses were found. They were located upon the dendrites of recipient motoneurons at 40 μm to 750 μm from the soma and ranged in size from 0.6 × 3.0 μm to 2.4 × 3.6 μm.
The results suggest that each main collateral of a corticomotoneuronal axon establishes very few synaptic contacts, and possibly only one, with the dendrites of recipient motoneurons. This small number of contacts per motoneuron is consistent with the small amplitudes of minimal and unitary corticomotoneuronal EPSPs recorded from forelimb and hand motoneurons.