Motoneuronal projection pattern of single C3-C4 propriospinal neurones.

The pattern of motoneuronal projection and termination of single C3-C4 propriospinal neurones in the forelimb segments C6-T1 of the cat was investigated by intra-axonal injection of horseradish peroxidase into stem axons. Twelve well-stained axons were used for analysis. Termination was observed in the estimated location of motor nuclei innervating pure shoulder muscles in 10 cases. Among motoneurones innervating shoulder, elbow, wrist, and digit muscles, projection and termination were observed in motor nuclei controlling muscles of two or three joints in the following combinations: shoulder + elbow, shoulder + wrist, shoulder + elbow + wrist, shoulder + elbow + digit, and elbow + wrist + digit. In one case it was difficult to exclude the possibility of projection and termination in motor nuclei controlling muscles at all four joints. These patterns of motoneuronal projection from C3-C4 propriospinal neurones are compatible with their function in mediating the descending command for visually guided target reaching movements with the forelimb. In addition, it was found that the C3-C4 propriospinal neurones project and terminate in the region of ventral and ventromedial motor nuclei, which innervate axial muscles acting on the trunk. This was confirmed by intracellular recording from presumed ventromedial motoneurones in the C6-C7 segments. It is postulated that the C3-C4 propriospinal neurones, in addition to their control of forelimb movements, provide for conjoint control of axial muscles to stabilize the trunk during target reaching.