Localization of spinal motor neurons supplying single ventral roots

The spinal cord location of motor neurons whose axons make up lumbar ventral roots L-5, L-6, and L-7 was investigated in the cat. Location was based on recording the extracellular action potentials from spinal motor neurons which had been activated antidromically by electrical stimulation of single ventral roots. The borders of the spinal segments (as defined by the rostral and caudal limits of the ventral root exit zone) and the sites recorded from were suitably marked, so that the territory occupied by the responding motor neurons could be determined. We showed that probably more certain localization of motor neurons could be achieved by using the positive polarity ("injury") antidromic responses which are recorded with needle electrodes of relatively large tip size. Negative polarity antidromic responses are recorded with fine-tipped electrodes. Results indicated that the origin of a single lumbar ventral root is from homolateral ventral horn cells and is mainly unisegmental, that is, from cells located within the segment of the root's attachment. There was, however, evidence of overlap of some motor cell bodies and/or dendritic processes into the caudal one-fourth of the adjacent more rostral segment. No difference was seen in the longitudinal extent of spinal motor cells supplying axons to the dorsal or ventral primary rami.