The distribution of dorsal root fibres on motor cells in the lumbosacral spinal cord of the cat, and the site of excitatory and inhibitory terminals in monosynaptic pathways

Abstract

The degeneration of cut, lumbosacral dorsal roots was traced by means of a silver method to the various sites of termination in the spinal cord of the cat. Particular attention was given to the pattern and distribution of dorsal root fibres terminating on motor cells. An attempt was made to correlate the synaptic organization of these monosynaptic pathways with the presence and distribution of short latency, direct excitation and inhibition (measured on the ventral roots) in comparable segments of the cord. Short latency excitation was found in the lower sacral segments to be correlated with dorsal root fibres terminating on cell bodies and dendrites of the motoneurons. Direct inhibition, largely or entirely uncontaminated by excitation, was identified in lower lumbar and upper and lower sacral segments. In these sites the terminal degeneration of the responsible dorsal root was limited to the dendrites and did not reach the cell bodies of the motoneurons. This consistent correlation between the spatial distribution of direct inhibition and terminals on motor dendrites suggests that this inhibition is monosynaptically and dendritically mediated, at least in part. Certain other interpretations are considered and relevant evidence obtained from intracellular microelectrodes placed in lower sacral motoneurones which were excited by ipsilateral and inhibited by contralateral dorsal root stimulation is discussed. Here, uncontaminated, monosynaptic excitation could be correlated with synaptic terminals on both cell bodies and dendrites of motoneurones.