Anatomical and physiological study of interneurons in an oligosynaptic cutaneous reflex pathway in the cat hindlimb

Morphological and physiological studies were undertaken in the cat lumbosacral spinal cord in order to examine individual last-order excitatory interneurons in the disynaptic pathway between low threshold superficial peroneal (SP) afferents and flexor digitorum longus (FDL) motoneurons. Last-order interneurons projecting to the FDL motor nucleus were visualized with activity-dependent trans-synaptic transport of lectin-conjugated horseradish peroxidase (WGA-HRP). The location of lumbar interneurons that responded at short latencies (less than 3 ms) to electrical stimulation of the SP nerve was further defined using extracellular recording. The projection of individual interneurons to motoneurons in the L6 segment was defined by spike-triggered averaging of population ventral root potentials (VRPs) using the sucrose gap technique. Measurements of the central latencies from the arrival of the SP afferent volley to interneuron firing, and from interneuron spike to the onset of the VRP EPSPs, confirm the existence of a low threshold, disynaptic SP pathway that excited motoneurons. These data provide useful latency constraints for the interpretation of the synaptic length of cutaneous reflex pathways in the cat hindlimb.