PRIMARY AFFERENT DEPOLARIZATION AND INHIBITORY INTERACTIONS IN SPINAL-CORD OF STINGRAY, DASYATIS-SABINA

Excitability changes in primary afferents and inhibitory interactions in evoked spinal cord activity were investigated in unanesthetized stingrays (Dasyatis sabina) with high cervical spinal transections. Primary afferent excitability increases following a conditioning stimulus to an adjacent segmental nerve were demonstrated with the Wall (31) technique. Stimulation of A.α,β and A-δ afferent fibers produced excitability increased in both A-α,β and δ-fibers of the adjacent segment. The excitability increase had a latency of about 10 ms, it peaked around 25 ms, and the change lasted more than 100 ms. The central afferent volley in A-α,β fibers and the N1- and late negative waves due to postsynaptic activity of dorsal horn interneurons were reduced by conditioning volleys in adjacent afferent nerves. The time course of the inhibition paralleled that of the excitability increases in afferent terminal arborizations, suggesting that the depression of postsynaptic activity is, at least in part, due to presynaptic inhibition.Reduction of evoked discharges and excitatory postsynaptic potentials was observed in recording from interneurons with a time course similar to that of the primary afferent depolarization (PAD). Conditioning volleys in afferents of adjacent peripheral nerves produced facilitation or inhibition of segmental reflexes.