ABSTRACT
Single units were recorded from the spinal cord of decerebrate dogfish (Scyliorhinus canicula) during pectoral fin reflexes (PFR) evoked by electrical pulse trains to the fin. The units were classified as primary afferent neurones, motoneurones or interneurones. Motoneurones discharged for limited (and various) periods during the reflex at latencies of 20 ms or more. There was no evidence for monosynaptic activation by primary afferents. Short-latency (S) units received monosynaptic input from fast-conducting afferents at latencies (<20 ms) appropriate for pre-motor interneurones. However, excitation of individual S-units by intracellular current injection never evoked motoneurone discharges, suggesting that convergence is necessary for motoneurone activation. Intracellular recordings from S-units which discharged for periods longer than the duration of the afferent volley generated by the fin stimulus showed that they receive other inputs in addition to those from primary afferent fibres. Intermediate-latency (I) units had similar properties to S-units except for a longer latency (>30ms), which ruled out monosynaptic excitation by fast-conducting afferents. Antidromic activation of S-and I-units by high spinal stimulation was rarely seen and orthodromic driving was also uncommon. A significant number of interneurones with latencies greater than 60 ms (L-units) were antidromically activated by high spinal stimulation. Their discharges were often long-lasting (>1 s) and we suggest that they may provide input to the cerebellum during the PFR.
