Myotypic Respecification of Regenerated Nerve- fibres in Cichlid Fishes

IN man and other mammals normal motor co-ordination is not restored, as a rule, after regeneration of a severed peripheral nerve-trunk (Sperry, 1945). The random misdirection of regenerating fibres into foreign muscles tends to prevent normal dissociated action within the re-innervated musculature. In contrast, larval amphibians have been found to show excellent recovery of motor function in the form of 'homologous or myotypic response' (Weiss, 1936, 1941) following the cutting and regeneration of limb-nerves, limb transplantation, and the crossconnecting of limb nerves to foreign muscles. Similarly, good restoration of muscle co-ordination has been observed in the pectoral fin of adult teleost fishes (Sperry, 1950, 1956). It has been suggested (Sperry, 1941, 1951) that such recovery is most easily explained in terms of a central readjustment of synaptic connexions to suit the altered pattern of peripheral innervation. Morphological or other direct evidence for such synaptic changes, however, has not been found. For further investigation of the problem fishes appear to offer promising possibilities in that the technical difficulties of staining the synaptic terminals in fishes appear to be somewhat less formidable than in the amphibians. The following study was undertaken to ascertain whether the type of motor recovery observed in the pectoral fin centres of adult teleosts might also occur in a cranial nucleus where the conditions for histological demonstration of synaptic end-feet are much more favourable. The experiment involves regeneration following surgical section and cross-union of branches of the mandibular ramus of cranial nerve V in the cichlid fish, Astronotus ocellatus (Aggassiz), with a study of the extent of subsequent functional recovery. The results demonstrate motor readjustment in the trigeminal system similar to that reported previously for the innervation of limb and fin musculature in amphibians and fishes respectively.