NEUROMUSCULAR ORGANIZATION IN THE SWIMMING SYSTEM OF THE PTEROPOD MOLLUSK CLIONE-LIMACINA

Swim motor neurons of the pteropod mollusc Clione limacina were identi fied by a combination of electrophysiological and morphological charac teristics. Two types of motor neurons were found, including small moto r neurons which are active during both slow and fast swimming and whic h innervated restricted fields of the ipsilateral wing. General excito r motor neurons have large cell bodies, innervate widespread fields an d are recruited into activity for fast swimming. Small motor neurons m onosynaptically innervate slow-twitch muscle cells, whereas general ex citors monosynaptically innervate both slow-twitch and fast-twitch mus cle cells. Activity in general excitors can centrally enhance that in small motor neurons because the neurons are electrically coupled. Neur omuscular recordings and lesion experiments indicate that a peripheral nerve network does not appear to play an important role in the spread of excitation throughout the muscle fields.