Neuromuscular Organization in the Swimming System of the Pteropod Mollusc Clione Limacina

ABSTRACT Swim motor neurons of the pteropod mollusc Clione limacina were identified by a combination of electrophysiological and morphological characteristics. Two types of motor neurons were found, including small motor neurons which are active during both slow and fast swimming and which innervated restricted fields of the ipsilateral wing. General excitor motor neurons have large cell bodies, innervate widespread fields and are recruited into activity for fast swimming. Small motor neurons monosynaptically innervate slow-twitch muscle cells, whereas general excitors monosynaptically innervate both slow-twitch and fast-twitch muscle cells. Activity in general excitors can centrally enhance that in small motor neurons because the neurons are electrically coupled. Neuromuscular 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.