Neural Control of the Buccal Muscle Movement in the African Giant Snail Achatina Fulica

Five pairs of neurones, R(L)-B1, R(L)-B2, R(L)-B3, R(L)-B4 and R(L)-B5, controlling buccal muscle movement, were identified in the buccal ganglia of the African giant snail Achatina fulica Ferussac. All these neurones fired during the radula retraction phase of rhythmic buccal activity. Neurones Bl, B2, B4 and B5 made direct excitatory connections to the radula retractor, and Bl, B2, B3 and B5 also made direct excitatory connections with the outer muscle of the buccal mass. Of these neurones, B4 had the most potent effect on contraction of the ipsilateral radula retractor. Physiological and pharmacological analyses suggested that the principal excitatory transmitter of B4 at the neuromuscular junctions was acetylcholine (ACh), although glutamate and aspartate also elicited the contraction. A pair of cerebral ganglion cells, v-RCDN and v-LCDN, was found to have modulatory effects on the muscle contraction evoked by B4 firing and ACh application. Morphological, physiological and immunohistochemical analyses suggested that the modulatory actions of v-CDN on muscle contraction are mediated by serotonin, which may be released from nerve terminals of v-CDN and act directly on the muscle. v-CDN also increased the activity of motoneurone B4. v-CDN shared several common features with serotonergic cerebral cells in other gastropod molluscs.