Proprioceptive inputs to large buccal motoneurons controlling pharyngeal expansion in Navanax

Large electrotonically coupled neurons (G and M cells) on the anterior surface of the buccal ganglion of the opistho branch mollusc, Navanax, are motoneurons causing pharyngeal expansion. Stretch of small muscles causing pharyngeal protraction or electrical stimulation of their nerves elicits unitary EPSP's in G and M cells which can summate to reach threshold. Pharyngeal inflation can decrease coupling between the motoneurons by activation of inhibitory synapses. Small neurons on the dorsal surface of the ganglion form inhibitory synapses on G and M cells, and send axons out the pharyngeal nerve as shown by electrical stimulation of the nerve. Pharyngeal stretch evokes abruptly rising action potentials in these neurons indicating that they are sensory. Feeding in Navanax involves pharyngeal protraction, followed by rapid pharyngeal expansion and then peristalsis with asynchronous expansions and contractions. The excitatory input from protractor muscles may be involved in the transition from the first to second stage. The small inhibitory neurons probably participate in the synaptic uncoupling to permit asynchrony in the contractions of peristalsis.