Fig. 1. Motor neurons MC and M3 control the timing of pharyngeal muscle action potentials. MC is an excitatory cholinergic neuron (Raizen et al., 1995; T. Niacaris and L. Avery, unpublished observations). Its firing triggers a pharyngeal muscle action potential via the release of acetylcholine, which acts on a muscle nicotinic receptor (Raizen et al., 1995; J. McKay, personal communication). MC thus controls the frequency of pharyngeal pumping. M3 is an inhibitory glutamatergic neuron. It fires during the action potential, releasing glutamate, which causes inhibitory postsynaptic potentials that can end the muscle action potential (Avery, 1993b; Dent et al., 1997; Li et al., 1997). M3 thus acts to decrease action potential duration. Serotonin acts to increase the activity or effect of both of these neurons on pharyngeal muscle, resulting in decreased action potential duration. Note that MC and M3 are actually located within the pharynx although they are shown here as external for clarity.