Fig. 1. Pharyngeal anatomy, behaviors, and neuronal stimulation in C. elegans. (A) The three muscle groups in the C. elegans pharynx. The corpus is large and anterior, the isthmus is narrow and in the middle, and the terminal bulb most posterior. There is a grinder in the terminal bulb for chewing bacteria. The lumen at rest is closed as shown. (B) Pumps are simultaneous contractions of entire muscle groups that open the pharyngeal lumen. Pumping occurs in the corpus, anterior isthmus, and terminal bulb in C. elegans (indicated by the open lumen). Peristalses are posteriorly moving contraction waves, and occur in the posterior isthmus in C. elegans (indicated by the arrows). (C) Stimulation of pumping and peristalsis by the two major excitatory pharyngeal neuron types in C. elegans, M4 and MC. M4 is required for posterior isthmus peristalsis. MC innervates the corpus and directly stimulates corpus pumping, which indirectly and simultaneously stimulates the anterior isthmus and terminal bulb to pump, since the anterior isthmus and terminal bulb are electrically coupled to the corpus via gap junctions.