(The following description is based on Avery (1993) and references therein.) A pump begins with a near-simultaneous contraction of the muscles of the corpus, anterior isthmus, and terminal bulb. The motion that gets food into the worm is called pumping ( Figure 2). Aside from the connections to the mouth and the intestine there are only two holes in the basal lamina, one on either side, through which the processes of a pair of extrapharyngeal neurons, the RIPs, pass to connect to the pharyngeal nervous system.

Pharyngeal neurons lie under the basal lamina that surrounds the entire pharynx and isolates it from the rest of the worm, in indentations of the muscle cell membrane. For more information about peptides and feeding see Franks et al. Neuropeptides are certain to play important roles, but they are still for the most part poorly understood. What we know of their functions is described below. The most important of the small transmitters are acetylcholine, glutamate, and serotonin. Pharyngeal neurons contain both neuropeptides and small-molecule neurotransmitters. Surprisingly, these three neuron types are sufficient for nearly normal behavior in the laboratory: even when all 11 other neuron types are killed together, the worm grows well and shows only subtle feeding abnormalities. MC and M3 control the timing of pumping: MC controls when a contraction starts ( Raizen and Avery, 1994 Raizen et al., 1995), and M3 when it ends ( Avery, 1993 Raizen and Avery, 1994). M4 is necessary and sufficient for normal isthmus peristalsis, and is essential for growth under some conditions ( Avery and Horvitz, 1987). Three of these 14 are particularly important for normal feeding.

The pharyngeal nervous system contains 20 neurons of 14 different types (8 bilaterally symmetric pairs and 6 unpaired) ( Albertson and Thomson, 1976). Side view of the pharynx, showing the main functional regions. Simplified generic cross-section through the pharynx, showing the geometric relationships between muscle cells, marginal cells, and the pharyngeal lumen. For instance, given all the high-quality food they can eat, worms eventually become satiated, stop eating and moving, and become quiescent.ДЄnatomy of the pharynx.

Food availability and quality also affect behavior in other ways. Given a choice, worms are capable of identifying and seeking out higher-quality food. Some types of bacteria are better at supporting growth than others. Isthmus peristalsis, a posterior moving wave of contraction of the muscle of the posterior isthmus, depends on a third motor neuron type.ДЇeeding motions are regulated by the presence and quality of food in the worm's environment. Pharyngeal muscle is capable of pumping without nervous system input, but during normal rapid feeding its timing is controlled by two pharyngeal motor neuron types. Pumping, the most visible and best understood of the two, is a cycle of contraction and relaxation that sucks in liquid from the surrounding environment along with suspended particles, then expels the liquid, trapping the particles. It accomplishes this through a combination of two motions, pumping and isthmus peristalsis. The pharyngeal muscle captures food-bacteria-and transports it back to the intestine. elegans feeding depends on the action of the pharynx, a neuromuscular pump that joins the mouth to the intestine.

Exessive speediness