Motor neuron M3 controls pharyngeal muscle relaxation timing in Caenorhabditis elegans

1. Previous work has shown that 12 of the 14 types of neurons in the Caenorhabditis elegans pharyngeal nervous system are collectively but not individually necessary for the trapping and transport of bacteria. The aim of these experiments was to determine the functions of individual neuron types by laser-killing combinations of neurons and looking at the effects on behavior. 2. The motor neuron M3 and the sensory neuron I5 are important in trapping bacteria, as shown by two observations. First, when M3 and I5 are both killed, trapping is inefficient in the isthmus (the middle section of the pharynx). Second, M3 is sufficient in the absence of the other 11 neuron types for normal trapping in the corpus (anterior pharynx). 3. M3 and I5 influence the timing of pharyngeal muscle motions. When M3 is killed, pump duration (the interval from the beginning of pharyngeal contraction to the end of relaxation) increases from 170 to 196 ms. This increase is at least partially due to a slower relaxation. Thus, M3 speeds up relaxation. Pump duration decreases to 159 ms when I5 is killed. When I5 and M3 are both killed, pump durations are long (192 ms), just as when M3 alone is killed. These observations, together with previous electron microscopic work showing synapses from I5 to M3, suggest that I5 slows down relaxation by inhibiting M3. 4. To explain these results, I propose that M3 and I5 promote bacterial trapping by regulating the relative timing of muscle relaxation in different regions of the pharynx.