Worm Breeder's Gazette 11(1): 62
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
In my initial survey of the pharyngeal neurons, I classified them into three groups: M4, MC, and the 12 remaining anatomical types of pharyngeal neurons, which I call the GREENs. None of the individual GREEN neuron types is necessary for superficially normal pumping, but when all 12 are killed, pumping becomes very abnormal. I have recently made some progress in figuring out what they are doing. During normal pumping, bacteria move posterior in the pharynx when the muscles contract, and are held where they are when the muscles relax again, so that they are efficiently transported towards the intestine. When all the GREENs were killed, the pharynx appeared to become slippery: bacteria slid anterior as well as posterior. When bacteria were abundant, the lumen of a GREEN pharynx became distended with them, and they were ground up and passed to the intestine only slowly. Seymour et al (J Zool 201: 527) showed by microcinematography that in normal pumping the contraction and relaxation of the corpus muscles are not precisely simultaneous along its length: both begin at the front. They proposed this has a function in trapping bacteria in the pharynx. I suspect that in the absence of the GREENs this fine timing goes awry, and as a result bacteria are not held back by the relaxing pharyngeal muscles. The GREEN neuron I understand best at present is M3. When M3 was killed, there was a subtle abnormality in pumping: the contractions lasted longer. Conversely, when all the GREENs except M3 were killed, contractions were very brief. Thus M3 hastens pharyngeal muscle relaxation during a pump, and is opposed by some other(s) of the GREENs. I5 is probably (one of) the GREEN(s) that opposes M3, since pumps became briefer when I5 alone was killed. When both M3 and I5 were killed, pumps were long, just as when M3 alone was killed, suggesting I5 may exert its effects on pump length via M3. In fact, I5 does synapse on M3. There was also a new effect when both M3 and I5 were killed: the isthmus became slippery. The timing of anterior isthmus contraction was abnormal in M3-I5 worms: it contracted just after the corpus contraction, instead of simultaneously with it, and remained contracted longer. M3 is also important for corpus function. In fact, when all the GREENs except M3 were killed, the corpus was not slippery. Other GREENs important for normal corpus function are I2, I6, M1, and possibly I4. M5 may also be important for isthmus function. However, I have not worked out the details for any of these. I have also learned something about GREEN function in a completely different way. I discovered previously that imipramine stimulates pumping in wild-type worms, but not in mutants that have reduced levels of serotonin (WBG 10(2): 39). The only serotonergic neuron in the pharynx is NSM, and the only pharyngeal neurons connected to the extrapharyngeal nervous system are I1 and M1. I therefore killed NSM, I1, and M1, thinking to remove any serotonin from the pharynx, and to sever its communication with extrapharyngeal serotonergic neurons that might be stimulating pumping. Sure enough, imipramine did not stimulate pumping in I1- M1- NSM- worms. I have not yet tested whether it is necessary to kill all three to get the effect.