Identification of excitatory and inhibitory motoneurons in the nematode Ascaris by electrophysiological techniques

A physiological preparation in which it is possible to record responses in muscle to stimulation of single motoneurons of the nematode Ascaris lumbricoides is described. With this preparation we have determined the physiological sign (E or I; excitatory or inhibitory) of the neuromuscular synapses of 21 identified motoneurons--12 are excitatory and 9 inhibitory. Ascaris motoneurons had previously been classified by morphological criteria into seven classes (Stretton, A. O. W., R. M. Fishpool, E. Southgate, J. E. Donmoyer, J. P. Walrond, J. E. R. Moses, and I. S. Kass (1978) Proc. Natl. Acad. Sci. U. S. A. 75: 3493-3497). Physiological studies were performed on members of five of these classes. Three classes of neurons (DE1, DE2, and DE3) are excitatory to dorsal muscle cells. Two classes (DI and VI) are inhibitory neurons which innervate the dorsal and ventral muscle cells, respectively. The motoneurons in Caenorhabditis elegans (White, J. E., E. Southgate, J. N. Thomson, and S. Brenner (1976) Philos. Trans. R. Soc. Lond. (Biol.) 275: 327-348) can be divided into seven morphological classes which are very similar to those in Ascaris. Based upon the structure-function correlation in Ascaris, we have predicted which motoneurons are excitatory and which are inhibitory in C. elegans.