Reciprocal inhibition in the motor nervous system of the nematode Ascaris: direct control of ventral inhibitory motoneurons by dorsal excitatory motoneurons.

In previous physiological experiments (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), we have shown that the dorsal cord of the nematode Ascaris lumbricoides includes the processes of three types of dorsal excitatory (DE) motoneurons and one type of ventral inhibitory (VI) motoneuron. Ultrastructural studies have revealed that the axons of the DE motoneurons make monosynaptic contacts with the dorsal processes of VI motoneurons. In this paper, we describe a physiological preparation with which to investigate the properties of these synapses. We show that activation of a DE neuron can excite a VI neuron producing inhibition in ventral muscle cells shortly after dorsal muscle cells are excited, thus mediating reciprocity between dorsal and ventral muscles. Each VI dendrite receives input from four or five DE neurons; activation of any one of these DE neurons is sufficient to activate the VI neuron.