Structure and physiological activity of the motoneurons of the nematode Ascaris.

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Abstract
The nervous system of the nematode worm A. lumbricoides contained about 250 nerve cells; of these the motoneurons consist of 5 segmental sets, each containing 11 cells. Morphologically, the motoneurons was divided into 7 different types. Their geometry was simple: some were unbranched, others have 1 branch point, and the most complex had 2. There was no neuropil in the nerve cords; synapses were made by axo-axonal contact or onto short spines. These features enabled study of the anatomy and physiology of the system with a degree of completeness that would be difficult in other systems. The physiological activity of 5 of the motoneurons was investigated; 3 excitatory and 2 inhibitory. The excitatory motoneurons received input from intersegmental interneurons. The inhibitory motoneurons did not receive input from the interneurons; instead they received input from the excitatory motoneurons in a circuit that could mediate reciprocal inhibition between the dorsal and the ventral musculature.