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

Abstract

The nervous system of the nematode worm Ascai contains about 250 nerve cells; of these, the motoneurons consist of five segmental sets, each containing 11 cells. Morphologically, the motoneurons can be divided into seven different types. Their geometry is simple: some are unbranched, others have one branch point, and the most complex have two. There is no neuropil in the nerve cords; synapses are made by axo-axonal contact or onto short spines. These features enable us to study the anatomy and physiology of the system with a degree of completeness that would be difficult in other systems. The physiological activity of five of the motoneurons has been investigated, three being excitatory and two inhibitory. The excitatory motoneurons receive input from intersegmental interneurons. The inhibitory motoneurons do not receive input from the interneurons; instead they receive their input from the excitatory motoneurons in a circuit that can mediate reciprocal inhibition between the dorsal and the ventral musculature.One of the outstanding problems in neurobiology is to understand the capabilities of assemblies of neurons in terms of the properties of the cells of which they are composed. In vertebrate nervous systems this problem has been approached in situations in which small numbers of cell types are arranged in a repeating pattern (e.g., cerebellum, retina); by studying the smallest fundamental repeating unit in these systems, the logical principles by which information is transformed might be inferred.An alternative approach is to select an invertebrate system in which there is only a small number of neurons. A number of interesting results have already emerged from the study of small assemblies of neurons in invertebrates (1-7). However, these assemblies still contain a large number of interacting components, and the geometry of the neurons in many cases is as complex as that found in vertebrates.In this paper we will describe the motor nervous system of the large parasitic nematode Ascaris lumicdes. This system has a small number of neurons with extremely simple geometry. This allows us to study the physiology and anatomy of the system with a degree of completeness that would be difficult in other systems.The salient advantages of the Ascaris nervous system are threefold.(i) Cell number. The entire nervous system contains only about 250 neurons; the motor nervous system that we will describe here is divided into five segments, each containing 11 motoneurons, and there are six nonsegmental interneurons traversing the segments. By contrast, in truly segmented animals such as the crayfish and the leech, each segmental ganglion contains several hundred cells and there are hundreds or thousands of neurons impinging upon each ganglion from other centers.

Evolution of pharyngeal behaviors and neuronal functions in free-living soil nematodes

“…applicability of C. elegans knowledge towards understanding other nematodes, and vice versa. For example, studies in the parasite Ascaris focused on anatomical and functional similarities to C. elegans in the ventral cord locomotory neurons (Johnson and Stretton, 1980;Stretton et al, 1978;Walrond et al, 1985), and studies in the parasite Strongyloides focused on similarities to C. elegans in the control of chemotaxis by amphid sensory neurons (Ashton et al, 1999;Forbes et al, 2004). We were interested instead in investigating nematode behaviors and their neuronal regulation explicitly for their evolutionary differences.…”

Actions of cholinergic drugs in the nematode Ascaris suum. Complex pharmacology of muscle and motorneurons.

“…Each class of motorneuron has several members arranged at reproducible positions along the worm, which have been arbitrarily assigned to different segments; each segment includes one each of the DE2, DE3, and DI motorneurons and two pairs of DE1 and VI motorneurons (Stretton et al, 1978). The majority of measurements were taken in the second DEI and VI pair of the second segment; likewise, experiments on DE2 and DI were done primarily in the second segment.…”

“…A cylindrical section of worm 2.5-3 cm in length, containing the commissures of the DE2-DI pair of motorneurons in the second segment and the first DE1-VI motorneuron pair of the third segment (Stretton et al, 1978), was removed from the intact animal under a dissecting microscope. The section was cut along the left lateral line, the intestine removed with fine forceps, and the body wall pinned flat, cuticle side down, in the Syigard chamber.…”

“…A cylindrical section of worm, 2.0-2.5 cm in length, containing the commissures of a selected motorneuron pair (Stretton et al, 1978; was prepared for microelectrode penetration of a motorneuron commissure. Because of the difficulty of recording from the DE3 motorneuron class, chiefly because of its very fine commissural process, this cell was not investigated.…”

“…In the experiments described in this article, we have focused primarily on dorsal motorneurons and on dorsal muscle, since it is innervated almost exclusively by commissural motorneurons (Stretton et al, 1978) that are accessible for electrophysiological experiments (Davis and Stretton, 1989a). We report the changes in input resistance induced by a series of cholinergic agents in synaptically isolated DE1, DE2, and DI motorneurons.…”