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.

We have developed a method for dissecting single neurons from the nematode Ascaris suum, in order to determine their peptide content by mass spectrometry (MS). In this paper, we use MALDI-TOF MS and tandem MS to enumerate and sequence the peptides present in the two neurons, ALA and RID, that comprise the dorsal ganglion. We compare the peptide content determined by MS with the results of immunocytochemistry and in situ hybridization of previously isolated peptides AF2, AF8 and 6 peptides encoded by the afp-1 transcript. We find complete agreement between the three techniques, which validates single neuron MS as a method for peptide localization. We also discovered and sequenced 6 novel peptides in the ALA neuron. Cloning of cDNAs and database searching of Genomic Survey Sequences showed that transcript afp-12 encodes peptide AF36 (VPSAADMMIRFamide), and afp-13 encodes AF19 (AEGLSSPLIRFamide), AF34 (DSKLMDPLIRFamide), AF35 (DPQQRIVTDETVLRFamide), and 3 non-amidated peptides (PepTT, PepTL, and PepGE). We have found no similarities with reported peptide expression in the nematode Caenorhabditis elegans. This method promises to be ideally suited for determining the peptide content of each of the 298 neurons in the nervous system of this nematode.

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In a study of the specificity of neuronal connections in lobster abdominal ganglia, the dye Procion Yellow M4RS was electrophoretically injected into identified cell bodies. This dye spreads into fine branches of cells, survives fixation and routine histological procedures, and permits the reconstruction of cell shapes through examination of serial sections of ganglia. Certain cells were found to have an internal bilateral symmetry. Repeated injection of the same cells in ganglia from different animals showed that cells have characteristic shapes and that the neuropil is highly structured. This method of dye injection should have general applicability in studies where a knowledge of the geometry of specific cells is important.