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The nervous system of Caenorhabditis elegans is arranged as a series of fibre bundles which run along internal hypodermal ridges. Most of the sensory integration takes place in a ring of nerve fibres which is wrapped round the pharynx in the head. The body muscles in the head are innervated by motor neurones in this nerve ring while those in the lower part of the body are innervated by a set of motor neurones in a longitudinal fibre bundle which joins the nerve ring, the ventral cord. These motor neurones can be put into five classes on the basis of their morphology and synaptic input. At any one point along the cord only one member from each class has neuromuscular junctions. Members of a given class are arranged in a regular linear sequence in the cord and have non-overlapping fields of motor synaptic activity, the transition between fields of adjacent neurones being sharp and well defined. Members of a given class form gap junctions with neighbouring members of the same class but never to motor neurones of another class. Three of the motor neurone classes receive their synaptic input from a set of interneurones coming from the nerve ring. These interneurones can in turn be grouped into four classes and each of the three motor neurone classes receives its synaptic input from a unique combination of interneurone classes. The possible developmental and functional significance of these observations is discussed.
One approach to the study of the function, development and genetic specification of nervous systems is to use an organism that is both amenable to genetic analysis and has a simple nervous system that may be completely characterized anatomically. The analysis of any structural alterations in behavioural mutants might provide some insight into both the functioning of the nervous system and the execution of the developmental program constructing it. The nematode Caenorhabditis elegans has been chosen as a suitable organism for this type of study and many behavioural mutants have been isolated and mapped (Brenner 1974). This paper is the second of a series on the structure of the nervous system of the wild type animals (Ward, Thornson, White & Brenner 1975). It describes the organization of the neuromuscular system mediating locomotion as determined by reconstructions from serial section electron micrographs.
Web adaptation, Chris Crocker, for Wormatlas, 2008