The Nerve Ring of the Nematode Caenorhabditis elegans: Sensory Input and Motor Output

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Table of contents   -   Abstract   -    Mat. & Methods   -    Observations   -    Discussion & Acknowledgments

Observations

General description - Detailed structure of the organs in C. elegans - Cephalic musculature - Neuropil

General description

The basic body plan of C. elegans conforms to the general description of nematode, given in a numher of accounts (Lee, '65; Crofton, '66; Bird, '71), and is that of a spindle-shaped tube with a thin superficial layer of longitudinally oriented muscles surrounding gut, reproductive, and passive supportive systems which fill nearly all of the interior. The central nervous system, located within the relatively short cephalic region, integrates input from the anterior senory organs and provides outpul to the rest of the body by means of peripheral nerve cords which extend the length of the animal and are connected with each other by circumferential connectives. The most important of the cords as judged by the number of nerve fibers contained is the ventral cord.

Figure 1

Fig. 1 - Photograph and schematic drawing of the image combining apparatus used to make 35 mm electron miscroscope cine films from 70 mm electron micrographs.

A, Amphid
AC, Ampidial connective
A.C, Amphidial cap process
AN, Amphidial neuron, nerve
AP, Amphidial pouch
A.P, Amphidial pocket process
Bu, Buccal cavity
C, Club-shaped dendritic ending, cell of the ventro lateral papillary organ
Cu, Cuticle
DC, Dorsal nerve cord
De, Deirid
Des, Desmosome
DN, Subdorsal papillary nerve
EC, Excretory canal
ES, Extracellular space
F, Finger processes of the amphid
Gl, Glial cytoplasm
H, Hypodermal cell
IL, Internal labial papillary sense organ
IL.C, Internal labial cap process
IL.N, Internal labial non-rootleted dendrite
IL.P, Internal labial pocket process
IL.R, Internal labial rootleted dendrite
LG, Lateral ganglion
L.N, Lateral papillary nerve
LSM, Lateral submedial sense organ
LSM.C, Lateral submedial cap process
LSM.P, Lateral submedial pocket process
M, Muscle fibers, processes
MP, Muscle plate
MSM, Medial submedial sense organ
MSM.C, Medial submedial cap process
MSM.P, Medial submedial pocket process
N, Neuron of the nerve ring
NR, Nerve ring
OE, Oesophagus
TB, Tubular body termination of the LSM organ
V, Vesicles
VG, Ventral ganglion
VL, Ventrolateral sense organ
VL.C, Ventrolateral cap process
VL.P, Ventrolateral pocket process
VN, Subventral papillary nerve
1, 1-cell process, cell body
2, 2-cell process, cell body
3, 3-cell process, cell body
60, 60-cell process, cell body
Figure 1

Fig. 2 - Cephalic region of C. elegans showing those structures of the nervous system discussed in the text. See preceding Abbreviations.

An overall survey of the nervous system of nematodes has been given most recently by Bird ('71). That portion discussed here is diagrammed schematically for C. elegans in figure 2. Basically it consists of the central brain mass of neuropil, called the nerve ring because it completely encircles the axially placed oesophagus, and associated cells and ganglia. The main input to the nerve ring, consistently observed in all species of nematodes and first described by Bütschli (1874), comes from six hexagonally placed aggregations of sensory organs on the snout by means of six longitudinal papillary nerves. In C elegans, the papillary nerves do not enter the ring anteriorly, as described in the light microscopic studies of other nematodes, but instead bypass it, then make a full U-turn and enter into the nerve ring neuropil from the rear (fig 28). Other sources of sensory input are (i) the cervical papillae or deirids, two laterally placed presumed pressure-sensitive organs located within the cuticle just posterior to the nerve ring; (ii) phasmids, presumed chemoreceptor organs of the tail which send processes to the nerve ring by means of the ventral cord; (iii) anal papillae. In addition, there are numerous extra receptors in the anal region of males which are presumably used during copulation.

Figure 1

Fig. 3 - A. Schematic perpendicular sectino through C. elegans at a region anterior to the beginning of the oesophageal musculature showing the distribution of the cephalic sensory organs. B. Scanning electron miscroscope micrograph of the anterior end of C. elegans (courtesy of S. Smith).

The anterior sense organs are located near the tip of the animal (fig 3) and are classically divided into two types. Various papillary (or setaceous) organs, assumed to be mechanoreceptors, are located on or near each of the six lips and are innervated by one or two sensory neurons. The amphids, assumed to be chemoreceptors because of their marked openings to the exterior of the animal, are located one on each of the lateral lips. They are larger than the papillary organs and are innervated by many more neurons. All organs are connected to the CNS by the six papillary nerves (fig 2), one to each lip, which contain the anterior processes of their centrally located bipolar neurons.


Web adaptation, Thomas Boulin, for Wormatlas, 2002