There are two main types of muscle in C. elegans: multiple sarcomere/obliquely striated (somatic) and nonstriated (also called single sarcomere). The multiple sarcomere muscles contain evenly distributed attachment points to the hypodermis and cuticle along their length (see Somatic muscle), whereas the majority of the nonstriated muscles have focal attachment points at their ends. The multiple sarcomere group is the most abundant muscle group, consisting of 95 body wall muscles; 14 of these are post-embryonically generated (see Somatic muscle). The nonstriated muscle group in hermaphrodites includes 20 pharyngeal muscles, 2 stomato-intestinal muscles, 1 anal sphincter muscle, 1 anal depressor muscle, 8 vulval muscles (all post-embryonically generated), 8 uterine muscles (all post-embryonically generated) and contractile gonadal sheath (see Nonstriated Muscle). In the male, instead of the vulval and uterine muscles and gonadal sheath, 41 specialized mating muscles are present, some having single sarcomeres and some being obliquely striated. Except for the pm1-pm5 cells of the pharynx, all muscle cells are mononucleate. After hatching, pm1 becomes a syncytial cell with 6 nuclei, and pm2-pm5 become binucleate syncytial cells (see Alimentary System - Pharynx).

Although most muscle contractions are generated by nerve transmission, three rhythmic behavior cycles in C. elegans are dependent on periodical contraction of certain muscle groups with recurrent intracellular Ca++ transients rather than excitation by neuronal transmission. These are: pharyngeal pumping behavior of the pharyngeal muscle (see Alimentary system - Pharynx), gonadal sheath contractions, and the defecation cycle involving three muscle groups: body wall (somatic) muscles near the head, posterior (somatic) body wall muscles and enteric muscles (i.e., anal depressor, sphincter, and stomato-intestinal muscles) (see Alimentary system - Rectum and Anus). Although they are generated by intrinsic motor activity, pharyngeal pumping and enteric muscle contractions are modulated by neurons.

The basic unit of the contractile apparatus in muscle is the sarcomere (MusFIG1A). In striated muscle these contractile units are repeated, giving the muscle its "striated" appearance. In vertebrates, a sarcomere is comprised of Z (Zwischenscheibe) discs located at each end of sarcomere; I (isotropic) bands, which correspond to thin filaments; A (anisotropic) bands, which correspond to thick filaments (including the thin filament overlap region); H (Heller) bands, which correspond to the central region of the A bands and M lines at the middle of the H bands where each myosin rod is joined end-to-end with its myosin rod neighbor (MusFIG 1). In the sarcomere, myosin-containing thick filaments are interdigitated with actin-containing thin filaments on either side. In C. elegans, the Z-disc analog is the dense body (DB), which functions to anchor and align thin filaments in striated muscles (MusFIG 1B; see Somatic Muscle). Thick filaments are attached to M-line analogs. Both the DB and the M-line analogs extend the entire depth of the lattice and anchor all filaments to the cell membrane and the underlying hypodermis and cuticle (see Somatic Muscle). In nonstriated muscles with single sarcomeres, large hemiadherens junctions (formerly called hemidesmosomes) connect each sarcomere at the muscle ends to body cuticle or specialized cuticle and/or to basal lamina to anchor the myofilaments (see Nonstriated Muscle). Some of the nonstriated muscles have myofilaments that are less well organized. Here, anchorages occur via small plaques and hemiadherens junctions distributed along the cell membrane similar to the organization of vertebrate smooth muscle (MusFIG 1A&B and MusFIG 1C&D, see Nonstriated Muscle).

The organization of the muscle filament lattice in C. elegans can be viewed by polarized light microscopy, to both assess the orientation of the filaments in wild type body muscles as they develop and to score for defects in mutant strains.

Excitation-contraction coupling (ECC) is the process by which an action potential triggers a muscle cell to contract. In vertebrates, myocytes respond to the excitation signal induced by their innervating motor neurons with a rapid depolarization, which is coupled with contraction of the muscle as its physiological response. The initial depolarization in the muscle caused by nerve transmission is a localized phenomenon and the depolarization signal is carried to the myofibrils deep within the cell body via sarcolemmal (cell membrane) invaginations called transverse (T) - tubules. T-tubules form a network of membranes that penetrate and span the cross section of each muscle cell, transmitting the depolarization signal uniformly throughout the muscle fiber (MusFIG 2). The lumena of the T-tubules are continuous with the extracellular fluid, and the membrane depolarization during an action potential diffuses across the T-tubule membrane. The T-tubules are close to the border between the A- and I-bands of the myofibrils and are in close apposition with cisternae formed by the Sarcoplasmic Reticulum (SR). This association is called a triad. T-tubules are essential structures for excitation-contraction coupling linking the depolarization of the action potential to Ca⁺⁺ release from SR where intracellular Ca⁺⁺ is sequestered (MusFIG 2B).

Depolarization in the T-tubule membrane leads to release of stored Ca⁺⁺ through the interaction of two proteins. A voltage sensor (dihydropyridine receptor [DHPR] which is a voltage-gated Ca⁺⁺ channel) in the T-tubule membrane changes conformation in response to the action potential (MusFIG 2A). This conformational change is transmitted to another Ca⁺⁺ channel (Ryanodine receptor [RyR]) on SR, causing it to open and allowing Ca⁺⁺ release from SR stores (Ahern et al., 2001). RyRs cluster in the junctions between SR and T-tubules. The direct mechanical interaction between DHPR and RyR is specific for excitation-contraction coupling in vertebrate skeletal muscle. Increased intracellular free calcium then binds to troponin-C (TN-C), part of the regulatory complex attached to the thin (actin) filaments of the sarcomere (Alberts et al., 2002). When Ca⁺⁺ binds to the TN-C, a conformational change in the regulatory complex relieves the tropomyosin blockage of the interaction between actin and the myosin head. A myosin ATPase located on the myosin head supplies energy for the movement between the myosin heads and actin. The actin and myosin filaments slide past each other (ratcheting) and shorten the sarcomere length (Alberts et al., 2002). One ratcheting cycle will last as long as the cytosolic Ca⁺⁺ remains elevated. At the end of contraction, Ca⁺⁺ is restored to sarcoplasmic reticulum by an ATP-dependent calcium pump.

In C. elegans, sarcoplasmic reticulum (SR) consists of a network of vesicular membranous organelles surrounding the myofilament lattice. The flattened vesicles of SR extend around dense bodies (DBs) and lay adjacent to the apical (hypodermal side) plasma membrane underneath the lattice, where they are localized randomly between DBs (Waterston, 1988) (MusFIG 1) (see Somatic Muscle). A gap of 12-14 nm separates the SR vesicles from the plasma membrane. No equivalent to the T-tubule system exists in C. elegans, possibly because the direct apposition of SR to the plasma membrane abrogates its utility (MusFIG 1A&B) (Waterston, 1988). In C. elegans, the ryanodine receptor (RYR) is encoded by the unc-68 gene (Maryon et al., 1996; Hamada et al., 2002). Its expression is seen in various muscles including body wall muscles, terminal bulb muscle of the pharynx, vulval and uterine muscles, diagonal muscles of male tail, and the anal sphincter and depressor muscles (Maryon et al., 1998). In somatic muscle, initiation ofunc-68 expression coincides with the first twitching movements of the embryo. Within the body wall muscle, UNC-68 is thought to be localized to SR vesicles, primarily between the rows of dense bodies in the A-band region (Maryon et al., 1998). In contrast to vertebrate muscle, UNC-68 functions to enhance motility but it is not essential for excitation-contraction (E-C) coupling in C. elegans because unc-68 null mutants are still able to propagate coordinated contraction waves, albeit weakly. Following excitatory (cholinergic) neurotransmission at the NMJs of C. elegans, opening of nicotinic AChR (ligand-gated ion channels) on muscle membrane is thought to initiate graded action potentials in muscle arms which then converge and propagate to the contractile compartment of the muscle (Richmond and Jorgensen, 1999; Jospin et al., 2002; Schafer, 2002). There are no voltage-activated Na⁺ channels in C. elegans and the graded action potentials are thought to be dependent on voltage-activated Ca⁺⁺ currents across the muscle plasma membrane through L-type channels. It is postulated that activation of these Ca⁺⁺ channels (similar to dihydropyridine receptor [DHPR] and encoded by the egl-19 gene) on the plasma membrane provides sufficient Ca⁺⁺ influx from extracellular space to directly initiate a contraction in the nematode body wall muscle where the sarcomeres are placed in close proximity to the plasma membrane (Lee et al., 1997; Maryon et al., 1998; Jospin et al., 2002).

Unlike other organisms where neurons send processes to their target muscle cells to make synapses, neuromuscular junctions (NMJs) of C. elegans are made by arms grown from muscle cells toward motor neurons (MusFIG 3) (Stretton, 1976; Sulston and Horvitz, 1977; Sulston et al., 1983; White et al., 1986; Dixon and Roy, 2005; Dixon et al., 2006). Muscle arms have simple structures made of a stalk and a bifurcated terminus that contacts the neuron. Similar to chemical synapses between neurons, NMJs are made en passant by the innervating neurons onto these muscle arms (White et al., 1986). In a process bundle, each motor neuron process sporadically moves to the outside of the bundle to become accessible to muscle arms in synaptic regions.

4.1 Somatic Muscle Arms

Among the 95 bodywall muscle (BWmu) cells, 16 head muscles have arms that synapse exclusively with the motor neurons of the nerve ring. Sixteen neck muscles have arms that extend both to the nerve ring and to the nearest nerve cord. The remaining 63 body muscles extend arms exclusively to the nearest nerve cord.

Muscle arm development is highly stereotypical. Each body wall muscle in the body usually begins by growing a single muscle arm during embryonic development (MusFIG 4). At hatching, muscle cells have on average 1.7 (+/- 0.8) arms per cell (Dixon and Roy, 2005; Dixon et al., 2006). The number of arms increases to three or more by the adult stage, with young adults averaging 4.0 (+/- 1.0) arms per cell (Hall and Hedgecock, 1991; Dixon and Roy, 2005). Individual muscle cells are observed to contain a stereotypical number of arms, and the muscles lying in rows closest to the dorsal and ventral nerve cords (ventral right and dorsal left quadrants) have significantly more arms than their contralateral homologs. In adult body muscles, individual muscle arms vary in size, shape and branchiness where they contact the longitudinal nerve cords. Viewed in thin sections, the nerve cords are covered by a muscle plate over much of their length, but there are bare patches where no arms contact the nerve cords. The majority of post-embryonic muscle arm outgrowth is coincidental with and dependent on the birth of 53 extra motor neurons and occurs during late-L1 to early-L2 stage (Dixon and Roy, 2005).

There are two suggested mechanisms of muscle arm development. First, during embryogenesis one-two arms are generated passively by migration of myoblasts away from their initial position next to the neurons (MusFIG 4) (Dixon et al., 2006). As myoblasts migrate to their final positions, their cell membranes stay in contact with a motor neuron process, thus generating muscle processes that stretch behind the migrating soma. In contrast, muscle arm growth during larval stages is thought to be an active extension process involving the actin cytoskeleton, extracellular matrix and guidance by chemoattraction (Dixon et al., 2006). Although the specific guidance cue for muscle arm extension is not yet known, the involvement of a guidance cue is supported by two lines of evidence. First, in a kinesin-defective (unc-104) mutant, in which anterograde transport of vesicles is disrupted, some of the dorsal body wall muscle arms extend towards the ventral cord, where dense core vesicles accumulate within motor neuron cell bodies (Hall and Hedgecock, 1991). In this mutant, it is suggested that the release of the muscle attractant occurs close to the neuron cell body where the vesicles are sequestered. Second, in unc-6 or unc-5 mutants, in which formation of motor neuron process bundles is erratic, body wall muscle arms extend to the lateral regions where the errant motor neuron processes are located (Hedgecock et al., 1990).

Development of muscle arms from the head and neck muscles to the nerve ring (NR) may occur principally by the passive extension mechanism. Early in embryogenesis, head and neck muscle cells directly surround the pharynx. It is suggested that when these muscle cells later migrate towards the periphery, as the first amphid axons extend to initiate the formation of the nerve ring, they leave an arm behind, next to the pharynx (MusFIG 4) (C. Norris, pers. comm.). The arms left behind from neck muscles, which are located posterior to the GLR cells, are then thought to grow anteriorly to reach the nascent nerve ring actively. The head and neck muscle arms together define a fairly precise topological map (both circumferential and antero-posterior) of motor neurons and their target muscles along the inner surface of the nerve ring (MusFIG 5A&B, MusFIG 5C-F, MusFIG 5G and MusMOVIE 1) (White et al., 1986). GLR cells are suggested to function as mesodermal scaffolding cells that guide the muscle arms to their appropriate territories for development of this motor map (see Somatic Muscle and GLR Cells). In the adult, the motor neuron axons that innervate the head and neck muscles are located in the innermost regions of the nerve ring except those of RIML/R motor neurons, which run more laterally within the nerve ring. The head muscle arms, which receive innervation from RIML/R, hence make small branches, which may penetrate the basal lamina in four places to contact RIML/R axons (MusFIG 5A).

In somatic muscle, the distal portions of muscle arms interdigitate abundantly in regions of neuromuscular junctions (MusFIG 6A-F, MusFIG 6G-K and MusMOVIE 2). The interdigitated muscle arms also make gap junctions to one another that are suggested to have a role in synchronous contractions of body muscles during embryonic elongation as well as in synchronizing the activity of left and right quadrants during normal body motion (Hall and Hedgecock, 1991) (see also Gap Junctions).

MusMOVIE 2: Interdigitation of neck muscle arms. Illustrations are reconstructions made from tracings of serial section TEMs (by Tylon Stephney) of neck muscles (based on [MRC] N2U series) (Liu et al., 2007). Reconstruction was created by Huawei Weng using Imaris software. Click on image to play movie.

4.2 Nonstriated Muscle Arms

Pharyngeal muscles do not extend muscle arms. No epithelial cells separate pharyngeal muscle from the pharyngeal nerves, placing many motor neurons in direct apposition to their target muscles for synaptic innervation. Some nerve bundles, such as M2 neurons, actually pass inside the muscles and make neuromuscular junctions to pharyngeal muscles inside the muscle cells (see Alimentary system - Pharynx).

Among sex-specific hermaphrodite muscles, the only obvious muscle arms are made by vm1R muscles. These extend arms to the ventral cord to receive synaptic input from the ventral cord motor neurons VA7, VB6, and VD7 (see Reproductive system - Egg-laying Apparatus ) (White et al., 1986). The muscle arms from the anal depressor muscle, anal sphincter muscle and two stomatointestinal cells are quite long. All arms must extend to the preanal ganglion where they receive synapses from the DVB neuron (see Nonstriated Muscle).

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