MESODERMAL ORGANS- Introduction to Muscle (Part I)

Muscle overview - Muscle arms - Structure of the contractile apparatus - Excitation-contraction coupling - Back to Contents

Muscle Overview

There are two types of muscle in C. elegans: multiple sarcomere/obliquely striated (somatic) muscle and nonstriated muscle. The multiple sarcomere muscles contain evenly distributed attachment points to the hypodermis and cuticle along their length (See Part II) whereas the majority of the nonstriated muscles have focal attachment points at their ends. The multiple sarcomere group is the most abundant muscle group and is constituted by 95 body wall muscles, 14 of which are postembryonically generated. The nonstriated muscle group includes 20 pharyngeal muscles, 2 stomato-intestinal muscles, 1 anal sphincter muscle, 1 anal depressor muscle, 8 vulval muscles (all postembryonically generated), 8 uterine muscles (all postembryonically generated) and contractile gonadal sheath. In the male, instead of the vulval and uterine muscles and gonadal sheath, 41 specialized mating muscles are included in this group. Except pm1-pm5 cells of the pharynx, all are mononucleate cells. After hatching, pm1 becomes a syncytial cell with 6 nuclei and, pm2-pm5 become binucleate syncytial cells (See Alimentary System-The Pharynx Part1a).
Though most muscle contractions are generated by nerve transmission, there are 3 rhythmic behavior cycles in C. elegans that 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 (Also see Alimentary System-The Pharynx Part1a), gonadal sheath contractions, and defecation cycle involving body wall (somatic) muscles near the head, posterior (somatic) body wall muscles and enteric muscles (i.e., anal depressor, sphincter and stomato-intestinal muscles) (Also see Alimentary system Part III).

Muscle Arms

Unlike other organisms where neurons send processes to their target muscle cells, nematode muscles extend cytoplasmic arms from their bodies to the neuron process bundles to make synapses with motor neurons (MusFIG1a and MusFIG1b; VL: ventral left, DL: dorsal left). Similar to neuron-neuron synapses, neuromuscular synapses are made en passant by the innervating neurons onto these muscle arms (White J. et al. 1976). The neuron process involved runs on the outside of the process bundle in synaptic regions where it becomes accessible to muscle arms.

In somatic muscle, muscle arms interdigitate abundantly in regions of neuromuscular junctions. Click here to see a reconstruction of how neck muscle arms interdigitate at the region of the dorsal nerve cord (For labels, refer to MusFIG2a-2d; Muscle arms are colored in red, yellow, light green, dark blue, turquoise and gray. Some of these muscle arms may belong to the same (right side or left side) muscle cells though they are shown in different colors here. Reconstructions of 4 selected dorsal nerve cord neurons are also shown in dark gray, pink, brown and dark green (MusFIG2a and MusFIG2d) to visualize their relation to the muscle arms. The remaining dorsal cord neurons did not contact muscle arms and were not traced. Cuticle is artistically rendered on MusFIG2a and is not actual. MusFIG2e shows an example of the TEMs from which the tracings were done. Tracings done by Tylon Stephney and reconstructions created by Huawei Weng from MRC/N2U TEMs using Imaris software).
The interdigitated muscle arms also make gap junctions to each other which are suggested to play a role in synchronous contractions of body muscles during embryonic elongation (Hall D. H. & Hedgecock E. 1991) as well as synchronizing the activity of left and right quadrants during normal body motion.

In the head, arms from the somatic head muscles run posteriorly until they reach the posterior nerve ring region (Fig. 5.3). The arms from each muscle row then make an anterior arc of about 45 o , extend centripetally to pass through the nerve ring layer and reach between the outside surface of the pharynx and the inner surface of the neural plate (MusFIG3-5. Note in MusFIG4 and MusFIG5 GFP is concentrated in nonsarcomeric portions of the muscle cell) (See a 3-D reconstruction of head muscles and muscle arms by R. Newbury & Moerman lab. Cell labels are shown in MusFIG3B-C. 3-D movie was created from confocal images of a strain expressing the GFP marker linked to the promoter for W05E10.4 using Zeiss LSM 5 Pascal software v. 3.2). These arms together define a topological map (both circumferential and antero-posterior) of the head muscles onto the inner surface of the nerve ring with a fairly precise mapping of motor neurons to their target muscles. During embryonic development, GLR cells are suggested to function as mesodermal scaffolding cells that guide the muscle arms to their appropriate territories (See GLR section). Early in development, head and neck muscles directly surround the pharynx, and it is suggested that these muscle cells migrate peripherally to the hypodermis leaving behind an arm next to the pharynx which then grows anteriorly through the GLR scaffold (Norris C. R. et al WBG14(5):37). In the adult, the motor neuron axons that innervate these 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 spurs which penetrate the basal lamina in four places to contact RIML/R axons (See Mind of a Worm Fig14 for NMJs in the nerve ring). The gap junctions that head muscle arms make at the inside of the nerve ring where they interdigitate is quite selective; gap junctions occur between arms only from muscle cells in adjacent quadrants, those from the same quadrant do not make gap junctions to each other here but only in the region of their cell bodies (bellies) (White J. et al., 1986).
In the neck, somatic muscles extend arms both to the nerve ring and the ventral and dorsal nerve cords where they receive synapses (White J. et al., 1986)( See part II MusTable3).

Muscle arms in the body contact the ventral and dorsal nerve cords where they interdigitate and spread along the basal lamina and receive input from cord motor neurons (MusFIG6 and MusFIG7; asterisks label muscle arms. Strain trIs30 contains chromosomally integrated pPRRF138.2(him4p::MB::YFP), pPRZL44(hmr-1b::DsRed2), pPR2.1(unc-129nsp::DsRed2) (Dixon S. J. & Roy P. (2005) Development, in press)). Each muscle quadrant extends arms to the nearest nerve cord (dorsal muscles to dorsal nerve cord and ventral muscles to ventral nerve cord) and no muscle arms are extended to the opposite cord (MusFIG1 and MusFIGFIG8). Each body wall muscle grows roughly a single muscle arm during embryonic development. At hatching, muscle cells have on average 1.7 (+/- 0.8) arms per cell (Dixon S. J. & Roy P. (2005) Development, in press). The number of arms increases to 3 or more by adult stage, such that in young adults there are on average 4.0 (+/- 1.0) arms per cell (Hall D. H. & Hedgecock E. 1991; Dixon S. J. & Roy P. (2005) Development, in press) (MusFIG9 and MusFIG10). Individual muscles are observed to extend a stereotypical number of arms and the muscles closest to the dorsal and ventral nerve cords (ventral right and dorsal left quadrants) have significantly more arms than their contralateral homologues (Dixon S. J. & Roy P. (2005) Development, in press). In adult body muscles, individual muscle arms vary in size, shape and branchiness where they contact the longitudinal nerve cords (MusFIG2 and MusFIG7-10). Viewed in thin sections, the nerve cords are covered by an extensive muscle arm plate over much of their length, but there are bare patches where no arms contact the nerve cords. The majority of postembryonic muscle arm outgrowth is coincidental with and dependent on the birth of 53 motor neurons and occurs during late L1 - early L2 stage. This suggests that neuron derived factors facilitate muscle arm extension (Dixon S. et al IWMA848, Dixon S. J. & Roy P. (2005) Development, in press). Also, a motor neuron derived attractant has been suggested to instruct muscle arms to their target location since, in a kinesin-defective (unc-104) mutant where anterograde transport of synaptic vesicles is disrupted, some of the muscle arms from dorsal body muscles reach towards the ventral cord where motor neuron cell bodies are located (Hall D. H. & Hedgecock E. 1991). In this mutant it is suggested that the release of the synaptic vesicle contents and among them possibly the muscle attractant occurs close to the cell body where the vesicles are sequestered.

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. Some nerve bundles pass inside the muscles and neuromuscular junctions to muscles are also made by some pharyngeal neuron processes, such as M2 neurons, as they travel inside the muscle cells during their dorso-ventral trajectories.
Among sex-specific hermaphrodite muscles, the only obvious muscle arms are made by vm1R muscles which are extended down to the ventral cord where vm1R receives synaptic input from ventral cord motor neurons, VA7, VB6 and VD7 (see EggFIG15 in Reproductive system-the egg-laying apparatus) (White J. et al., 1986). In males, sex specific muscles make arms in many instances, but are often rather short.
The muscle arms from anal depressor muscle, anal sphincter muscle and two stomatointestinal cells are quite long. All must extend to the preanal ganglion where they make synapses with the DVB neuron (see RectFIG1 in PartIII).

Structure of the contractile apparatus

The basic unit of the contractile apparatus in muscle is the sarcomere. 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 corresponding to thin filaments, A (Anisotropic) bands corresponding to thick filaments (including the thin filament overlap region), H (Heller) bands which correpond to the central region of the A bands and M lines at the middle of the H bands where each myosin rod is joined laterally to its myosin rod neighbor. In the sarcomere, myosin-containing thick filaments are interdigitated with actin-containing thin filaments on either side (MusFIG11). In C. elegans the Z-disc analog is the dense body (DB) which functions to anchor and align thin filaments in striated muscles (See Muscle Part II). 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 Muscle Part II). In nonstriated muscles with single sarcomeres, large hemidesmosomes connect each sarcomere at the muscle ends to body cuticle or specialized cuticle and/or to basal lamina to anchor the myofilaments (see Part III). Some of the nonstriated muscles have myofilaments that are less well organized where anchorages occur via small plaques and hemidesmosomes distributed along the cell membrane similar to the organization of vertebrate smooth muscle (See Part III) (MusFIG12).
The organization of the muscle filament lattice in C. elegans can be viewed by polarized light microscopy, both to assess their orientation in wild type body muscles as they develop and to score for defects in mutant strains.

Excitation-contraction coupling

EXCITATION-CONTRACTION (E-C) COUPLING IN VERTEBRATE MUSCLE: 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 (MusFIG13 and MusFIG14). 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. Close to the border between the A- and I-bands of the myofibrils, the T-tubules 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 (MusFIG14).
Depolarization in the T-tubule membrane leads to release of stored Ca++ through the interaction of two proteins. A voltage sensor (voltage-gated Ca++ channel) in the T-tubule membrane, dihydropyridine receptor (DHPR), changes conformation in response to the action potential (MusFIG13). This conformational change is transmitted to another Ca++ channel, the Ryanodine receptor (RyR), on SR, causing it to open, and allowing Ca++ release from SR stores (Ahern, C.A., et al. (2001). Biophy. Journal 81:3294-3307). RyR 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 (See Alberts B. et al (2002), Molecular Biology of the Cell-Muscle Contraction).  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 (racheting) and shorten the sarcomere length (See Alberts B. et al (2002), Molecular Biology of the Cell, FIG16-74).  One racheting cycle will last as long as the cytosolic Ca++ remains elevated. At the end of contraction, Ca++ is restored to SR by an ATP-dependent calcium pump.

EXCITATION-CONTRACTION (E-C) COUPLING IN C. elegans MUSCLE: In C. elegans SR is a network of vesicular membranous organelles surrounding the myofilament lattice. The flattened vesicles of SR extend around dense bodies and lay adjacent to the apical (hypodermal side) plasma membrane underneath the lattice where they are localized randomly between dense bodies (Waterston R. H., 1988) (MusFIG15. Also See Part II). A gap of 12-14 nm separates them 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 the need for a T-tubule system (Waterston R. H., 1988). In C. elegans RYR is encoded by unc-68 gene (Maryon E. B. et al, 1996; Hamada T. 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, the anal sphincter and depressor muscles (Maryon E. B. et al, 1998). In somatic muscle, initiation of unc-68 expression coincides with the first twitching movements of the embryo. Within 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 E. B. et al, 1998) (See Muscle Part II). In contrast to vertebrate muscle, UNC-68 functions to enhance motility but is not essential for E-C coupling in C. elegans since 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 (Jospin M. et al, 2002; Schafer W. R., 2002; Richmond J. E. and Jorgensen E. M.,1999). 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 membrane through L-type channels. It is postulated that activation of these voltage-gated Ca++ channels (similar to DHPR and encoded by 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 (Maryon E. B. et al, 1998; Lee R. Y. N. et al, 1997; Jospin M. et al, 2002). It must be noted that in a separate class of muscle, the spicule protractor muscles of male, UNC-68 and EGL-19 seem to differentially promote distinct (periodic vs prolonged) contractile behaviors. Although both channels have been shown to be required for nicotine-induced protraction, levamisole-induced protraction is facilitated by UNC-68 whereas arecoline induced protraction is promoted by EGL-19 (Garcia L. R. et al, 2001).

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