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1 Sphincter Muscles

As described in Male Muscles - Overview, the sphincter (sph), like the anal depressor, is a sexually dimorphic muscle, common to both sexes but modified during male larval development (MaleAlimFIG 1).

MaleAlimFIG 1 Sphincter muscles
MaleAlimFIG 1: Sphincter muscles. Illustration of the adult male tail region featuring the sphincter muscle (sph), left lateral view. Sexually dimorphic muscles are underlined.

In the adult male the alimentary tract and reproductive tract are joined to a common chamber, the cloaca, which opens to the environment at the anus (described in detail in The Proctodeum). In contrast to the hermaphrodite anus, the male cloaca is constitutively open presumably to permit free movement of the spicules (housed within) and sperm during mating behavior. The principal seal for the male alimentary tract is instead at the sphincter (MaleAlimFIG 2).

MaleAlimFIG 2 Positioning of the sphincter
MaleAlimFIG 2: Positioning of the sphincter. A. Epifluorescent image from transgenic animal expressing the egl-15 reporter gene (FGF receptor), lateral right medial plane. The sphincter muscle (sph) and nucleus are proximate to the spicules and rectal gland. (Strain source: C. Branda and M. Stern.) B. Diagram showing the position of the sphincter muscle in the male tail. (PAG) Pre-anal ganglion.

The male sphincter is tonically contracted and must be relaxed (through inhibitory GABA transmission) during defecation. This response to GABA is opposite to that observed in larval males and in adult hermaphrodites and may reflect a change in receptor expression in the muscle (Reiner and Thomas, 1995). The male sphincter also differs morphologically from larval male and hermaphrodite sphincters. It is enlarged (hypertrophied) but still contains a single sarcomere. It has a dorsal process that attaches to the dorsal roof and that contains myofilaments (MaleAlimFIG 3). Thus hyper-contraction of the sphincter during ejaculation not only clamps off the intestine but also draw it away from the vas deferens, possibly allowing sperm to pass more freely (Sulston et al., 1980).

MaleAlimFIG 3 Ultrastructure of the sphincter muscle
MaleAlimFIG 3: Ultrastructure of the sphincter muscle. A. Low-power TEM featuring the sphincter muscle. (PAG) Pre-anal ganglion; (v/dsrL/R) ventral and dorsal spicule retractors, (v/dspL) ventral and dorsal spicule protractors; (d/vBWM) dorsal and ventral body wall muscle (Image source: N2Y [MRC] 596-24.) B. TEM of boxed area in 3A showing close up of the myosin and actin filaments within the sphincter muscle. (N) Nucleus. (Image source: N2Y [MRC] 942-L.)

2 Cell List

Anal sphincter muscle
mu sph
Anal depressor muscle
mu anal

3 References

Branda, C.S. and Stern, M.J. 2000. Mechanisms controlling sex myoblast migration in Caenorhabditis elegans hermaphrodites. Dev. Biol. 226: 137-51. Article

Garcia, L.R., Mehta, P. and Sternberg, P.W. 2001. Regulation of distinct muscle behaviors controls the C. elegans male's copulatory spicules during mating. Cell 107: 777-788. Article

Liu, K.S. and Sternberg, P.W. 1995. Sensory regulation of male mating behavior in Caenorhabditis elegans. Neuron 14: 79-89. Article

Reiner, D.J. and Thomas, J.H. 1995. Reversal of a muscle response to GABA during C. elegans male development. J. Neurosci. 15: 6094-6102. Article

Sulston, J.E., Albertson, D.G. and Thomson, J.N. 1980. The Caenorhabditis elegans male: Postembryonic development of nongonadal structures. Dev Biol. 78: 542-576. Article

This chapter should be cited as: Lints, R. and Hall, D.H. 2009. Male alimentary system, defecation muscles. In WormAtlas.  doi:10.3908/wormatlas.2.3
Edited for the web by Laura A. Herndon. Last revision: May 22, 2013.