MusFIG 18: Stomatointestinal muscles and anal depressor muscle.

A. Graphic rendition of enteric (anal depressor, anal sphincter, and stomatointestinal) muscles, which function in defecation (from left lateral side). These three muscles are located at the tail just posteriorly to the intestine. They all receive direct innervation via muscle arms in the preanal ganglion region from the DVB neuron and indirect input from the AVL neuron, which makes gap junctions to DVB.
B. A pair of stomatointestinal muscles wrap around the posterior of the intestine. The ventral portions of these muscles include some longitudinal, randomly arranged contractile filaments similar to vertebrate smooth muscle. The thin processes that encircle the posterior intestine contain myofilaments that attach to the body wall via electron-dense attachments (not shown). Epifluorescent image from a larval-stage transgenic animal expressing the reporter gene mig-2::GFP, left lateral view. (Image source: C. Kenyon and H. Thieringer.)
C. The anal depressor muscle has a large, asymmetrically placed nucleus (arrow), close to the dorsal side of the H-shaped muscle, DIC image, left lateral view.
D. The anal depressor muscle is a single sarcomeric muscle that attaches to the dorsal wall of the rectum on the ventral side and the dorsal hypodermis on the dorsal side. (Int) Intestine. Epifluorescent image from a transgenic animal expressing a GFP-tagged reporter gene in the anal depressor and anal sphincter muscles (toroidal portion out of plane of focus), left lateral view. (Strain source: Z.-W. Wang and B. Chen.)
E. The myofilaments are organized as two parallel sheets on the right and left sides of the H-shaped anal depressor muscle. The single sarcomere of each sheet covers the length of the muscle. In this view, the thin filaments of the right side of the cell are in focus. Epifluorescent image from a transgenic animal expressing the reporter gene unc-27::GFP. (Strain source: L. Jia and S.W. Emmons.)

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