Smooth Muscle in the GI system
The muscle tissue of the GI tract is smooth muscle, except for that in the oral cavity (lips, tongue, palate) and upper esophagus.
In most regions of the tract, smooth muscle forms a substantial two-layered muscularis externa.
Smooth muscle also forms a delicate muscularis mucosae at the deep boundary of the mucosa (i.e., between lamina propria and submucosa).
Unlike striated muscle, smooth muscle consists of individual cells (leiomyocytes), each cell with its own nucleus. The physiology of smooth muscle also differs substantially from that of striated muscle.
Neurotransmitter activitation of smooth muscle is fairly diffuse, without discrete, well-defined neuromuscular junctions. The motor endings of autonomic axons, where neurotransmitter is released, are not closely associated with individual smooth muscle fibers.
Electrical activitation of smooth muscle is passed from cell to cell by gap junctions.
Smooth muscle of the gut can generate intrinsic rhythmic contraction, independent from direct neural control. Input from the autonomic nervous system increases or decreases the level of this spontaneous activity. (In certain other locations, such as the iris of the eye, neural control is more direct and precise.)Consult your textbook for more information, including molecular details of contraction and control mechanisms.
Each smooth muscle cell (or "muscle fiber") is just a few microns in diameter but a couple hundred microns long. The nucleus is also quite elongate, compared to any other cell types.
Because the cells that comprise smooth muscle fibers are typically packed closely together, individual cells are usually difficult to resolve microscopically, especially at low magnification.
In contrast, individual striated muscle fibers are conspicuous as individual units. Each fiber has so many nuclei that any random cross section typically displays at least one and often several nuclei, normally located around the periphery of the fiber.
The appearance, or visual texture, of smooth muscle varies dramatically depending on the orientation of the fibers with respect to the plane of section.
Note especially the appearance of smooth muscle nuclei. These nuclei appear very long (and often wiggly) when cut in longitudinal section, small and round in cross section, and somewhere in between in oblique section. (The full length of smooth muscle nuclei is only apparent when the plane of section is perfectly aligned with the long axis of the cells.)
A very small bundle of smooth muscle can be particularly inconspicuous when cut in cross section. In that case, the cell nuclei appear as small round dots whose apparent size (about 3-4µm) and consistent shape are unlike those most other cells.
In cross-sectioned smooth muscle, only a fraction of the fibers display nuclei. Recall that each smooth muscle fiber is a single cell, each with its own nucleus. In each such cell, the nucleus occupies only a small proportion of the cell's length. So nuclei appear only in a similarly small proportion of the fibers cut by any single section.
Collagen resembles smooth muscle, in being fibrous and eosinophilic (i.e., pink staining with H&E). However, note that connective tissue can usually be distinguished by the collagen fibers' irregular size and orientation, and by the haphazard shape and distribution of fibroblast nuclei.
Smooth muscle fibers usually occur in bundles in which all fibers (and the associated nuclei) have approximately the same size and orientation.
Collagen fibers, in contrast, tend to be various in size and orientation (except in specialized cases, such as tendons).
Subtle differences in color can sometimes distinguish smooth muscle from collagen in specimens stained with H&E, but the quality of difference varies from specimen to specimen. Texture (size, shape, orientation of fibers and associated cell nuclei) usually provides more reliable distinction. When it is important to make this distinction quickly and reliably, a stain that is selective for collagen may be chosen (e.g., one of the trichromes).
Comments and questions: firstname.lastname@example.org
SIUC / School
of Medicine / Anatomy / David
Last updated: 16 May 2022 / dgk