While there are major differences among the organs and systems in which smooth muscle plays a major part, the structure of smooth muscle is quite consistent at the tissue level and even more similar at the cellular level. Several typical arrangements of smooth muscle occur in a variety of locations.
The variety of smooth muscle tasks—regulating and promoting movement of fluids, expelling the contents of organs, moving visceral structures—is accomplished by a few basic types of tissue structures. All of these structures are subject, like skeletal muscle, to the requirement for antagonistic actions: If smooth muscle contracts, an external force must lengthen it again. The structures described below provide these restoring forces in a variety of ways.
Circular Organization: Blood Vessels. The simplest smooth muscle arrangement is found in the arteries and veins of the circulatory system. Smooth muscle cells are oriented in the circumference of a vessel so that shortening of the fibers results in reducing the vessel's diameter. This reduction may range from a slight narrowing to a complete obstruction of the vessel lumen, depending on the physiological needs of the body or organ. The orientation of the cells in the vessel walls is helical, with a very shallow pitch. In the larger muscular vessels, particularly arteries, there may be many layers of cells and the force of contraction may be quite high; in small arterioles, the muscle layer may consist of single cells wrapped around the vessel. The blood pressure provides the force to relengthen the cells in the vessel walls. This type of muscle organization is extremely important because the narrowing of a blood vessel has a powerful influence on the rate of blood flow through it (see Chapters 12 and 15). This circular arrangement is also prominent in the airways of the lungs, where it regulates the flow of air.
A further specialization of the circular muscle arrangement is a sphincter, a thickening of the muscular portion of the wall of a hollow or tubular organ, whose contraction has the effect of restricting flow or stopping it completely. Many sphincters, such as those in the gastrointestinal and urogenital tracts, have a special nerve supply and participate in complex reflex behavior. The muscle in sphincters is characterized by the ability to remain contracted for long periods with little metabolic cost.
Circular and Longitudinal Layers: The Small Intestine.
Next, in order of complexity, is the combination of circular and longitudinal layers, as in the muscle of the small intestine. The outermost muscle layer, which is relatively thin, runs along the length of the intestine. The inner muscle layer, thicker and more powerful, has a circular arrangement. Coordinated alternating contractions and relaxations of these two layers propel the contents of the intestine, although most of the motive power is provided by circular muscle (see Chapter 26).
Complex Fiber Arrangements. The most complex arrangement of smooth muscle is found in organs such as the urinary bladder and uterus. Numerous layers and orientations of muscle fibers are present and the effect of their contraction is an overall reduction of the volume of the organ. Even with such a complex arrangement of fibers, coordinated and organized contractions take place. The re-lengthening force, in the case of these hollow organs, is provided by the gradual accumulation of contents. In the urinary bladder, for example, the muscle is gradually stretched as the emptied organ fills again.
In a few instances, smooth muscles are structurally similar to skeletal muscles in their arrangement. Some of the structures supporting the uterus, for example, are called ligaments; however, they contain large amounts of smooth muscle and are capable of considerable shortening. Pilomotor muscles, the small cutaneous muscles that erect the hairs, are also discrete structures whose shortening is basically unidirectional. Certain areas of mesentery also contain regions of linearly oriented smooth muscle fibers.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.