The physiological properties of smooth muscle
Manuel, Elizabeth Bacon
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Although smooth muscle is one of the important tissues of the body, widely different views have been expressed on even its most fundamental characteristics. The study of smooth muscle has been complicated by its spontaneous activity, and double innervation. The purpose of this paper is to present some of the more recent findings concerning this subject. The smooth muscle cell is a spindle-shaped fiber with a single, oval, centrally placed nucleus. The size of the fibers varies greatly according to their environment. The fibers are bound together by a dense connective tissue network. There is no true cell membrane as is found in striated muscle. Myofibrils have been seen in some preparations (Tiegs, 1924) but these have been called merely artifacts as the result of coagulation during fixation. Because of the larger size of the fibrils in the pregnant uterus than in the nonpregnant, and because in regions of contraction the fibrils are thicker, Tiegs argues that they cannot be artifacts. The difficulty experienced in teasing smooth muscle apart is attributed to the presence of intercellular protoplasmic bridges. Evidences of a syncitial arrangement have been found in the uterus (Bozler, 1939) and the smooth muscle in the walls of the small blood vessels (Fulton & Lutz, 1942); but not in the nictitating membrane (Cannon, 1933). Smooth muscle is innervated from the autonomic nervous system which is composed of efferent fibers distributed to the effector organs through interposed peripheral neurones. As a general rule each organ is innervated from both the sympathetic and parasympathetic systems; and the two systems have opposite effects. The nerve endings may enter the cell and terminate near the nucleus (Tiegs, 1924), or closely adhere to the outside of the cell. The number of cells in a given tissue which are innervated is variable. In the ciliary muscle there is a numerous arrangement of nerve fibers (Boeke, 1932); and in the stomach only about one cell in every hundred is innervated (Stohr, 1931). Bozler has suggested a division of smooth muscle into two types; First, visceral smooth muscle which is characterized by a syncitial arrangement of fibers. Examples are the muscle of the intestine and uterus. The second type is multi-unit smooth muscle and is found in the blood vessels and the nictitating membrane. It is composed of small units activated by motor nerves. The coordinated activity of visceral smooth muscle is explained by the syncitial arrangement of fibers which affords a mechanism for the conduction of nervous impulses. Bozler (1941) has found that the spontaneous contractions characteristic of visceral smooth muscle are due to bursts of impulses conducted within the muscle. Cocainized preparations of the small blood vessels of the retrolingual membrane of the frog respond with constriction to a direct stimulation. Since the area is the same as that which contracted in innervated preparation the conduction of the impulse must by myogenic (Fulton & Lutz, 1942) The electrical changes accompanying the spontaneous contractions have been recorded by Cannon & Rosenblueth (1937). They show a one-to-one ratio for the intestine, stomach, and ureter signifying rhythmic twitches. Other muscles, uterus and retractor penis, show a many-to-one relationship denoting a state of tetanus. According to Cannon & Rosenblueth (1937) smooth muscle is electrically unexcitable. The changes produced, by stimulation are caused by activation of the nerve fibers in the muscle, or by the injurious effects of the current. Anoxia causes an increase in gastric motility, first by the production of epinephrine which inhibits the stomach by stimulating the splanchnic nerves; and second, on a vagospastic pylorospastic basis up to a critical threshold when the tome of the musculature is decreased (Van Liere, et al., 1936). The fibers of multi-unit smooth muscle are grouped into small units and are activated by motor nerves. The motor units; are more variable in extent than those found in striated muscle. Fulton & Lutz (1942) have suggested a syncitial arrangement for vascular smooth muscle. The electrical response to single shocks has been recorded by Cannon & Rosenblueth (1937). The response is made up of three components which are interpreted as denoting the depolarization of the surface of key and adjacent cells and the diffusion of a chemical mediator. The small blood vessels have contractility independent of the supplying vessels. The size of the capillary is controlled by innervated modified smooth muscle cells which act like a sphincter at the origin of the capillary. Nervous impulses may be transmitted at the synapse by electrical or chemical mediation. Both theories have strong and weak points in their favor. When the sympathetic nerves are stimulated a substance resembling adrenine is released. It diffuses into the blood stream and is carried to other parts of the body. The sympathetic system is arranged to work as a unit and to bring about diffuse effects. This system is reinforced by the discharge of local and circulating sympathin. There are two types of sympathin with different effects. Sympathin E is liberated from the smooth muscle which contracts upon stimulation of the sympathetic nerves; and I from that inhibited. Acetylcholine is liberated when parasympathetic nerves are stimulated. In contrast to sympathin acetylcholine is very rapidly hydrolysed in the tissues. It is evident from the points summarized in this paper that smooth muscle is composed of dissimilar groups with varying characteristics. Because of this fact each of the two types must fee studied separately; and assumptions based on evidence from one type cannot be transferred to the other.
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