Smooth muscle Those muscles which surround the hollow viscera, the digestive tract, the bronchi, the urogenital system, the arteries, the veins, the ducts of the externally secreting glands, the muscles in the interior of the eyes for visual accommodation, and those which erect the hairs in some animals are not striated, neither are they under the control of the will nor attached to the skeletal system. They are, therefore, designated as nonstriated, involuntary, or smooth muscle. They are composed of relatively small spindle-shaped cells which are united together to form thin layers of muscle. Each muscle cell contains a single elongated nucleus and a number of myofibrillæ which run longitudinally through its cytoplasm and are said to be continuous from one cell to another. These are the contractile elements of the cells. Smooth muscle receives its nerve supply from the involuntary nervous system and is hence removed from the control of the will. Properties and activities of smooth muscle The distinguishing characteristic of smooth muscle is its power to develop and maintain a condition of tonus, that is, a condition of maintained shortening and increased tension. Under normal conditions it is never in a complete state of relaxation; indeed, it is difficult to say what its natural length really is. It is so constituted that it is capable of maintaining this low grade of activity continuously without the intervention of fatigue. The walls of the hollow organs accommodate themselves to the volume of their contents, by virtue of this tonus, without any marked alteration of the internal pressure. Ordinarily the smooth muscle in the walls of these organs spontaneously adapts its length to the volume without any change in tension unless changes in pressure are developed rapidly. This type of muscle is very susceptible to stretching; if this occurs slowly, the muscle yields; if more rapidly, it serves as a stimulus and the muscle contracts. The action of smooth muscle is very sluggish, more so than that of other types of muscle, and may require a minute or more for a single contraction. The action of smooth muscle is frequently spontaneous and rhythmical. This property of rhythmical contractility is possessed also by heart muscle, but distinguishes these two types of muscle from the voluntary variety. When a strip of smooth muscle is suspended from a clamp, the free end being attached to a movable lever, it may begin contracting spontaneously and rhythmically and may continue to do so for an indefinite time. Whether the automaticity is due primarily to a property of the muscle or to the nerve tissue to be found within it, cannot be definitely stated. Movements of internal structures which are brought about by contractions, relaxations, and changes in tonus of the smooth muscle are exhibited by the gastro-intestinal tract, vascular apparatus, excretory system, and other structures, which although less obvious are equally important. So fundamental are they to the welfare of the individual that nature has kept them removed from the control of the will. For example, once the ingested food passes to the pharynx it becomes subjected to a series of reflex responses from which there is no recall. The series of movements of the alimentary canal which follow, propel the food before them as the appropriate stimuli incurred by its presence and the state of its digestion arise. Similarly the appropriate digestive juices are poured into the canal at the call of their adequate stimuli. These responses are, as compared to the contractions of the voluntary muscles, very stereotyped and exact, but nevertheless adequate and adapted to the needs of the organism. In the circulatory system, we have in the body a closed system of tubes and a fluid transportation system (blood) with the heart as the pump to force and direct the flow of the blood through the arteries to the capillaries. The veins serve as a means of return of the blood to the heart. The circulatory system is concerned with the immediate needs of the body cells. For this reason the capillaries are of first importance, since in them the circulating blood comes in rather intimate relation with the tissues. The larger vessels serve merely as conduits to and from the capillaries.		Home Classes of levers Varieties of muscle Smooth muscle The blood supply to muscles The neurone Cardiac muscle On the nomenclature of skeletal muscles The nerve supply to muscles Structure of a skeletal muscle Central nervous system The reflex arc and reflex action Physiologic properties of muscle Mechanical Changes The effects of temperature on muscular contraction Muscle glycogen, its source and fate The reaction of muscles Oxygen consumption of muscle Determination of the oxygen consumption of muscle The viscosity factor in muscle and muscle efficiency Significance of the recovery heat Production of Electricity