A muscle is an organ of movement made up of many muscle cells that uses its contractive ability to move the bones or, as a hollow muscle or sphincter muscle does, make cavities smaller and close openings. The individual muscle cells are joined together by connective tissue membranes or sheaths, which in turn merge to form tendons and are thus able to transfer forces (Fig 7-33).
Muscle tissue is a contractile tissue, which means that its cytoplasm (known as sarcoplasm) has special fibers (myofibrils) that are able to contract. Based on differences in structure and very different physiologic characteristics, three separate types of muscle tissue are described: smooth muscle tissue, cardiac muscle tissue, and striated muscle tissue.
Smooth muscle cells are oval or spindle-shaped cells 20 to 500 |jm in length. These muscle cells have only one nucleus, which is always centrally located. In terms of microstructure, the myofibrils of the sarcoplasm run parallel to the long axis of the cell. Each smooth muscle cell is sheathed in a delicate membrane (sarcolemma) made up of reticulin fibers, while the ends of the cell merge into very fine tendons. This type of muscle tissue is only able to contract very slowly but displays excellent endurance. This is why smooth musculature is found in vessel walls, the intestinal walls, the airways, and other places where prolonged, rhythmic contractions are involved. This musculature is therefore also described as visceral musculature.
A typical form of work is the peristaltic movement in the digestive tract, which propels food forward in the intestinal tube. Smooth musculature may also form powerful rings of muscle (sphincter muscles) that serve as opening and closing muscles (eg, pyloric part of the stomach). Another special characteristic of the smooth musculature is that it is controlled by the autonomic nervous system, which means that it is not influenced voluntarily. Autonomic conduction pathways, which are activated by certain stimulating substances (epinephrine/adrenaline and acetylcholine), may thus be found in the muscle layers of the smooth musculature.
Cardiac musculature (myocardium) is a specific type of striated musculature that nevertheless bears strong similarities to smooth musculature. Different cells join together in the myocardium to form a network, and holes in the network are filled with loose connective tissue. The cell nuclei lie in the middle of the cells and are surrounded by transverse striated (striped) myofibrils. The individual muscle cells are meshed in what are known as intercalated discs and are cemented with intercellular substance, as if a kind of cell fusion were taking place. The myocardial cells are also enclosed by a sheath of reticulin fibers.
The conduction system of the cardiac musculature lies within a connective tissue sheath close to the lining membrane of the heart (endocardium). This autonomic control system produces a strong, rhythmic, and involuntary contraction of the cardiac musculature, but this contraction can also be influenced by the autonomic nervous system and by hormonal regulation.
Striated muscle cells are fibers measuring a few millimeters to 10 cm in length, with a great number of cell nuclei at their periphery; as many as 100 nuclei may be found in one cell. The myofibrils lie in the middle of the cell and, as a result of their arrangement into light and dark segments, give the impression of crosswise stripes (transverse striation) (Fig 7-34).
The striated muscle fibers are also encased in a membrane (sarcolemma) made of ultrathin reticulin fibers, which again form tendons at the ends. Muscle fibers with varying proportions of sarcoplasm and mitochondria (sarcosomes) can be differentiated. Muscle fibers with a lot of sarcoplasm but few mitochondria appear light in color and only work for a short time, but very quickly; muscle fibers with little sarcoplasm and a lot of mitochondria appear dark and, at a moderate contraction speed, can work for a relatively long time.
Skeletal musculature made up of striated muscle fibers is the main constituent of the active locomotor system. It is controlled by the somatic nervous system, so it is subject to a person's will. The striated muscle cells react rapidly but tire very quickly. They can be stimulated by electrical impulses and can be influenced by heat and cold. The masticatory musculature, as part of the active locomotor system, is made up of striated muscle fibers.