After studying this chapter, you should be able to:
Differentiate the major classes of muscle in the body.
Describe the molecular and electrical makeup of muscle cell excitation–contraction coupling.
Define elements of the sarcomere that underlie striated muscle contraction.
Differentiate the role(s) for Ca2+ in skeletal, cardiac, and smooth muscle contraction.
Appreciate muscle cell diversity and function.
Muscle cells can be excited chemically, electrically, and mechanically to produce an action potential that is transmitted along their cell membranes and leads to activation of a contractile mechanism. The contractile protein myosin and the cytoskeletal protein actin are abundant in muscle, where they are the primary structural components that bring about contraction. Muscle can be divided into three types: skeletal, cardiac, and smooth, although smooth muscle is not a homogeneous single category. The basic properties of these three groups of muscle cells will be discussed in this chapter.
SKELETAL MUSCLE MORPHOLOGY
Skeletal muscle is made up of individual muscle fibers that are the “building blocks” of the muscular system. Most skeletal muscles begin and end in tendons, and the muscle fibers are arranged in parallel between the tendinous ends, so that the force of contraction of the units is additive. Each muscle fiber is a single cell that is multinucleated, long, cylindrical, and surrounded by a cell membrane, the sarcolemma (Figure 5–1). There are no syncytial bridges between cells. The muscle fibers are made up of myofibrils, which are divisible into individual filaments. These myofilaments contain several proteins that together make up the contractile machinery of the skeletal muscle.
Mammalian skeletal muscle. (A) A single muscle fiber surrounded by its sarcolemma has been cut away to show individual myofibrils. The cut surface of the myofibrils shows the arrays of thick and thin filaments. The sarcoplasmic reticulum with its transverse (T) tubules and terminal cisterns surrounds each myofibril. The T tubules invaginate from the sarcolemma and contact the myofibrils twice in every sarcomere. Mitochondria are found between the myofibrils and a basal lamina surrounds the sarcolemma. (B and C) Structural elements of myofibril shown in detail (see also Figure 5–2).
The contractile mechanism in skeletal muscle largely depends on the proteins myosin-II, actin, tropomyosin, and troponin. Troponin is made up of three subunits: troponin I, troponin T, and troponin C. Other important proteins in muscle are involved in maintaining the proteins that participate in contraction in appropriate structural relation to one another and to the extracellular matrix.
Differences in the refractive indexes of the various parts of the muscle fiber are responsible for the characteristic cross-striations seen in skeletal muscle when viewed under the microscope. The parts ...