Muscle physiology is the study of muscle function. A muscle is a bundle of fibers that contract to produce heat, posture, and motion, either of internal organs or of the organism itself. Muscle physiology studies the physical, mechanical, and biochemical aspects of muscles in development, fiber structure, muscle structure, contraction, and strength-building.
The body has three types of muscle: cardiac, smooth, and skeletal. Skeletal muscle is a voluntary muscle, or a muscle that can be consciously controlled, characterized by even striations, or stripes. Skeletal muscle attaches to bones to effect movement of the skeleton for purposes such as posture and locomotion. Smooth muscle is an involuntary muscle, marked by a lack of striations, that effects movement in the internal organs. Cardiac muscle is an involuntary, unevenly striated muscle that composes the heart and causes its contractions, or the beating of the heart.
Understanding the physiology of skeletal muscle requires a basic grasp of its structure. Skeletal muscles typically attach to bones via tendons and often appear in antagonistic pairs, so that when one muscle contracts, the other lengthens. The muscle itself is made up of a bundle, or fascicle, of long, cylindrical cells called muscle fibers. Each fiber contains many string-like structures called myofilaments that sit within the sarcoplasm, a fluid similar to cytoplasm which is held in by the fiber’s sarcolemma, or membrane. The myofilaments contain contractile structures called myofibrils, whose elements repeat geometrically to create functional units called sarcomeres.
Each sarcomere contains overlapping thick filaments, composed of myosin molecules, and thin filaments, composed of actin, troponin, and tropomyosin molecules. Sliding filament theory of contraction proposes that, during contraction, the myosin binds to the molecules of thin filament to pull the thin filaments over or under the thick filament. The sarcomere becomes shorter as a whole, though no element of the fiber is actually shrinking in size. The binding of molecules responsible for this contraction is stimulated by a release of calcium ions from the sarcoplasm. The calcium is released in response to an electrical impulse called an action potential sent from a neuron to a muscle through a neuromuscular synapse.
Smooth muscle physiology differs from skeletal muscle physiology because smooth muscles do not have sarcomeres, explaining the lack of striations in smooth muscle. Instead, smooth muscle contracts as a single unit, with electrical impulses being communicated from cell to cell through gap junctions. These electrical impulses are communicated by neurons stemming from the autonomic nervous system. Some smooth muscle may contract spontaneously, without stimulus from a neuron, due to the presence of pacemaker cells, which can create their own electrical impulses. Like skeletal muscle, contractions occur from the binding and sliding of thick filaments with thin filaments in response to a release of calcium within the muscle fiber.
Cardiac muscle physiology is similar to the physiology of skeletal muscle in several ways. Cardiac muscle contracts in response to elevated levels of calcium and is also striated; indicating that it also uses sarcomeres as its contractile unit. Like smooth muscle and unlike skeletal muscle, cardiac muscle does not need to be innervated at every fiber because it can communicate electrical signals from cell to cell. This communication is achieved through intercalated discs, a feature unique to cardiac muscle.