Responsible for creating movement, circulating substances, and storing glycogen, the muscular system is a complex network of nearly 700 distinct organs in the human body [1]. Each one is composed of one of the three types of contractile tissue: skeletal, smooth, or cardiac muscle. All three types of muscle generate motion through contraction and relaxation, stimulated by the myofilament proteins myosin and actin, but with different anatomy [2]. Both skeletal and cardiac muscles exhibit a striated, or striped, appearance, while smooth muscle appears homogeneous under the microscope [2, 3]. Moreover, skeletal muscles are innervated by the somatic nervous system, allowing them to be controlled consciously, while cardiac and smooth muscles cannot be moved voluntarily due to their connection with the autonomic nervous system [4]. Besides these similarities, each type of muscle exhibits unique cellular makeup, functions, and physiology.
Skeletal muscle, the most common type, composes around 40% of normal human body weight [1]. Muscles in this category — e.g., the biceps and hamstrings — attach to tendons, allowing them to produce the movements of the body. Each muscle contains thousands of muscle fibers organized into bundles, or fascicles, and each muscle fiber contains thousands of tiny parallel threads called myofibrils [1, 5]. Myofibrils, the cellular units of skeletal muscle, house the mitochondria and multiple nuclei in the sarcolemma, a specialized cytoplasm [5]. Each myofibril is divided into sarcomeres, small segments containing the myofilaments actin and myosin and separated by jagged borders, or “Z-discs” [6]. To produce movement, the sarcomeres contract, pushing the discs within a myofibril closer together and causing actin to bind to myosin [5, 7]. The myosin molecules then release stored energy and lurch forward, therefore converting chemical energy into mechanical [7]. To protect the fibers, skeletal muscles have three layers of connective tissue — the endomysium, perimysium, and epimysium — which support the organ while also supplying it with blood vessels and motor nerves [5].
Smooth muscle, however, exhibits a different anatomy. Found lining the walls of internal organs, blood vessels, and the pupil, this type of muscle contracts at a slower but steadier rate [8]. Its significant elasticity enables it to remain in a state of partial contraction for long periods of time, a necessary condition for maintaining pressure in the gastrointestinal tract and blood vessels [8]. Smooth muscle cells contain only one nucleus; additionally, they lack fibers and sarcomeres, instead resembling the shape of a spindle [1]. Because the myofilaments actin and myosin are not arranged in a regular pattern, smooth muscle appears non-striated and homogenous [8, 9]. The myofilaments each form continuous chains stretching from one muscle cell to another, creating a network of smooth muscle cells [9]. The autonomic nervous system innervates smooth muscle, allowing the body to maintain life-sustaining functions without conscious thought.
Finally, cardiac muscle, also known as the myocardium, exists only within the middle layer of the heart. Involuntary and striated, cardiac muscle comprises a complex of muscle cells, each of which has one or two nuclei, a significant number of mitochondria, and a connective extracellular matrix of capillaries [10]. Like skeletal muscle, the myofibrils of cardiac muscle are divided into sarcomeres, but they are linked by connective tissues called intercalated discs [11]. Contraction of the heart muscle utilizes actin and myosin like all muscle types, but this muscle can trigger contraction without the stimulation produced by a nerve impulse, unlike skeletal and smooth muscle; however, the autonomic nervous system can influence the behavior of the muscle [4, 11]. Coronary arteries supply the cardiac muscle with oxygen, which in turn enables the heart to generate the pressure needed to pump blood throughout the body.
Overall, each type of muscle is essential to the human body, but it is important to remember the differences in function and anatomy between the types of muscle cells — skeletal muscle cells, responsible for movement, are voluntary, striated, cylindrical, and multinucleate; smooth muscle cells, responsible for digestion and carrying nutrients, are involuntary, non-striated, spindle-shaped, and mononucleate; and finally, cardiac muscle cells are involuntary, striated, interconnected, and mono- or binucleate.
References
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