Science, Tech, Math › Science Facts About Muscle Tissue It is the most abundant tissue in most animals, including humans Share Flipboard Email Print This is a colored scanning electron micrograph (SEM) of a skeletal, or striated, muscle fiber. It consists of a bundle of smaller fibers called myofibrils, which are crossed by transverse tubules (green) that mark the division of the myofibrils in to contractile units (sarcomeres). STEVE GSCHMEISSNER/Science Photo Library/Getty Images Science Biology Anatomy Basics Cell Biology Genetics Organisms Physiology Botany Ecology Chemistry Physics Geology Astronomy Weather & Climate By Regina Bailey Biology Expert B.A., Biology, Emory University A.S., Nursing, Chattahoochee Technical College Regina Bailey is a board-certified registered nurse, science writer and educator. Her work has been featured in "Kaplan AP Biology" and "The Internet for Cellular and Molecular Biologists." our editorial process Regina Bailey Updated August 21, 2019 Muscle tissue is made of "excitable" cells that are capable of contraction. Of all the different tissue types (muscle, epithelial, connective, and nervous), muscle tissue is the most abundant tissue in most animals, including in humans. Muscle Tissue Types Muscle tissue contains numerous microfilaments composed of the contractile proteins actin and myosin. These proteins are responsible for movement in muscles. The three major types of muscle tissue are: Cardiac Muscle: Cardiac muscle is so named because it is found in the heart. Cells are joined to one another by intercalated discs, which allow the synchronization of the heartbeat. Cardiac muscle is branched, striated muscle. The heart wall consists of three layers: epicardium, myocardium, and endocardium. Myocardium is the middle muscular layer of the heart. Myocardial muscle fibers carry electrical impulses through the heart that power cardiac conduction. Skeletal Muscle: Skeletal muscle, which is attached to the bones by tendons, is controlled by the peripheral nervous system and associated with the body's voluntary movements. Skeletal muscle is striated muscle. Unlike cardiac muscle, the cells are not branched. Skeletal muscle cells are covered by connective tissue, which protects and supports muscle fiber bundles. Blood vessels and nerves run through the connective tissue, supplying muscle cells with oxygen and nerve impulses that allow for muscle contraction. Skeletal muscle is organized into several muscle groups that work in coordination to perform body movements. Some of these groupings include head and neck muscles (facial expressions, chewing, and neck movement), trunk muscles (moving the chest, back, abdomen, and vertebral column), upper extremity muscles (moving the shoulders, arms, hands, and fingers), and lower extremity muscles (moving the legs, ankles, feet, and toes).Visceral (Smooth) Muscle: Visceral muscle is found in various parts of the body including the blood vessels, the bladder, and the digestive tract as well as in many other hollow organs. Like cardiac muscle, most visceral muscle is regulated by the autonomic nervous system and is under involuntary control. Visceral muscle is also called smooth muscle because it doesn't have cross striations. Visceral muscle contracts slower than skeletal muscle, but the contraction can be sustained over a longer period of time. Organs of the cardiovascular, respiratory, digestive, and reproductive systems are lined with smooth muscle. This muscle can be described as rhythmic or tonic. Rhythmic, or phasic, smooth muscle contracts periodically and spends most of the time in a relaxed state. Tonic smooth muscle remains contracted for the majority of the time and only relaxes periodically. Other Facts About Muscle Tissue Adults have a certain number of muscle cells. Through exercise, such as weight lifting, the cells enlarge but the overall number of cells does not increase. Skeletal muscles are voluntary muscles because we have control over their contraction. Our brains control skeletal muscle movement. However, reflex reactions of skeletal muscle are an exception. These are involuntary reactions to external stimuli. Visceral muscles are involuntary because, for the most part, they are not consciously controlled. Smooth and cardiac muscles are under the control of the peripheral nervous system.