muscle tissue Flashcards
(4 cards)
Skeletal muscle
Skeletal muscle tissue is packaged by connective tissue sheets into organs called skeletal muscles, which are attached to the skeleton. These muscles, which can be controlled voluntarily (or consciously), form the flesh of the body, the so-called muscular system (see Chapter 6). When the skeletal muscles contract, they pull on bones or skin. As a result, gross body movements or changes in our facial expressions occur.
The cells of skeletal muscle are long, cylindrical, and multinucleate, and they have obvious striations (stripes). Because skeletal muscle cells are elongated to provide a long axis for contraction, they are often called muscle fibers.
Cardiac muscle
Cardiac muscle (covered in more detail in Chapter 11) is found only in the heart wall. As it contracts, the heart acts as a pump to propel blood through the blood vessels. Like skeletal muscle, cardiac muscle has striations, but cardiac cells have only a single nucleus and are relatively short, branching cells that fit tightly together (like clasped fingers) at junctions called intercalated discs. These intercalated discs contain gap junctions that allow ions to pass freely from cell to cell. This ties the cardiac cells into a functional syncytium (syn = together, cyt = cell), resulting in rapid conduction of the electrical signal to contract across the heart. Cardiac muscle is under involuntary control, which means that we cannot consciously control the activity of the heart. Can you imagine having to “tell” your heart to beat like you “tell” your legs to walk?
Smooth (visceral) muscle
Smooth (visceral) muscle is so called because no striations are visible. The individual cells have a single nucleus and are tapered at both ends. Smooth muscle is found in the walls of hollow organs such as the stomach, uterus, and blood vessels. As smooth muscle in its walls contracts, the cavity of an organ alternately becomes smaller (constricts when smooth muscle contracts) or enlarges (dilates when smooth muscle relaxes) so that substances are mixed and/or propelled through the organ along a specific pathway. Contractions of smooth muscle are longer and slower than the other two muscle types. Peristalsis (per”ĭ-stal′sis), a wavelike motion that keeps food moving through the small intestine, is typical of its activity.
