Named for a mouse: the tissue that flexes like one
Muscle is one of the four basic animal tissues, and vertebrates have three kinds: skeletal, cardiac and smooth. The word itself is a joke about mice. Latin musculus means little mouse, because a flexed biceps looks like a mouse’s back, and the Greek word mys means both mouse and muscle.
All muscle tissue works through two contractile proteins, actin and myosin, regulated by troponin and tropomyosin. Skeletal and cardiac muscle are striped under the microscope because their sarcomeres sit in tidy rows; smooth muscle has no stripes. Skeletal muscle is the voluntary kind, tied to bone by tendons, with fibres from a few millimetres to several inches long and 10 to 100 micrometres wide. Heart and smooth muscle work without conscious control, and the different types respond in different ways to signals such as acetylcholine, adrenaline and nitric oxide.
Skeletal fibres come in slow-twitch and fast-twitch forms. Slow type I muscle is red with myoglobin, crowded with capillaries and mitochondria, and built for sustained aerobic work. Of the fast types, IIa is still aerobic, IIx is the quickest and strongest in humans but tires within short anaerobic bursts, and IIb is pale, glycolytic white muscle, the main fast type in rodents. A single mutation in lab mice cuts IIb muscle sharply and roughly halves hind-limb muscle mass. Muscle is about 15 percent denser than fat, at roughly 1.06 kilograms per litre.
Cardiac muscle exists only in the heart wall, where cells usually have one nucleus and join at branching intercalated discs; the coronary arteries keep them supplied. Smooth muscle lines the gut, airways, bladder, uterus and blood vessels, and even raises body hair through the arrector pili. In single-unit smooth muscle a whole sheet contracts together, while multiunit tissue gives finer, graded control.
Invertebrate muscle is sorted by striping pattern into transverse, oblique and smooth, and arthropods have no smooth muscle at all. In vertebrate embryos every muscle comes from paraxial mesoderm, segmented into somites whose myotome sends out myoblasts that fuse into fibres.
Source: Muscle
