Muscles and the structure of muscle fibres

Muscle is a tissue formed from mesoderm. It makes up 40-50% of an adult's body weight. Muscle can respond to a stimulus (excitability), shorten (contractility), be stretched (extensibility), and spring back (elasticity). Tighten your own biceps and feel it go hard: that is contraction.

There are three types:

• Skeletal muscles: attached to the bones. Under a microscope they show stripes (striated), and we control them at will (voluntary). They are used for locomotion and to keep our posture.
• Visceral muscles: in the walls of hollow organs like the gut and blood vessels. They have no stripes (smooth, nonstriated) and work on their own (involuntary). They move food along the gut and gametes along the reproductive tract.
• Cardiac muscles: found only in the heart. They are striated and branched, and involuntary.

A skeletal muscle is built of bundles (fascicles) held together by a layer of connective tissue, the fascia. Each bundle contains many muscle fibres. Each fibre is wrapped in a membrane, the sarcolemma ("sarco" means muscle), around the sarcoplasm (its cytoplasm). One fibre has many nuclei, because it formed by many cells fusing: it is a syncytium. Its ER, the sarcoplasmic reticulum, stores calcium ions.

Each fibre is packed with long threads, myofibrils, with alternating light and dark bands:

• Light I bands (isotropic), made of the protein actin (thin filaments). A dark line, the Z line, cuts each I band in half and holds the thin filaments.
• Dark A bands (anisotropic), made of the protein myosin (thick filaments). An M line in the middle holds the thick filaments together.

The part of a myofibril between two Z lines is a sarcomere: the functional unit of contraction. When a muscle shortens, the Z lines come closer together. At rest, the ends of the thin filaments overlap the ends of the thick ones; the middle of the A band, where they do not overlap, is the H zone.