Skeletal muscle, also called striated muscle because of its striped appearance under the microscope, is used to move the skeleton. Skeletal muscles are under the direct control of the nervous system and can produce contractions ranging from quick twitches to powerful sustained tension. Individual muscle cells called muscle fibers are among the largest cells in the human body. body.
Ranging from 10 to 100 micrometers in diameter, each muscle fiber runs the entire length of the muscle, which may be as much as 35 centimeters or about 14 inches in a human thigh. Each muscle fiber, in turn, contains many individual contractile subunits, myofibrils, extending from one end of the fiber to the other. Each cylindrical myofibril is surrounded by sarcoplasmic reticulum, a complex of membranes forming a network of interconnected hollow tubes. The sarcoplasmic reticulum contains a fluid rich in calcium ions.
Deep indentations of the muscle cell membrane, called transverse or T-tubules, extend down into the muscle fiber, passing very close to portions of the sarcoplasmic reticulum. This arrangement of T-tubules and sarcoplasmic reticulum is crucial, as we will see, to controlling muscle contraction. Myofibrils contain subunits called sarcomeres made up of precise arrangements of actin and myosin filaments. Sarcomeres are attached end-to-end throughout the length of the myofibril, and their junction points are called Z-lines. Attached to the Z-lines are strands of actin and two accessory proteins that form the thin filaments.
Suspended between the thin filaments are thick filaments composed of the protein myosin. The alternating thick and thin filaments give myofibrils their striped appearance. Small arms, called cross bridges, extend from the strands of myosin and contact the thin filaments.
The actin protein of the thin filament is formed from a double chain of subunits resembling a twisted double strand of pearls. Each subunit has a binding site for a myosin cross bridge. In a relaxed muscle, however, these sites are covered by two accessory proteins.
which prevent the myosin cross bridges from attaching to the thin filaments. But when a muscle contracts, the accessory proteins on the thin filaments are moved aside, so myosin cross bridges can attach to the binding sites on the actin subunits of the thin filaments.