Structure of Skeletal Muscle

A skeletal muscle -- so called because most skeletal muscles are attached to and move some part of the skeleton -- is built as a hierarchy of progressively finer structures, each nested inside the one before it.

A whole muscle, such as the biceps, is made of a number of muscle bundles or fascicles, each enclosed by a connective-tissue sheath. Each fascicle is in turn made of many individual muscle fibres running parallel to one another along the length of the muscle. A single muscle fibre is a long, cylindrical, unbranched cell that, unusually, is multinucleate (contains many nuclei lying just under its surface) because it is formed during development by the fusion of many separate embryonic cells (myoblasts). The plasma membrane of a muscle fibre is called the sarcolemma, and its cytoplasm is called the sarcoplasm; the sarcoplasm is rich in an extensive network of tubules and sacs called the sarcoplasmic reticulum, which is the muscle fibre's specialised store of calcium ions -- calcium that plays the key triggering role in contraction, described later in this chapter.

Packed lengthwise inside the sarcoplasm of every muscle fibre are hundreds to thousands of thread-like myofibrils, running the full length of the fibre. Each myofibril is not a smooth, uniform thread but is made of a long chain of repeating structural and functional units placed end to end, and each such unit is called a sarcomere. It is the sarcomere that actually shortens during contraction, and because thousands of sarcomeres are arranged in series along a myofibril, and thousands of myofibrils run in parallel through a fibre, even a very small shortening of each individual sarcomere adds up to a substantial, useful shortening of the whole muscle. …