Formerly known as sarcostyles, myofibrils are long, bundled tubes of cytoskeleton that run the length of striated muscle fibers. Like all cytoskeletons, myofibrils function in cellular support, movement, and intra-cellular transport. To facilitate this purpose, they are made up of long chains of regular, repeating units known as sarcomeres. These units house the contractile apparatus of the cell. Two microfilaments, primarily composed of actin and myosin, interact within the sarcomeres to produce cellular contraction — enabling movement of the cell, the muscle, and the entire organism.
The two microfilaments that make up the myofibril are generally referred to as thick and thin filaments. Thick filaments are composed mostly of myosin protein and reside near the center of the sarcomere. Thin filaments are made up of three proteins, most notably actin, and sit at the outer edges of the sarcomere. The border between sarcomeres is known as the Z line, a dark band of material that acts as a base for the thin filaments.
Muscle cells themselves are analogous to other cells in a number of ways, however, their increased size and high degree of specialization results in many of their attributes being given names that are particular to muscle cells. This usually involves the use of the prefix 'sarco-.' The cytoplasm of a muscle cell, therefore, becomes the sarcoplasm; the endoplasmic reticulum is known as the sarcoplasmic reticulum; and the cellular membrane is often termed the sarcolemma.
Myofibrils reside within the sarcoplasm and typically occupy most of the space within the muscle cell. Running parallel with the myofibrils are infoldings of sarcolemma known as transverse tubules, or T tubules. These internal channels primarily provide a pathway for neurons. Following the same pathways as other structures within the cell, a specialized organelle known as sarcoplasmic reticulum runs alongside the T tubules. The sarcoplasmic reticulum acts as a storage system for calcium ions.
When a T tubule carries an electrical signal, known as an action potential, into the muscle fiber, the sarcoplasmic reticulum responds by releasing calcium ions into the sarcoplasm. Once they are moving freely through the sarcoplasm, the calcium ions are able to bind to specialized structures on the actin and myosin proteins within the myofibrils. In doing so, they pull the thin filaments toward the center of the sarcomere, effectively shortening the whole unit. This process is known as the sliding filament model of muscular contraction.