Myofibrils

contractile filaments in the protoplasm of striated muscle fibers of the skeletal musculature, myocardium, and muscles with double diagonal striation. They range in diameter from 0.5 to several microns.

In cross section myofibrils are round, angular, or oval. Most of them are made up of very fine protein filaments called myofilaments, or protofibrils. There are two types of myofilaments: thick myofilaments, which consist primarily of myosin and are about 1,500 nanometers (nm) long and 10–15 nm in diameter; and thin myofilaments, which consist primarily of actin and are 1,000–1,100 nm long and 5–8 nm in diameter. Other proteins found in myofilaments are tropomyosin B, which occurs in thin myofilaments of all kinds; tropomyosin A, or paramyosin, which is present in the thick myofilaments of muscles with double diagonal striation; α-actinine and β-actinine; and troponin.

Thin myofilaments adhere to the Z band, a complex intertwining of protein filaments. The portion of a myofibril that lies between two Z bands is called the sarcomere. Thick myofilaments make up the anisotropic band (the A band), the solid, birefringent portion of a myofibril. Thin myofilaments are partly wedged in between the thick myofilaments (the zone of overlap). The sections of sarcomeres that are located on either side of the Z membranes and that contain only thin myofilaments are referred to as the isotropic, or I band. The central zone of the A band, which lacks thin myofilaments, is called the H zone. An M band consisting of short (40-nm) M-threads running along the longitudinal axis of the myofibril can usually be seen in the center of the H zone. The length of the M-threads equals the width of the M band. On both sides of the M band are the H subzones, narrow (approximately 130 nm) bands lighter than the rest of the H zone. Distributed evenly over the entire length of the thick myofilaments are projections (“bridges”) that are evidently the ends of myasin molecules that branch off the myofilaments. The H subzones appear to be lighter because the middle of the thick myofilaments lacks the myosin bridges.

The hypothetical structure of myofibrils is open to criticism. For example, when the myofibrils are stretched a great deal, the thin myofilaments should emerge completely from the A band, and the sarcomere should fragment. However, this does not happen, possibly because of the existence of a third type of myofilament—“ultrathin filaments,” which connect the Z bands.