A myofiber is a multinucleated single muscle cell (see Figure 1, below). Physically, they range in size from a under a hundred microns in diameter and a few millimeters in length to a few hundred microns across and a few centimeters in length. The cell is densely packed with contractile proteins, energy stores and signaling mechanisms.

Figure 1: Myonuclei identified along the length of an isolated muscle fiber. Nuclei were stained with a florescent dye that binds to DNA. (Micrograph kindly provided by V. Reggie Edgerton (University of California, Los Angeles).

The myofiber is the smallest complete contractile system. As such, it requires subsystems for metabolism, excitation and contraction.

Metabolism

Metabolism in muscle cells is much like metabolism in any cell. Nutrients are brought into the cell and oxidized to release energy. The most common examples of this are glycolysis and fatty acid breakdown. Muscles make use of other energy intermediaries, as well.

Excitation

For effective force production, contraction must be excited along the whole length of a fiber simultaneously. The contraction signal is spread rapidly along the fiber by means of the T-tubule system, which signals the rapid release of calcium ions from the sarcoplasmic reticuluum (SR). As soon as the contraction signal ends, ATP-driven calcium pumps begin sequestering almost all the intracellular calcium in this SR.

Contraction

Within each myofiber are a network of myofibrils. These fibrils contain the proteins that do the actual force production. It is because of these fibrils that skeletal muscle demonstrates its characteristic striated pattern. An extensive network of proteins binds each myofibril to its neighboring fibril and to the cell membrane.

Maintenance

Each fiber is associated with a population of satellite cells. Sometimes referred to as adult myoblasts, these cells are capable of dividing and fusing with existing fibers. This fusion appears to be an essential part of fiber growth and hypertrophy.