The Structure of Muscle

The structure of muscle can be viewed at the electron micrograph level shown in Figure 8.9a. The muscle tissue is composed of bundles of muscle cells called muscle fibers. Within a muscle fiber are myofibrils, which are also arranged in bundles. Individual myofibrils contain the structurally distinct regions described below. Myofibrils have thin filaments composed of actin and thick filaments composed of myosin. Arrangement of the thick and thin filaments in a myofibril produces the distinctive pattern in Figure 8.9a and Figure 8.15.

Muscle contraction can be explained by the sliding filament model on the basis of the structures illustrated in Figure 8.9a and described as follows:

Sarcomere - A basic unit of contraction in a myofibril. The bounds of the sarcomere are defined by the Z lines.

Z line - A region within an I band that defines the end of a sarcomere.

I band - A portion of the sarcomere which contains the Z line and which contains thin filaments only. During muscular contraction, the region of the I band will shrink as the thin filaments are pulled towards the center of each respective sarcomere by the thick filaments.

A band - A region of a myofibril composed of thick filaments and thin filaments. At the center of the A band, the H zone provides a space for the thick filaments to pull the thin filaments towards. The overall size of the A band remains the same during muscular contraction (Figure 8.10), but the H zone size diminishes.

H zone - A region in the center of the sarcomere of the myofibril composed almost exclusively of thick filaments. During muscular contraction, the H zone shrinks in size as thin filaments are pulled in to fill the space of the H zone.

Thin filaments are composed of a polymer of actin (called F-actin) arranged in a helix, tropomyosin (a fibrous protein that exists as elongated dimers lying along, or close to, the groove in the F-actin helix), and three small proteins called troponins I, C, and T. The presence of tropomyosin and the troponins inhibits the binding of myosin heads to actin unless calcium is present at a concentration of about 10^-5M. In resting muscle, calcium concentrations are approximately 10^-7M, so new cross bridges between the thick and thin filaments cannot occur.

Signals from the nervous system that cause muscle to contract are conveyed to the sarcoplasmic reticulum via the transverse tubules (Figure 8.15). Upon receipt of the signal from the nerves, the sarcoplasmic reticulum releases calcium, which signals the muscles to contract.

Striated Muscle - Striated muscle can be divided into two categories - red muscle, designed for relatively continuous use, and white muscle, employed for occasional, often rapid motions. Red muscle owes its dark color to abundant heme proteins. It is well supplied with blood vessels and, therefore, with hemoglobin. It has many mitochondria with cytochromes and it has large stores of myoglobin. Red muscle depends heavily on aerobic metabolism in mitochondria, so the primary energy source in red muscle is the oxidation of fat. White muscle, on the other hand, relies on glycogen as a primary energy source.

INTERNET LINK: Skeletal Muscle Structure