Sangeetha Pulapaka

A single skeletal muscle is made up of many muscle fibres (muscle cells), each of which is divided into many myofibrils. Myofibrils contain sarcomeres, repeating units of thick and thin filaments. These filaments are the contractile apparatus of muscles

Huxley proposed the sliding filament theory of contraction. According to the theory, a sarcomere shortens when its thin filaments slide along its thick filaments as shown in the below figure.

High-magnification electron micrographs show that the myosin filaments are rod shaped with globular end ( called the myosin head). The head can form a cross-bridge with acitin, the protein in the thin filaments. When attached to actin, a myosin head can change shape and slide the actin further along the myosin. During a muscle contraction, these cross-bridges are formed and broken down repeatedly upto 100 times per second, causing the sarcomere to shorten by a rachet-like mechanism. The mechanism can only shorten a sacromere. It cannot actively return the sarcomere to its original length ; muscle elongation is usually brought by the action of antagonistic muscles, Calcium ions are required for cross-bridges to form and the breakdown of ATP provides the energy needed by the rachet mechanism. The combined actions of millions of sarcomeres can contract  a whole muscle to about half of its resting length. Contraction is started by a nerve impulse which triggers the availability of calcium ions and ATP.

The role of calcium ions

When a muscle is at rest, calcium ions are not present in the sarcoplasm, because they are stored in the sarcoplasmic reticulum, fine membrane-bound channels in the muscle fibres. In the absence of calcium ions in the sarcoplasm, tropomyosin ( a protein in thin filaments) prevents the myosin heads from attaching onto actin by blocking the binding sites. When a muscle is stimulated sufficiently by nerve impulses calcium ions are released form the sarcoplasmic reticulum and combine with troponin (another protein in thin filaments) causing the tropomyosin to change shape and unblock the binding sites.

Calcium ions spread through the sacroplasm, when the muscle contracts enabling myosin heads to bind onto actin (another protein). Energy from the breakdown of ATP enables the heads to take up a new position.