Physiology of Muscle Contraction
- The thick myofilaments are composed of a protein called myosin. Each myosin filament has small regular projections known as crossbridges. The crossbridges lie in a radial fashion around the long axis of the myofilament. The rounded heads of the crossbridges lie in apposition to the thin myofilaments.
- The thin myofilaments are composed of a complex protein called actin, arranged in a double stranded coil. The actin filaments also contain two additional proteins called troponin and tropomysin.
- Contraction of skeletal muscle tissue occurs as actin and myosin myofilaments slide past one another, causing the sarcomeres to shorten.
- Many sarcomeres are joined end-to-end to form myofibrils. Shortening of the sarcomeres causes myofibrils to shorten, thereby causing the entire muscle to shorten.
- The sliding of actin myofilaments past myosin myofilaments during contraction is called the sliding filament model of muscle contraction.
- In a resting muscle fibre the myosin crossbridges are prevented from combining with the actin filaments by the presence of troponin and tropomysin.
- When a nerve impulse reaches a muscle fibre it is conducted over the sarcolemma and in to the T-tubules, then to the sarcoplasmic reticulum. The sarcoplasmic reticulum releases calcium ions into the sacrcoplasm. The liberated calcium ions combine with troponin causing it to push tropomysin away from the receptor sites on the actins filaments. The myosin crossbridges interact with the actin receptor sites and pull the actins myofilaments toward the centre (H-zone) of each sarcomere. The bond between the myosin crossbridges and actin breaks down under the influence of enzymes and the crossbridges are then free to rejoin with other actin receptor sites. The actin filaments do not shorten but slide past the myosin filaments overlapping them so that the Z lines are drawn toward each other, shortening the sarcomere. As each sarcomere shortens the whole muscle fibre contracts.
Figure – Contraction of the muscle fibres
- Relaxation of the muscle fibres occurs when the calcium ions are actively reabsorbed by the sarcoplasmic reticulum thus allowing troponin and tropomysin to again inhibit the interaction of the actins and myosin filaments.
Figure – Relaxation of the muscle fibres
Summary of events in the contraction of a muscle fibre
Figure – Events in the Contraction of a Muscle Fibre
The Sliding Filament Model of Contraction
When signaled by a motor neuron, a skeletal muscle fiber contracts as the thin filaments are pulled and then slide past the thick filaments within the fiber’s sarcomeres. This process is known as the sliding filament model of muscle contraction. The sliding can only occur when myosin-binding sites on the actin filaments are exposed by a series of steps that begins with Ca++ entry into the sarcoplasm.
Tropomyosin is a protein that winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin from binding to myosin. Tropomyosin binds to troponin to form a troponin-tropomyosin complex. The troponin-tropomyosin complex prevents the myosin “heads” from binding to the active sites on the actin microfilaments. Troponin also has a binding site for Ca++ ions.
To initiate muscle contraction, tropomyosin has to expose the myosin-binding site on an actin filament to allow crossbridge formation between the actin and myosin microfilaments. The first step in the process of contraction is for Ca++ to bind to troponin so that tropomyosin can slide away from the binding sites on the actin strands. This allows the myosin heads to bind to these exposed binding sites and form cross-bridges. The thin filaments are then pulled by the myosin heads to slide past the thick filaments toward the center of the sarcomere. But each head can only pull a very short distance before it has reached its limit and must be “re-cocked” before it can pull again, a step that requires ATP.