The sliding filament theory

The sliding filament theory

1. According to the theory, a sarcomere shortens when its thin filaments slide along its thick filaments. High-magnification electron micrographs show that the thich myosin are rod shaped with a globular end (myosin head). The head can form a cross-bridge with actin. When attached, a myosin head can change shape and slide the actin further along the myosin. The mechanism can only shorten a sarcomere. It cannot actively return the sarcomere to its orginal length. Muscle elongation is usually brought about by the action of antagonstic muscles.

2. Calcium ions are required for cross-bridges to form, and the breakdown of ATP provides the energy needed by thr rachet mechanism. The combined actions of millions of sarcomeres can contract a whole mucle to about half its resting length. Contraction is started by the nerve impulse which triggers the release of calcium ions and the generation of ATP

3. When a muscle is at rest, calcium ions are not present in the sarcoplasm because they are stored in the sarcoplasmic reticulium, fine membrane-bound channels in the muscle fibres. In the absence of calium ions in the sarcoplasm, tropomyosin prevents myosin heads from attaching onto actin by blocking the binding sites

4. Calcium ions are released from the sarcoplasmic reticulium at the end of a sequence of events which begins when an action potential reaches a neuromusclular junction. When the graded potential in the sarcolema exceeds the threshold level, an action potential sweeps across the msucle fibre and passes into mebrane tubles called T-tubules or transverse tubles that fold inwards from the sarcolema Where the T tubules make contact with the sarcoplasmic reticulium, the action potential causes the sarcoplamic reticulium to release calcium ions into the sarcoplasm. Calcium ions spread through the sarcoplasm, enabling myosin heads to bind onto the actin. Energy from the breakdown of ATP enables the heads to take up a new postion

5. When the action potentials stop arriving, calcium is actively pumped back into the sarcoplasmic reticulium, tropomyosin blocks the myosin head binding sites on the actin and the muscle relaxes