A theory that explains how muscles contract. Each sarcomere (the functional unit of the muscle) contains overlapping thin (see actin) and thick (see myosin) filaments that can be interconnected by cross bridges. According to the theory, a shortening of sarcomere length is brought about by the two types of filaments sliding past each other by means of a ratchet-like mechanism of the cross bridges. Strong intermolecular forces occurring between the myosin head and cross bridge, cause the head to tilt. By means of this so-called power stroke, the thin filaments are pulled into the space between the thick filaments in each sarcomere. Contraction is triggered by a stimulatory nerve impulse that causes an action potential to spread across the sarcomere. The action potential causes calcium ions to be released around the filaments, enabling the cross bridges from myosin to attach onto the actin (in the absence of calcium, the attachment sites are blocked by tropomyosin. Adenosine triphosphate provides the energy used by the ratchet mechanism. See also rigor complex.
Sliding-filament theory
