The Muscular System Muscle Contraction • Action potentials travel along T tubules to the sarcoplasmic reticulum, where they cause the release of calcium ions. • Calcium ions, released from the sarcoplasmic reticulum, bind to the actin myofilaments, exposing attachment sites. • Myosin forms cross-bridges with the exposed actin attachment sites. • The myosin molecules bend, causing the actin molecules to slide past; this is the sliding filament mechanism. This process requires ATP breakdown. • A muscle twitch is an all-or-none response to an action potential that exceeds the threshold; it consists of a lag phase, contraction phase, and relaxation phase. • Tetanus occurs when stimuli occur so rapidly that a muscle does not relax between twitches. • Small contraction forces are generated when small numbers of motor units are recruited, and greater contraction forces are generated when large numbers of motor units are recruited.
This is the end of the preview.
access the rest of the document.