Each muscle fibre bundle( fibre, bundle ) may be up to 2cm long, but less than 0.1mm in diameter. Inside the muscle fibre is the cytoplasm containing mitochondria vacuole( mitochondria, vacuole ) and the other organelles found in a cell. Within each muscle fibre there are also numerous myofibrils myosins( myofibrils, myosins ); each is composed of repeated contractile units blocks( repeated contractile units, blocks ) called sarcomeres.

Contractions Extensions( Contractions, Extensions ) are brought about by co-ordinated sliding of these protein filaments within the muscle cell sarcomeres tendons( sarcomeres, tendons ). The proteins overlap and give the muscle fibre its characteristic striped spotted( striped, spotted ) (striated) appearance under the microscope. When the muscle contracts, the actin myosin( actin, myosin ) moves between the myosin actin( myosin, actin ) - this shortens lengthens( shortens, lengthens ) the length of the sarcomere and hence the length of the muscle ligament( muscle, ligament ).

When a nerve impulse hormone( nerve impulse, hormone ) arrives at a neuromuscular junction, calcium sodium( calcium, sodium ) ions are released from the sarcoplasmic endoplasmic( sarcoplasmic, endoplasmic ) reticulum. The calcium ions diffuse are actively transported( diffuse, are actively transported ) through the sarcoplasm. This initiates the movement of protein filaments. Calcium ions attach to the troponin tropomyosin( troponin, tropomyosin ) molecule causing it to move. As a result, the tropomyosin troponin( tropomyosin, troponin ) on the actin filament shifts its position, exposing myosin-binding sites myosin heads( myosin-binding sites, myosin heads ) on the actin filaments. Myosin heads bind with myosin-binding sites on the actin filament, forming cross-bridges ionic interactions( cross-bridges, ionic interactions ). When the myosin heads bind to the actin, ADP and P ATP( ADP and P, ATP ) on the myosin head are is( are, is ) released. The myosin myosin head( myosin, myosin head ) changes shape, causing the myosin head myosin( myosin head, myosin ) to nod forward. This movement results in the relative movement of filaments; the attached actin myofibril( actin, myofibril ) moves over the myosin. An ATP DNA( ATP, DNA ) molecule binds to the myosin head - this causes the myosin head to detach from attach to( detach from, attach to ) the actin. An ATPase ATP synthase( ATPase, ATP synthase ) on the myosin head hydrolyses the ATP, forming ADP and P. This hydrolysis synthesis( hydrolysis, synthesis ) causes a change in the shape of the myosin head. It returns to its upright position. This enables the cycle to start again