Muscle fibers are comprised of elongated myofibrils. A myofibril is made of repeating chains of sarcomeres which have no nucleus but have myosin and actin filaments which are able to slide past each other during muscle contraction. As a result, Z discs may move closer together and the H zone may shorten, but the actin and myosin filaments always remain the same size. Muscle contraction occurs when an action potential reaches the end of a motor neuron. Using neurotransmitters (primarily acetylcholine) at the neuromuscular junction, the action potential is transferred to the muscle fibre as the sarcoplasmic reticulum rapidly releases Ca 2+ ions. These calcium ions cause conformational changes in the myosin and actin, exposing binding sites. Myosin heads then bind to sites on the actin and form relatively stable cross-bridges. By reforming these cross-bridges at different spots on the actin filament using ATP, the actin filaments at either side of the H zone move closer together towards the M band. ATP causes the release of myosin heads from actin, whereas its conversion to ADP + Pi changes the angle of the myosin head. As this cycle repeats, the sarcomeres are shortened, allowing for movement of skeletal muscles.
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