Sliding wire model of muscle contraction. The actin filaments slide beyond the myosin filaments towards the center of the sarcomere. The result is a shortening of the sarcomere without changing the length of the filament. In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts.  This happens when the force generated by the muscle exceeds the load that counteracts its contraction. With the exception of reflexes, all skeletal muscle contractions occur as a result of conscious exertion that comes from the brain. The brain sends electrochemical signals via the nervous system to the motor neuron, which innervates several muscle fibers.  In some reflexes, the contraction signal may come from the spinal cord through a feedback loop with gray matter. Other actions such as locomotion, breathing and chewing have a reflex aspect: contractions can be triggered both consciously and unconsciously. Watch this video animation of the muscle contraction of the transverse bridge. Muscle contraction usually stops when signals emanate from the end of the motor neuron, which repolarizes the sarcolemma and T tubules and closes the voltage-controlled calcium channels in the SR. The Ca++ ions are then pumped into the SR, forcing tropomyosin to protect (or cover) the binding sites at the actin strands.
A muscle can also stop contracting when it lacks ATP and gets tired (Figure 2). The number of transverse bridges formed between actin and myosin determines the tension that a muscle fiber can create. Transverse bridges can only form where thick, thin filaments overlap, allowing myosin to bind to actin. As more transverse bridges form, more myosin will pull on the actin and create more tension. Figure 4. Contraction of skeletal muscles. (a) The active center on actin is exposed because calcium binds to troponin. b) The myosin head is attracted to actin, and myosin binds actin to its actin binding site, forming the transverse bridge. c) During the coup de force, the phosphate produced during the previous contraction cycle is released. This causes the myosin head to rotate towards the center of the sarcomaer, after which the ADP and attached phosphate group is released. d) A new ATP molecule attaches to the myosin head, loosening the transverse bridge.
e) The myosin head hydrolyzes ATP to ADP and phosphate, which brings the myosin back to the tense position. Muscle contraction is described by the sliding filament contraction model. ACh is the neurotransmitter that binds to the neuromuscular compound (NMJ) to trigger depolarization, and an action potential moves along the sarcolemma to trigger the release of calcium by SR. Actin sites are exposed after Ca++ enters the sarcoplasm from its SR memory to activate the troponin-tropomyosin complex so that the tropomyosin moves away from the sites. The bridging of myposin heads docking at actin binding sites is followed by “Power Stroke” – the sliding of thin filaments through thick filaments. Performance shots are determined by the ATP. Ultimately, sarcomeres, myofibrils, and muscle fibers shorten to create movement. With substantial evidence, Hugh Huxley formally proposed the sliding filament mechanism and is variously referred to as the oscillating cross bridge model, the cross bridge theory or the cross bridge model.   (He himself preferred the name “Swinging Crossbridge Model” because, as he recalled, “it was, after all, the 1960s.” ) He published his theory in the June 20, 1969 issue of Science under the title “The Mechanism of Muscular Contraction.”  According to his theory, filament sliding occurs by cyclic fixation and detachment of myosin on actin filaments. Contraction occurs when myosin pulls the actin filament towards the center of the A-band, detaches from the actin and creates a force (blow) to bind to the next actin molecule.  This idea was later widely proven and is better known as the transition cycle.  When an event changes the permeability of the membrane to Na+ ions, they enter the cell.
This changes the tension. This is an electrical event called action potential that can be used as a cellular signal. Communication between nerves and muscles takes place via neurotransmitters. The action potentials of neurons cause the release of neurotransmitters from the synaptic terminal into the synaptic cleft, where they can then diffuse through the synaptic cleft and bind to a receptor molecule on the motor end plate. The end plate of the motor has junctional folds – folds in the sarcolemma that create a large area for the neurotransmitter to bind to the receptors. Receptors are actually sodium channels that open to allow Na+ to pass through the cell when they receive a neurotransmitter signal. Contractile accumulations of actin and myosin, which resemble small versions of muscle fibers, are also present in non-muscle cells. As in muscle, the actin filaments in these contractile arrangements are interdigitated with bipolar filaments of myosin II, consisting of 15 to 20 molecules of myosin II, which create a contraction by pushing the actin filaments relative to each other (Figure 11.26). .