How does muscle contraction work

When a a sarcomere b contracts, the Z lines move closer together and the I band gets smaller. The A band stays the same width and, at full contraction, the thin filaments overlap. When a sarcomere shortens, some regions shorten whereas others stay the same length.

A sarcomere is defined as the distance between two consecutive Z discs or Z lines; when a muscle contracts, the distance between the Z discs is reduced. The H zone—the central region of the A zone—contains only thick filaments and is shortened during contraction.

The I band contains only thin filaments and also shortens. The A band does not shorten—it remains the same length—but A bands of different sarcomeres move closer together during contraction, eventually disappearing. Thin filaments are pulled by the thick filaments toward the center of the sarcomere until the Z discs approach the thick filaments. The zone of overlap, in which thin filaments and thick filaments occupy the same area, increases as the thin filaments move inward.

The motion of muscle shortening occurs as myosin heads bind to actin and pull the actin inwards. This action requires energy, which is provided by ATP. Myosin binds to actin at a binding site on the globular actin protein. ATP binding causes myosin to release actin, allowing actin and myosin to detach from each other. The enzyme at the binding site on myosin is called ATPase. The myosin head is then in a position for further movement, possessing potential energy, but ADP and Pi are still attached.

If actin binding sites are covered and unavailable, the myosin will remain in the high energy configuration with ATP hydrolyzed but still attached. If the actin binding sites are uncovered, a cross-bridge will form; that is, the myosin head spans the distance between the actin and myosin molecules. Pi is then released, allowing myosin to expend the stored energy as a conformational change. The myosin head moves toward the M line, pulling the actin along with it. As the actin is pulled, the filaments move approximately 10 nm toward the M line.

This movement is called the power stroke, as it is the step at which force is produced. As the actin is pulled toward the M line, the sarcomere shortens and the muscle contracts. This energy is expended as the myosin head moves through the power stroke; at the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the cross-bridge formed is still in place, and actin and myosin are bound together.

ATP can then attach to myosin, which allows the cross-bridge cycle to start again and further muscle contraction can occur Figure 6.

Watch this video explaining how a muscle contraction is signaled. With each contraction cycle, actin moves relative to myosin. View this video animation of the cross-bridge muscle contraction. When a muscle is in a resting state, actin and myosin are separated. Furthermore, X-ray diffraction experiments allowed to provide experimental evidence for the postulated structural difference between attached weak-binding and attached strong-binding cross-bridges.

Finally, recent studies have confirmed the prediction of Eisenberg and Greene that the rate limiting step in vitro determines the rate of force generation in muscle. Abstract Muscle contraction occurs when the thin actin and thick myosin filaments slide past each other. ATP is critical for muscle contractions because it breaks the myosin-actin cross-bridge, freeing the myosin for the next contraction.

Muscles contract in a repeated pattern of binding and releasing between the two thin and thick strands of the sarcomere. ATP first binds to myosin, moving it to a high-energy state.

ADP and Pi remain attached; myosin is in its high energy configuration. With each contraction cycle, actin moves relative to myosin.

The muscle contraction cycle is triggered by calcium ions binding to the protein complex troponin, exposing the active-binding sites on the actin. As soon as the actin-binding sites are uncovered, the high-energy myosin head bridges the gap, forming a cross-bridge. Once myosin binds to the actin, the P i is released, and the myosin undergoes a conformational change to a lower energy state.

When the actin is pulled approximately 10 nm toward the M-line, the sarcomere shortens and the muscle contracts. At the end of the power stroke, the myosin is in a low-energy position. After the power stroke, ADP is released, but the cross-bridge formed is still in place. ATP then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site. ATP can then attach to myosin, which allows the cross-bridge cycle to start again; further muscle contraction can occur.

Therefore, without ATP, muscles would remain in their contracted state, rather than their relaxed state. Tropomyosin and troponin prevent myosin from binding to actin while the muscle is in a resting state. Describe how calcium, tropomyosin, and the troponin complex regulate the binding of actin by myosin.

The binding of the myosin heads to the muscle actin is a highly-regulated process. When a muscle is in a resting state, actin and myosin are separated. To keep actin from binding to the active site on myosin, regulatory proteins block the molecular binding sites. Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation, which prevents contraction in a muscle without nervous input.

The protein complex troponin binds to tropomyosin, helping to position it on the actin molecule. To enable muscle contraction, tropomyosin must change conformation and uncover the myosin-binding site on an actin molecule, thereby allowing cross-bridge formation. Troponin, which regulates the tropomyosin, is activated by calcium, which is kept at extremely low concentrations in the sarcoplasm.

If present, calcium ions bind to troponin, causing conformational changes in troponin that allow tropomyosin to move away from the myosin-binding sites on actin. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction. Muscle contraction : Calcium remains in the sarcoplasmic reticulum until released by a stimulus.

Calcium then binds to troponin, causing the troponin to change shape and remove the tropomyosin from the binding sites. Cross-bridge cling continues until the calcium ions and ATP are no longer available. The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm. Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the muscle cell to relax.

During stimulation of the muscle cell, the motor neuron releases the neurotransmitter acetylcholine, which then binds to a post-synaptic nicotinic acetylcholine receptor. A change in the receptor conformation causes an action potential, activating voltage-gated L-type calcium channels, which are present in the plasma membrane.

The inward flow of calcium from the L-type calcium channels activates ryanodine receptors to release calcium ions from the sarcoplasmic reticulum. This mechanism is called calcium-induced calcium release CICR. It is not understood whether the physical opening of the L-type calcium channels or the presence of calcium causes the ryanodine receptors to open.

The outflow of calcium allows the myosin heads access to the actin cross-bridge binding sites, permitting muscle contraction. Excitation—contraction coupling is the connection between the electrical action potential and the mechanical muscle contraction.

Excitation—contraction coupling is the physiological process of converting an electrical stimulus to a mechanical response. It is the link transduction between the action potential generated in the sarcolemma and the start of a muscle contraction. Excitation-contraction coupling : This diagram shows excitation-contraction coupling in a skeletal muscle contraction. The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells.

A neural signal is the electrical trigger for calcium release from the sarcoplasmic reticulum into the sarcoplasm. Each skeletal muscle fiber is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. The area of the sarcolemma on the muscle fiber that interacts with the neuron is called the motor-end plate. A small space called the synaptic cleft separates the synaptic terminal from the motor-end plate.

Because neuron axons do not directly contact the motor-end plate, communication occurs between nerves and muscles through neurotransmitters. Neuron action potentials cause the release of neurotransmitters from the synaptic terminal into the synaptic cleft, where they can then diffuse across the synaptic cleft and bind to a receptor molecule on the motor end plate. The motor end plate possesses junctional folds: folds in the sarcolemma that create a large surface area for the neurotransmitter to bind to receptors.

Acetylcholine ACh is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate. Once released by the synaptic terminal, ACh diffuses across the synaptic cleft to the motor end plate, where it binds with ACh receptors. This reduces the voltage difference between the inside and outside of the cell, which is called depolarization. As ACh binds at the motor end plate, this depolarization is called an end-plate potential.

The depolarization then spreads along the sarcolemma and down the T tubules, creating an action potential. Troponin is attached to the protein tropomyosin within the actin filaments, as seen in the image below.

When the muscle is relaxed tropomyosin blocks the attachment sites for the myosin cross bridges heads , thus preventing contraction. When the muscle is stimulated to contract by the nerve impulse, calcium channels open in the sarcoplasmic reticulum which is effectively a storage house for calcium within the muscle and release calcium into the sarcoplasm fluid within the muscle cell. Some of this calcium attaches to troponin which causes a change in the muscle cell that moves tropomyosin out of the way so the cross bridges can attach and produce muscle contraction.

In summary the sliding filament theory of muscle contraction can be broken down into four distinct stages, these are;. Muscle activation: The motor nerve stimulates an action potential impulse to pass down a neuron to the neuromuscular junction. This stimulates the sarcoplasmic reticulum to release calcium into the muscle cell. Muscle contraction: Calcium floods into the muscle cell binding with troponin allowing actin and myosin to bind.

The actin and myosin cross bridges bind and contract using ATP as energy ATP is an energy compound that all cells use to fuel their activity — this is discussed in greater detail in the energy system folder here at ptdirect. Recharging: ATP is re-synthesised re-manufactured allowing actin and myosin to maintain their strong binding state 4.

Relaxation: Relaxation occurs when stimulation of the nerve stops. Calcium is then pumped back into the sarcoplasmic reticulum breaking the link between actin and myosin. Actin and myosin return to their unbound state causing the muscle to relax.