Mechanism of muscle contraction and its regulation

1. The contraction begins with a nerve impulse. In the synapse, acetylcholine is released. It stimulates sarcolemma, depolarizes the membrane and generates action potential at its surface.

2. The action potential spreads deep into the muscle fiber reaches the sarcoplasmic reticulum and promotes the release of calcium ions from the reticulum in the sarcoplasm.

3. Calcium ions activate ATPase centers in the heads of myosin. ATP is cleaved, ADP, and P_i remain on myosin. Interaction site of the myosin heads is blocked by troponin.

4. Calcium ions bind to troponin and unlock the myosin heads. Myosin head is free to rotate and when it reaches the desired position it is associated with F-actin, forming with the axis the fibril angle of 90°.

5. Myosin heads and the active sites of actin form crosslinking adhesions – an actin-myosin complex.

6. ADP and P_i are split off from the heads of myosin; the released energy is used for the conformational changes of contractile proteins.

7. Myosin heads are bent. The angle the myosin with the axis of fibrils is changed from 90° to 45°. Tension is created between thick and thin filaments. Thin filament moves to the direction of the sarcomere.

8. The new molecule of ATP binds to the myosin – F-actin complex.

9. Myosin – F-actin complex has a low affinity for actin, and myosin head is separated from the F-actin. There is relaxation of the muscle (Fig. 10).

Fig. 10. The mechanism of muscle contraction

The overall contraction process is the result of summing a large number of adhesion formation along the entire length of myofibrils involved in the process of contraction the excited muscle. Calcium ions play a key regulatory role. Myofibrils interact with ATP and contracts at C(Ca) = 10^-6-10^-5 M. If the excitation ceases, the concentration of calcium ions in the sarcoplasm is decreased, and the heads of the myosin filaments stop to attach actin filaments. In the presence of ATP, the muscle relaxes, and its length reaches the initial phase. If there isn’t ATP synthesis (anoxia, death, gassing), the muscle stiffness is developed. The myosin cross bridges attached to actin filaments, resulting in immobility of the muscles.

Date: 2016-04-22; view: 1093