What stops the muscle contraction

Muscle contraction

Muscle contraction, Contraction (shortening) of muscles. Every M. is the result of numerous asynchronous shortening of the individual muscle cells or fibers of a muscle, ultimately their subcellular contractile elements, the myofibrils, and it occurs in all muscle types with ATP consumption due to the same basic molecular processes (contractile proteins, muscle proteins), which are best examined in the highly ordered striated muscles of vertebrates. The M. is accompanied by a cyclic binding and loosening of cross-bridges between actin and myosin filaments (actins, myosin). The latter, with their movable and laterally protruding HMM heads, shimmy along the surrounding actin filaments, so that actin and myosin filaments increasingly slide between each other (sliding filament mechanism).

The individual contraction cycle proceeds as follows: In the initial state of the resting ATP-rich muscle fiber, actin and myosin filaments are separate and can passively slide along each other (muscle relaxation), since the actin is displaced from a possible bond to the myosin heads by ATP. The ATP-loaded myosin molecules, in turn, are pretensioned in an energy-rich stretched conformation. Since the corresponding myosin binding sites on actin are sterically blocked by tropomyosin / troponin, the actin-activatable myosin-ATPase (adenosine triphosphatase) remains inactive. A momentary increase in the approx2+-Concentration in the sarcoplasma of approx. 10-8 on 10-5mol / l leads to the allosteric rearrangement of the troponin-tropomyosin complexes on the actin filament and releases the myosin binding sites on the actin, thus indirectly activating the myosin ATPase, which then forms the active enzyme as actomyosin ATPase (electromechanical coupling). This requires Mg as a cofactor2+. With hydrolysis of ATP to ADP + Pi Each myosin head can now bind to the closest actin, converting the bias energy of the myosin into kinetic energy, so that the myosin molecules snap back into their lower-energy, more angled conformation and pull the bound actin filaments along by an amount of around 10 nm. A renewed binding of ATP releases the actomyosin complexes again and prepares the next cycle of contraction. The cycles follow one another as long as there is enough ATP in the muscle and they are controlled by briefly changing Ca2+-Concentrations in the sarcoplasm.

In the resting state, the calcium remains stored in the cisterns of the sarcoplasmic reticulum (L-system). Its release takes place in response to a nervous stimulus. The nerve impulses trigger a calcium outflow through a short-term change in permeability. Immediately after the nervous impulse has subsided, calcium pumps in the membranes of the L-system move the Ca.2+ back to the L-cisterns until the next cycle is triggered.

Exceeding the muscle-type-specific contraction frequency due to excitation impulses following one another too quickly leads to energy-consuming continuous contraction (tetanus, cramp) of the individual muscle.

Even though the contraction of smooth muscle cells proceeds according to the same principle, the Ca is absent there2+-dependent tropomyosin-troponin control system, and the regulation of the sequence of the individual contraction cycles happens calcium-independently in a manner not yet known. The energy for the M. is provided in rapidly working, but also quickly tiring (white) muscle fibers mainly through glycolysis of carbohydrates to lactate, in the less rapidly working, but permanently resilient, myoglobin-rich (red) muscle fibers, however, are aerobic oxidative phosphorylation obtained. The depletion of ATP in muscle fibers after overload leads to the temporary formation of solid actomyosin complexes and causes the fibers concerned to freeze (muscle soreness), which causes rigor mortis when the ATP production ceases completely after the cell has died.