What Is Muscle Contraction?

Muscle contraction is the contractile response of muscles to stimuli. In a narrow sense, it refers to the contraction of vertebrate skeletal muscles by transmitting active potentials. A single active potential produces a single contraction, and repeated active potentials produce tetanic contraction. Most cases of muscle contraction that do not pass the active potential are caused by non-transmissive depolarization. Depolarization is limited to local muscles and is transient, which is called local contraction. Depolarization is called contraction if it is throughout and persistent in the muscle. The continuous contraction seen in smooth muscles is generally called spasticity, but many are still accompanied by repeated active potentials or continuous depolarization. However, the sustained contraction seen in the bivalve shell muscle and the like does not change the potential, and this contraction is due to the gate structure.

Muscle contraction is a basic characteristic of muscle tissue and refers to the mechanical response of muscle fibers after receiving a stimulus. The maintenance of body posture, spatial movement, complex movements, and breathing exercises are all achieved through muscle contraction activities ^[1]

According to the myofilament sliding theory, the basic process of muscle contraction is: muscle cells generate action potentials, causing

Increased in concentration,

By binding to troponin C, troponin undergoes a conformational change, weakening the binding of troponin to actin, and tropomyosin undergoes a conformational change, exposing the binding site on actin, the bridge and muscle Actin binds, and the bridge is twisted, dragging the thin filament toward the center of the thick filament. Through the process of the cross-bridge cycle consisting of the cross-bridge's binding, twisting, dissociation and recombination, and then twisting, the thin filaments continue to slide and the sarcomere shortens. The energy during muscle contraction comes from the energy released from the hydrolysis of ATP ^[2]

(I) Isotonic contraction and isometric contraction Isotonic contraction refers to the contraction form in which the muscle is contracted, the main manifestation is that the length changes but the tension is basically unchanged.
2. Isometric contraction refers to a contraction form in which the muscle is contracted, mainly showing that the tension changes and the length is basically constant. ^[2]
(B) single contraction and tetanic contraction A single contraction is a contraction and relaxation of the muscle after a short stimulus. The systolic process is divided into three periods: latency, systole, and diastole.
2. Tonic contraction means that the muscles are continuously stimulated. When the stimulation frequency reaches a certain level, the last contraction falls on the sum of the previous contractions and a strong and continuous contraction occurs. If the stimulation frequency is low, the last contraction occurs during the diastole of the previous contraction process, which is called incomplete tonic contraction, and the jagged curve is described in the experiment. If the stimulus frequency is high, the last contraction occurs during the previous contraction period, which is called complete tonic contraction, and the smooth curve is described.
In human muscle activity, both isotonic contraction and isometric contraction exist. Under the regulation of the nervous system, the muscle can quickly adjust the intensity of the contraction by the sum of the contractions. There are two forms of the sum: the sum of the number of motor units and the sum of frequency effects (that is, the frequency of motor neurons' impulses can affect the form and intensity of muscle contraction, such as tonic contraction) ^[2]