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What is muscle contraction?

Meaning of Muscle Contraction

Muscle contraction refers to actin slipping over myosin in muscle cells, allowing the body to move. Muscle fibers contain the contractile protein filaments of actin and myosin, arranged side by side. These filaments are repeated throughout the muscle fiber, forming the sarcomere.

Concomitantly, the H band, formed only by the thick filaments (myosin), also decreases, since these filaments overlap thin filaments (actin). This will result in a large shortening of the sarcomere.

Functioning

Muscle contraction depends on the availability of calcium ions and muscle relaxation depends on the absence of these ions. Calcium ion flux is regulated by the sarcoplasmic reticulum (SR), for rapid performance of muscle contraction cycles. RS is a network of cisternae of the smooth endoplasmic reticulum, which surrounds and separates groups of myofilaments into cylindrical bundles.

When depolarized, RS passively releases calcium ions to the thick and thin filaments. When repolarized, the RS transports the calcium ion to the cisternae, interrupting contractile activity.

Uniform contraction of each muscle fiber is the responsibility of the T tubule system. This system consists of a complex network of tubular invaginations of the muscle fiber sarcolemma.

Motor nerves control the normal contraction of skeletal muscle fibers. Branching within the connective tissue of the perimysium at this innervation site, the nerve loses its myelin sheath and forms the dilation found within a depression in the muscle fiber surface .

This structure is called the neural plate or myoneural junction , where the axon has numerous mitochondria and synaptic vesicles, and releases acetylcholine, which diffuses across the synaptic cleft, the motor endplate, and binds to specific receptors for the sarcolemma of the junctional fold. .

A nerve fiber can innervate a single muscle fiber, or branch out and innervate up to 160 muscle fibers, forming a motor unit. The number of motor units in a given muscle is related to the delicate movement required of the muscle.

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