Lihaskontraktsioon

Lihaskontraktsioon is the process by which muscle fibers shorten, generating force and movement. This complex physiological event is fundamental to all voluntary and involuntary muscle actions in the body. The primary mechanism involves the interaction of actin and myosin filaments within the muscle cell.

The process is initiated by a nerve impulse transmitted from a motor neuron to the muscle fiber at the neuromuscular junction. This signal triggers the release of calcium ions from the sarcoplasmic reticulum. Calcium ions play a crucial role by binding to troponin, a protein complex on the actin filaments. This binding causes a conformational change in tropomyosin, another protein, which exposes the myosin-binding sites on the actin filaments.

Myosin heads, which are energized by ATP, can now bind to these exposed sites on the actin. This binding forms cross-bridges. The myosin heads then pivot, pulling the actin filaments towards the center of the sarcomere, the basic contractile unit of the muscle. This sliding filament mechanism results in the overall shortening of the muscle fiber.

The cycle of cross-bridge formation, pivoting, and detachment, fueled by ATP, continues as long as calcium ions are present and nerve stimulation persists. When the nerve impulse ceases, calcium ions are pumped back into the sarcoplasmic reticulum, and the muscle fiber relaxes, returning to its resting length. Different types of muscle contractions exist, including isometric (muscle length does not change) and isotonic (muscle length changes).