Mechanism of Muscle Contraction

Muscle contraction involves a series of steps that allow the actin and myosin filaments to slide past each other which results in the shortening of the sarcomere. Following are the steps involved in muscle contraction:

Excitation

The process of muscle contraction begins with excitation, which involves the initiation of an electrical impulse called an action potential. This action potential originates from a motor neuron in the brain or spinal cord and travels until it reaches the neuromuscular junction, also known as the motor end plate. It is the region where the motor neuron connects with the muscle fiber.

Neuromuscular Junction

At the neuromuscular junction, the action potential triggers the release of a neurotransmitter called acetylcholine into the synaptic cleft. Acetylcholine binds to receptors on the sarcolemma, which leads to the generation of another electrical impulse along the sarcolemma.

Transmission by T-tubules

The electrical impulse propagates along the sarcolemma and goes throughout the muscle cell through invaginations in sarcolemma called T-tubules. These T-tubules allow the electrical impulse to reach the interior of the muscle fiber and ensure simultaneous activation of the whole muscle cell.

Release of Calcium Ions

The transmembrane signal in the T-tubules results in the release of calcium ions from the sarcoplasmic reticulum. The calcium ions diffuse into the cytoplasm

Calcium-Binding to Troponin

Within the muscle fiber, the calcium ions bind to a regulatory protein called troponin present in thin filaments. This binding causes a conformational change and exposes the myosin-binding sites on the actin filaments making it ready for contraction.

Cross-Bridge Formation

ATP is hydrolyzed into ADP and inorganic phosphate in the presence of Mg²⁺ and Ca²⁺ ions. When ADP is removed, the head of meromyosin binds strongly with myosin binding sites on the actin filaments. This is known as Cross-Bridge Formation or Actomyosin interaction or Power Stroke.

Sliding of Filaments

When inorganic phosphate is removed, the head of the meromyosin moves backward and it pulls thin filaments inside, towards the center of the sarcomere. Thin filaments slide over thick filaments and thick filaments do not slide. This sliding movement shortens the sarcomeres.

Detachment of Cross-Bridge

After the power stroke, ATP is formed using high-energy phosphate like phosphocreatine. In this process, Creatinine (excretory product) is formed as a by-product. This ATP binds with the ATP binding site on the head of meromyosin. This results in the detachment of actin and myosin.

These steps like attachment, power stroke, detachment, and reactivation of myosin heads repeat and this cycle continues as long as the calcium concentration remains high and ATP is available.