Muscle contraction

Voluntary and involuntary movements in animals are mainly by contraction of muscles, namely the skeletal muscle, smooth muscle and cardiac muscle. Skeletal muscle fibres are made up of many myofibrils, which are the basic contractile elements of the muscle cell. Myofibrils are made up of repeating units known as sarcomeres, which cause striation in the fibre.

Each sarcomere consists of overlapping thick and thin filaments respectively known as myosin and actin. The sliding of actin and myosin filaments changes the degree of overlap between them and causes contraction. During contraction,the sarcomere shortens considerably but the length of the actin and myosin filaments stays the same.

When an action potential is triggered in the sarcoplasmic membrane, it travels down the T tubules, activating Ca²⁺ channels in the sarcopalsmic reticulum. There is an influx of Ca²⁺ ions in the cytosol, which initiates the contraction of the myofibrils.

The actin filament has two associated proteins, tropomyosin  and troponin.Tropomyosin binds along the groove of the actin helix. Troponin is trimeric and made up of troponin T, I and C. Troponin T and I are inhibitory and bind to actin and tropomyosin, blocking the myosin binding sites on the actin molecule. Troponin C binds to Ca²⁺ molecules and causes troponin T and I to release their hold on actin.The tropomyosin molecule moves to reveal the myosin binding sites and the myosin heads bind and rotate using energy from ATP hydrolysis, to cause contraction .