Question

Contrast the role of Ca²⁺ in the contraction of a skeletal muscle fiber and a smooth muscle cell.

Short answer

In skeletal muscles, the contraction occurs as Ca²⁺attaches with troponin, whereas Ca²⁺binds with calmodulin in smooth muscles. Another difference in the role of Ca²⁺in skeletal and smooth muscles contraction is the source of Ca²⁺.

The skeletal muscles derive Ca²⁺ from sarcoplasm only whereas, the Ca²⁺ for smooth muscles is from both sarcoplasmic reticulum and extracellular fluid.

Step-by-step solution

Skeletal and smooth muscles

Skeletal muscles are voluntary muscles that allow the movements of body parts. They have sarcomeres providing striations to the muscle. This type of muscle attaches to bones by tendons.

Smooth muscles are involuntary muscles that lack sarcomeres and hence no striations. This type of muscle is present in internal visceral organs such as the stomach and intestine.

Contraction of muscles

Both skeletal and smooth muscles follow the sliding filament process consisting of actin and myosin to undergo contraction. Both the muscles require Ca²⁺ for contraction. This ion helps to establish interactions between actin, myosin, and calcium-binding proteins.

Contrasting role of Ca2+ in the contraction of skeletal muscle fiber and a smooth muscle cell

The role of Ca²⁺in the contraction of skeletal and smooth muscles varies depending on two factors. They are the site of attachment of calcium and the source of calcium.

In skeletal muscles, the site of attachment forCa²⁺is troponin. This attachment of Ca²⁺with troponin moves tropomyosin away from myosin-binding sites on actin and results in skeletal muscle contraction.

In smooth muscles, thesite of attachment forCa²⁺is calmodulin. Calmodulin helps to activate the myosin light chain kinase and allows the myosin head phosphorylation. It enables cross bride formation and sliding of actin and myosin filaments.

In skeletal muscle contraction, the source of Ca²⁺ is in the sarcoplasmic reticulum only. On the other hand, smooth muscle contraction derives Ca²⁺ from both sarcoplasmic reticulum and extracellular fluid.