Calcium and the mechanism of contraction

In skeletal muscle, calcium binds to troponin and causes the repositioning of tropomyosin. As a result, the myosin-binding sites on the actin become uncovered and crossbridge cycling takes place. Although an increase in cytosolic calcium is also needed in smooth muscle, its role in the mechanism of contraction is very different. Three major steps are involved in smooth muscle contraction:

• Calcium binding with calmodulin

• Activation of myosin kinase

• Phosphorylation of myosin

Upon entering the smooth muscle cell, Ca++ ions bind with calmodulin, an intracellular protein with a chemical structure similar to that of troponin. The resulting Ca++-calmodulin complex binds to and activates myosin kinase. This activated enzyme then phosphorylates myosin. Crossbridge cycling in smooth muscle may take place only when myosin has been phosphorylated.

Relaxation of smooth muscle involves two steps:

• Removal of calcium ions

• Dephosphorylation of myosin

Calcium ions are actively pumped back into the extracellular fluid as well as the sarcoplasmic reticulum. When the concentration of calcium falls below a certain level, steps one and two of the contractile process reverse. Calcium no longer binds with calmodulin and myosin kinase is no longer activated.

The dephosphorylation of myosin requires the activity of myosin phosphatase. Located in cytoplasm of the smooth muscle cell, this enzyme splits the phosphate group from the myosin. Dephosphorylated myosin is inactive; crossbridge cycling no longer takes place and the muscle relaxes.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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