Ionic Strengths

By Terri Sota

Calcium has made a name as a builder of strong bones and teeth, but is deserving of credit for so many supporting roles. Not only does calcium facilitate nerve transmission and regulate blood pH, it is also at the heart of muscle function. Stored in the muscle cells in minute quantities (99 percent stored in bone and teeth), calcium enables the chemical reactions that create force and enact movement.

Muscle fibers contain many filament-filled myofibrils (cylinders of proteins). These filaments do the "heavy lifting" of motion and are of two types: thick strands made of myosin (protein) and thin filaments made of actin (also a protein). Two other proteins — tropomyosin and tropomin — are the molecular switches that enable the thin filaments to slide along the thick ones, causing muscle tension and contractions.

Voluntary muscle contraction is controlled by the central nervous system. When stimulated, the brain sends an electrical signal (action potential) that prompts the release of a neurotransmitter (chemical message), which crosses the gap (synapse) between nerve and muscle cell. This message initiates another action potential, which enters the muscle cell and opens the gates of the sarcoplasmic reticulum, the membranous organelle responsible for storing and pumping calcium ions. Once released, the ions flow into the cytoplasm and bind to troponin-tropomysin molecules located within the thin filaments. This action displaces tropomyosin and exposes actin-myson binding sites where thick filaments can grab onto the thin, pulling them forward in a sliding motion. As a result, contractions occur simultaneously down the length of the myofibril, and the muscle moves as the brain intended. Once the action .potential has passed, the calcium gates close and calcium pumps on the reticulum remove calcium from the cytoplasm.

Calcium is not produced by the body and must be obtained from foods or supplements. Dairy products contain the highest concentration per serving of absorbable calcium, but there are plenty of other sources available if milk, cheese and yogurt are scarce or contraindicated. The recommended requirement varies with age. Children between the ages of four and eight need 1,000mg; those between nine and 18 require 1,300mg. According to the Institute of Medicine, most people in the U.S. get enough calcium with the exception of teenage girls. Bone loss occurs at an accelerated rate for women mid-life, and without adequate bone mass built in their youth, older women are at greater risk for osteoporosis.

Classroom Discussion

• Ask students to keep a food diary for a week; compute their average daily calcium intake
• Have students plan real-world menus, which would ensure proper calcium consumption