When you hear the word calcium, bone health is likely your first thought. No doubt, calcium is a key component of strong bones but its presence in your muscles enables movement. Contraction and relaxation of your muscles occur because of rapidly changing concentrations of calcium inside your muscle cells, a biochemical process referred to as the calcium cycle. Too much or too little calcium in your blood may cause muscular symptoms due to disruption of the calcium cycle.
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Muscle Cell Anatomy
To understand the calcium cycle, you need to know a bit about muscle cell anatomy. Strands of fibers called myofilaments are the distinguishing feature that separates muscle cells from other types of cells in your body. A muscle contraction represents shortening of the myofilaments with individual cells that comprise the muscle. A tiny tube-like network called the sarcoplasmic reticulum, or SR, surrounds each myofilament. In a relaxed state, the SR contains a high concentration of calcium. The SR controls muscle contraction and relaxation by regulating calcium flow inside your muscle cells.
The Calcium Cycle
Muscle contraction begins with an electrical "go" signal from your brain. The signal stimulates the SR to open its calcium gates, flooding the myofilaments with calcium. The sudden increase in calcium concentration sets off an energy-consuming chain reaction that causes the myofilaments to change shape and shorten. The simultaneous shortening of thousands of microscopic myofilaments leads to muscle contraction.
When the "go" signal stops, the SR calcium gates close and calcium pumps quickly return the mineral from the myofilaments to the interior of the SR. As the calcium concentration around the myofilaments falls, your muscle returns to a relaxed state.
Fast- and Slow-Twitch Fibers
Your muscles contain different types of fibers, broadly categorized as fast or slow twitch. As the name implies, fast-twitch fibers contract and react more rapidly than slow-twitch fibers. Your slow-twitch fibers, however, have greater endurance than fast-twitch fibers, rather like the tortoise and the hare. The SR in fast-twitch fibers is more developed than in slow-twitch fibers, enabling faster calcium release calcium in response to a "go" signal. Your muscles contain a mixture of fast- and slow-twitch fibers, although one or the other predominates in any given muscle.
A low blood calcium level, or hypocalcemia, causes muscle irritability. With this condition, you may experience muscle cramps in your back and legs. Twitching of your facial muscles may also occur. An abnormally high blood calcium level, or hypercalcemia, causes muscle sluggishness and weakness. Muscular symptoms typically do not occur with hypercalcemia unless your blood calcium level is critically elevated.