Although it's not the first place you may think of calcium working in your body, muscle cells rely on calcium as an essential component to function properly. Calcium assists the mechanism in which your muscle fibers work to contract and produce movement. Calcium is involved every time your brain signals for a muscle to produce movement.
Microscopic Anatomy
To understand how calcium works in the muscular system, you must have a concept of how the muscles are arranged. Each muscle is made of muscle fibers, and inside each muscle fiber is a myofibril. Each myofibril is composed of the filaments that allow the muscle to contract. A muscular contraction occurs when a myosin filament attaches itself to an actin filament. The myosin pulls the actin, which shortens it and contracts the muscle.
Function
When your brain sends a signal to your muscle to move, the first step to movement is for your muscle to release calcium. When calcium is released, it travels through the muscle fiber and into the myofibril, where it interacts with the myofilaments and proteins that allow for a contraction. Myosin attaches to actin at a specific binding site. This binding site is blocked by a protein, tropomyosin, that must be moved for a contraction to occur. Calcium binds to troponin, which is attached to tropomyosin. When calcium attaches to troponin, it is able to move the tropomyosin to reveal the binding site for myosin. Once the binding site is open, myosin attaches to actin and a contraction occurs.
Storage and Release
Within each muscle fiber are numerous myofibrils, a sarcoplasm, a sarcoplasmic reticulum and transverse tubules. These organelles work in concert to produce a contraction. Your muscles store needed calcium within the sarcoplasmic reticulum. When a nerve innervates the muscle, the transverse tubule carries the electrical current from the nerve through the entire muscle fiber, also signaling for the calcium channels to open and release stored calcium into the myofibril.
Fatigue
Like any component to the muscular system, any alterations in calcium can have a negative effect. Fatigue may be caused by a lack of calcium or a malfunction in the release or reuptake of calcium. If there is not enough calcium available, actin and myosin may not be able to bond as effectively as they normally would. If reuptake of calcium by the sarcoplasmic reticulum is slowed, an abnormally long contraction may occur. If the sarcoplasmic reticulum does not effectively take in calcium released, wandering calcium may travel into the mitochondria and interfere with its ability to produce energy.
References
- LRN.org; Human Anatomy and Physiology: Muscular System; James Grass, Ph.D.; April 10, 2011
- "Anatomy and Physiology"; Kenneth S. Saladin; 2004
- "Exercise Physiology"; George A. Brooks, Thomas D. Fahey, Kenneth M. Baldwin; 2005
