What is calcium’s role in muscle contraction?
| Calcium isn’t just important for bone, it is also the key factor that initiates muscle contraction. When skeletal muscle fibers become excited, calcium channels open and calcium floods in and around the myofibrils and bathes the sarcomeres. Calcium then interacts with specific proteins associated with actin and induces sarcomere contraction. The contraction of one muscle fiber is really the net result of the shortening of all the tiny sarcomeres in each myofibril within that cell. Further, the contraction of the muscle itself is the net result of contraction and shortening of muscle fibers that make up that muscle.
Skeletal muscle cells have another unique characteristic. They contain an organelle called the sarcoplasmic reticulum which is actually a modified version of the endoplasmic reticulum found in other cells. This organelle stores large quantities of calcium. In fact, when a skeletal muscle cell is stimulated, most of the calcium that bathes the sarcomeres actually comes from the sarcoplasmic reticulum. |
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What powers muscle contraction?
| In order for muscle fibers to contract, a lot of ATP must be used (see Sarcomere Contraction Figure). Some of the energy released from ATP is used to power the contraction. Interestingly, ATP is also necessary for a contracted muscle cell to “relax” as well. When the muscle is no longer being stimulated, ATP helps the thick and thin filaments to dissociate from each other so that each sarcomere can return to a relaxed (or unstimulated) position. Also, ATP is necessary to pump calcium out of intracellular fluid of the muscle fiber. Calcium is either pumped out of the cell or more likely into sarcoplasmic reticulum organelles. |  |
If ATP is deficient, muscle fibers become locked in a contracted state called rigor. Rigor mortis occurs when the human body dies as the integrity of muscle cell membranes decrease. This allows calcium to leak into the contracting regions of muscle fibers from the extracellular fluid and from within the sarcoplasmic reticulum. As a result, calcium bathes myofibrils and contraction is invoked. Usually there is enough ATP in these dying cells to power the contraction. The dying cell would then remains locked in a contracted state.
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