Answer:
Since calcium is an ion that allows the contraction of the sarcomere, if the calcium does not leave the sarcomere the muscle contraction is maintained and the sarcomere is unable to relax.
Explanation:
Calcium (Ca²⁺) is necessary for muscle contraction to occur. Once the muscle fiber receives the signal that starts the contraction, cytoplasmic Ca²⁺ levels increase —by intracellular Ca²⁺ release and extracellular Ca²⁺ input—causing muscle contraction.
If for some circumstance the cytoplasmic Ca²⁺ levels do not decrease, muscle contraction would be maintained with no possibility of relaxation. This is a brief summary of the process by which Ca promotes muscle contraction:
- Tropomyosin is a molecule that blocks the bond between actin and myosin.
- Ca²⁺, found in the sarcoplasmic reticulum, exits the sarcoplasm and binds to troponin C, whose interaction with tropomyosin leaves actin and myosin free to bind.
- Ca²⁺, in a high concentration, promotes the union of the filaments, shortening the muscular fiber and producing the contraction.
The Ca²⁺ pumps of the sarcoplasmic reticulum and the Na⁺/Ca²⁺ exchanger decrease the levels of intracellular Ca, by storing it again in the reticulum or taking it to the extracellular space, respectively. This decreases intracellular Ca²⁺ concentrations.
If this process does not occur properly, Ca²⁺ levels remain elevated and muscle contraction is maintained, without relaxation occurring.
