Biological Systems MCAT Practice Exam 2026 – The Comprehensive All-in-One Guide for Exam Success!

During repolarization in muscle cells following contraction, calcium is sequestered back into the sarcoplasmic reticulum. This process is essential for muscle relaxation after contraction. When a muscle cell is stimulated, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm, allowing actin and myosin filaments to interact and cause contraction. To end this contraction and initiate relaxation, calcium must be removed from the sarcoplasm.

The sarcoplasmic reticulum actively pumps calcium ions back into its lumen, reducing the calcium concentration in the cytoplasm. This decrease in calcium concentration leads to the detachment of myosin from actin, allowing the muscle fiber to return to its resting state.

The other choices describe processes that do not occur during repolarization. While sodium is involved in the action potential and membrane depolarization, it is not directly linked to the calcium dynamics during repolarization. The synthesis of acetylcholine relates to the formation of this neurotransmitter rather than the mechanisms of muscle contraction and relaxation. Thus, the primary event during repolarization is indeed the resequestration of calcium back into the sarcoplasmic reticulum.