Question

the most important factor in decreasing the intracellular concentration of calcium ion after contraction is
active transport of calcium into the synaptic cleft.
diffusion of calcium out of the cell.
active transport of calcium across the sarcolemma.
diffusion of calcium into the sarcoplasmic reticulum.
active transport of calcium into the sarcoplasmic reticulum.

Explanation:

After muscle contraction, the sarcoplasmic reticulum (SR) plays a crucial role in decreasing intracellular calcium ion (\(Ca^{2+}\)) concentration. The SR actively transports \(Ca^{2+}\) back into its lumen using a \(Ca^{2+}\)-ATPase pump (active transport, as it requires energy - ATP).

• Calcium in the synaptic cleft is related to neurotransmitter release at the neuromuscular junction, not post - contraction intracellular \(Ca^{2+}\) regulation.
• Diffusion of \(Ca^{2+}\) out of the cell is not the main mechanism for reducing intracellular \(Ca^{2+}\) after contraction.
• Active transport across the sarcolemma (cell membrane of muscle fiber) is not the primary way to sequester \(Ca^{2+}\) after contraction.
• Diffusion into the SR is not sufficient as the concentration gradient (higher \(Ca^{2+}\) in the cytoplasm than in the SR after contraction) would oppose simple diffusion, and active transport is needed.

Answer:

active transport of calcium into the sarcoplasmic reticulum.