Question

multiple choice question

when excitation of a muscle fiber stops, the sr _______

reabsorbs calcium
releases calcium

Explanation:

Identify the physiological state

Using the Muscle Contraction Initiation knowledge point, excitation of a muscle fiber involves action potentials that trigger the release of calcium ions (\(\text{Ca}^{2+}\)) from the sarcoplasmic reticulum (SR) into the sarcoplasm, initiating contraction.

Determine the effect of stopping excitation

Using the Sarcoplasmic Reticulum Anatomy knowledge point, when motor neuron stimulation and muscle fiber excitation cease, active transport pumps (SERCA) in the SR membrane actively pump calcium back inside. This reabsorption of calcium decreases sarcoplasmic calcium levels, leading to muscle relaxation.

Answer:

• reabsorbs calcium (Correct answer)
• releases calcium