Which of the Following Events Occurs in a Muscle That

Question

Which of the following events occurs in a muscle that is relaxing?
A) The muscle fiber shortens.
B) AChE is present in the synaptic cleft.
C) Released calcium ions are present.
D) Myosin heads bind to active sites on the thin filaments.
E) Hydrolysis of ATP occurs.

Accepted Answer

AChE (acetylcholinesterase) is present in the synaptic cleft during muscle relaxation, which degrades any remaining acetylcholine and stops the stimulation of the muscle fiber.

Answer: C 
 Released calcium ions are present during muscle relaxation, but their concentration decreases as they move back into the sarcoplasmic reticulum due to active transport pumps.

Answer: E 
 Hydrolysis of ATP occurs during relaxation, as the myosin heads use ATP to detach from the active sites on the thin filaments and return to their original conformation.  A is incorrect because muscle fibers do not shorten during relaxation, D is incorrect because myosin cannot bind to active sites during relaxation.