Question

The factor(s) that determine the duration of a twitch in various types of fibers is the speed of the

A) removal of Ca₂₊ ions from the sarcoplasm only.
B) hydrolysis of ATP only.
C) movement of Na+ across their membranes only.
D) removal of Ca₂₊ ions from the sarcoplasm and hydrolysis of ATP.
E) removal of Ca₂₊ ions from the sarcoplasm and hydrolysis of ATP and movement of Na+ across their membranes.

Answer 1

The duration of a muscle twitch is predominantly determined by the speed at which Ca2+ ions are removed from the sarcoplasm and the rate of ATP hydrolysis.

Step-by-step explanation:

The factors that determine the duration of a twitch in various types of muscle fibers are mainly the removal of Ca2+ ions from the sarcoplasm and the hydrolysis of ATP. When the neurotransmitter acetylcholine (Ach) is released by a motor neuron, it triggers an action potential that leads to the release of Ca2+ ions from the sarcoplasmic reticulum (SR) into the sarcoplasm. This release of calcium ions initiates muscle contraction. The contraction continues as long as Ca2+ ions remain in the sarcoplasm and ATP is present to fuel the cross-bridge cycling between actin and myosin. For the muscle to relax, these Ca2+ ions must be pumped back into the SR, which requires ATP. Furthermore, the speed of contraction is also affected by how quickly the ATPase on myosin can hydrolyze ATP, with faster hydrolysis resulting in more rapid contractions.