Question

How is coordinated muscle relaxation achieved at the scale of a sarcomere?

A) Independent relaxation of sarcomeres
B) Uniform cooperative switching of regulatory states
C) Global ATP flooding for simultaneous relaxation
D) Sequential relaxation of adjacent sarcomeres

Answer 1

Coordinated muscle relaxation at the scale of a sarcomere is achieved through sequential relaxation of adjacent sarcomeres, a process in which ATP-driven pumps move calcium out of the sarcoplasm back into the sarcoplasmic reticulum.

Step-by-step explanation:

Coordinated muscle relaxation at the scale of a sarcomere is achieved through sequential relaxation of adjacent sarcomeres. When the motor neuron stops releasing acetylcholine (ACh) at the neuromuscular junction, the muscle fiber repolarizes. This closes the gates in the sarcoplasmic reticulum (SR), causing ATP-driven pumps to move calcium (Ca^2+) out of the sarcoplasm and back into the SR. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fiber loses tension and relaxes.