Answer
Fig. 11.13
The energy of ATP is needed to pump Ca++ into the sarcoplasmic reticulum (SR)-this is an active transport process.
Work Step by Step
ATP is also needed to induce the myosin heads to release actin molecules so that the sarcomere can relax.
At threshold a muscle goes through a quick cycle of contraction and relaxation ( a twitch). The contraction phase is relatively brief because the SR quickly resorbs Ca++ ions. As the Ca++ concentration falls in the cytoplasm, myosin releases the thin actin filaments and the muscle relaxes.
