The first to occur when a motor neuron stops sending an impulse to a muscle is the pumping of calcium ions out of the cytoplasm and back into the sarcoplasmic reticulum.
When a nervous impulse stops, these things happen:
- The membrane of the sarcoplasmic reticulum is no longer permeable to calcium (the calcium gates closed). Therefore, no calcium diffuses out.
- The calcium pump in the membrane starts transporting the calcium BACK into the sarcoplasmic reticulum.
- Calcium ions leave the binding sites on toponin molecules, making the troponin returns to its original shape and position. So does the tropomyosin. That makes the myosin head no longer in contact with actin, which leads to the muscle stops contracting (relaxing).
Learn more about sarcoplasmic reticulum at https://brainly.com/question/1540451
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