Then comes a brief period of contraction, when the myosin heads are binding, and pulling, and releasing, over and over, and the muscle fibers contract.
They're divided lengthwise into segments called sarcomeres, which contain two even tinier strands of protein — two different kinds of myofilaments called actin and myosin.
A sarcomere contains both thin filaments, made up mostly of two light and twisty actin strands, and thick filaments, composed of thicker, lumpy-looking myosin strands.
That tension means myosin and the calcium pumps are burning up the muscle cells' ATP, and the finite supply of ATP is what makes it impossible to maintain vigorous muscle activity indefinitely.
But soon the fiber slides back down into the relaxation period, when the calcium gets pumped back into the sarcoplasmic reticulum, and the actin and myosin stop the binding cycle, and the muscle relaxes.
If another action potential travels down before that can happen, even more calcium gets released, which ends up exposing more actin for myosin to bind to, and that means more force in that fiber.
You'd also re-learn that your skeletal muscles are constructed like a rope made of bundles of protein fibers, and that the smallest strands are your actin and myosin myofilaments.
That binding causes another shape change. But this time, it causes the myosin to release from the actin, in a tear-jerking scene like some microscopic re-creation of the finale from Titanic.
Because this is when the myosin breaks down its new molecule of ATP into ADP and a phosphate, which moves it into the armed position yet again, getting it ready for its next rendezvous.
因为这是肌球蛋白将其新的 ATP 分子分解成 ADP 和磷酸盐的时候,这使它再次处于武装状态,为下一次会合做好准备。
When they bind, the myosin releases all that stored energy, and — in the excitement of it all — the myosin changes shape. It pulls on its precious actin strand, kind of like pulling a rope hand over fist.
So the calcium latches on to the troponin and causes it to pull the other bodyguard protein — the tropomyosin — away from the sites on the actin strands that the myosin really wants to get its paws on.
When a myosin head does that, it moves into an extended position, kinda like a stretched spring — still holding on to the ADP and phosphate, and still storing the energy that was released when they were broken apart.