The duration of a contraction depends on the duration of stimulation at the neuromuscular junction. The ACh released after a single action potential arrives at the synaptic terminal does not remain intact for long. Whether it is bound to the sarcolemma or is free in the synaptic cleft, the released ACh is rapidly broken down and inactivated by AChE. Inside the muscle fiber, the permeability changes in the SR are also very brief. A contraction will therefore continue only if additional action potentials arrive at the synaptic terminal in rapid succession. When they do, the continual release of ACh into the synaptic cleft produces a series of action potentials in the sarcolemma that keeps Ca²⁺ levels elevated in the sarcoplasm. Under these conditions, the contraction cycle will be repeated over and over.

If just one action potential arrives at the neuromuscular junction, Ca²⁺ concentrations in the sarcoplasm will quickly return to normal resting levels. Two mechanisms are involved in this process: (1) active Ca²⁺ transport across the cell membrane into the extracellular fluid and (2) active Ca²⁺ transport into the SR. Of the two, transport into the SR is the more important. Virtually as soon as the calcium ions have been released, the SR returns to its normal permeability and begins actively absorbing Ca²⁺ from the surrounding sarcoplasm. As Ca²⁺ concentrations in the sarcoplasm fall, (1) calcium ions detach from troponin, (2) troponin returns to its original position, and (3) the active sites are re-covered by tropomyosin. The contraction has ended.

Once the contraction has ended, the sarcomere does not automatically return to its original length. Sarcomeres shorten actively, but there is no active mechanism for reversing the process. External forces must act on the contracted muscle fiber to stretch the myofibrils and sarcomeres to their original dimensions. We will describe those forces in a later section.

Before you proceed, review the entire sequence of events from neural activation through excitation–contraction coupling to the completion of a contraction. Table 10-1 provides a summary of the contraction process, from ACh release to the end of the contraction.