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When its work is done, a muscle fiber relaxes and returns to its resting length. This is achieved by the steps shown in figure 11.11.

14. Nerve signals stop arriving at the neuromuscular junction, so the synaptic knob stops releasing ACh.

15. As ACh dissociates (separates) from its receptor, acetylcholinesterase breaks it down into fragments that cannot stimulate the muscle. The synaptic knob reabsorbs these fragments for recycling. All of this happens continually while the muscle is being stimulated, too; but when nerve signals stop, no new ACh is released to replace that which is broken down. Therefore, stimulation of the muscle fiber by ACh ceases.

16. Active transport pumps in the sarcoplasmic reticulum (SR) begin to pump Ca2+ from the cytosol back into the cisternae. Here, the calcium binds to a protein called calsequestrin (CAL-see-QUES-trin) and is stored until the fiber is stimulated again. Since active transport requires ATP, you can see that ATP is needed for muscle relaxation as well as for muscle contraction (see insight 11.2).

17. As calcium ions dissociate from troponin, they are pumped into the SR and are not replaced.

18. Tropomyosin moves back into the position where it blocks the active sites of the actin filament. Myosin can no longer bind to actin, and the muscle fiber ceases to produce or maintain tension.

A muscle returns to its resting length with the aid of two forces: (1) like a recoiling rubber band, the series-elastic components stretch it; and (2) since muscles often occur in antagonistic pairs, the contraction of an antagonist lengthens the relaxed muscle. Contraction of the triceps brachii, for example, extends the elbow and lengthens the biceps brachii.

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