Answer
Muscle Relaxation
When nerve stimuli stop arriving at the neuromuscular junction, the synaptic knob stops releasing acetylcholine (ACH). Acetylcholinesterase (ACHe) , breaks down the ACH in the synaptic cleft into molecular fragments. These fragments ( choline, acetyl-CoA ) are reabsorbed by the synaptic knobs for reuse.
Work Step by Step
During excitation-contraction coupling the sarcolemma releases and reabsorbs Ca++, simultaneously. When neural stimulation ceases, Ca++ release stops, but reabsorption continues.. In the sarcoplasmic reticulum (SR) Ca++ binds to the protein calsequestrin which enables the SR to store large amounts of Ca++ without the formation of calcium salts.
The reabsorption of calcium by the SR results in a decrease of [Ca++] in the sarcoplasm. One consequence of this decline in sarcoplasmic levels of Ca is the dissociation of Ca++ ions from troponin. This loss-- without replacement-- causes the tropomyosin complex to recover the conformation (initial) in which it is able to block the active sites on actin filaments.
This blocking of actin sites means the myosin can no longer bind actin filaments and the muscle fiber loses tension. However for the muscle to return to initial resting length it must be stretched by some other force. Thus after the flexion of the elbow the flexor muscles ( brachialis, biceps brachii) only return to original length under the stretching of the elbow extender ( triceps brachii) or the pull of gravity on the fore arm.
