Final answer:
Acetylcholine is broken down by acetylcholinesterase after it initiates muscle contraction, to prevent continuous muscle stimulation. The byproducts are reabsorbed or removed from the synaptic area. This coordinated process ensures muscle contraction and relaxation following precise control by the nervous system.
Step-by-step explanation:
After neurotransmitters such as acetylcholine(ACh) have bound to receptors on the muscle fiber at the neuromuscular junction, initiating a muscle contraction, they are quickly broken down to terminate the signal and prevent continuous muscle contraction. The enzyme acetylcholinesterase is responsible for this breakdown, removing acetylcholine from the synaptic cleft. The byproducts of this degradation are then reabsorbed by the presynaptic neuron, recycled, or further broken down and removed from the synaptic area. In the case of a functioning motor neuron, the release of acetylcholine leads to a series of events that allow an action potential to move along the sarcolemma. This action potential triggers calcium release from the sarcoplasmic reticulum, which then allows the actin-myosin cross-bridging necessary for muscle contraction. After the contraction, the calcium ions return to their storage sites within the sarcoplasmic reticulum, and the muscle relaxes. The sliding filament theory describes this process of muscle contraction and relaxation in more detail.