Final answer:
Acetylcholinesterase is an enzyme that breaks down the neurotransmitter acetylcholine within the synaptic cleft, thereby terminating the signal transmission between neurons. The correct answer is A. An enzyme.
Step-by-step explanation:
Acetylcholinesterase is an enzyme that plays a crucial role in the cholinergic system, which is the neurotransmitter system of acetylcholine. This system includes not only acetylcholine itself, which is a neurotransmitter responsible for transmitting signals across the synaptic cleft, but also the receptors it binds to and the enzyme acetylcholinesterase. The primary function of acetylcholinesterase (AChE) is to break down acetylcholine in the synaptic cleft, effectively terminating the signal transmission between neurons.
During the process of neurotransmission, acetylcholine (ACh) is released from the presynaptic neuron. It binds to ACh receptors on the post-synaptic membrane to relay the neural signal. Once acetylcholine has completed its function, acetylcholinesterase degrades it into choline and acetate (acetic acid), ensuring that the signal is only transient and allowing the neuron to reset and be ready for the next signal.
Therefore, the correct answer to the question is A. An enzyme.