Final answer:
Neurotransmitters are predominantly broken down by enzymes, a process essential for the regulation of neural communication and the preparation of the postsynaptic membrane for subsequent signals.
Step-by-step explanation:
Many neurotransmitters are broken down by enzymes in the synaptic cleft to terminate the signal. The removal of the neurotransmitter from the synaptic cleft is crucial for the postsynaptic membrane to reset and be ready to receive the next signal. This breakdown can prevent continuous stimulation of the postsynaptic neuron and allow for the precise control of neural communication.
There are several mechanisms for clearing neurotransmitters from the synaptic cleft: diffusion away from the area, degradation by enzymes, or reuptake by the presynaptic neuron. Drugs that target these processes can either enhance or diminish neurotransmission. For instance, some Alzheimer's drugs inhibit acetylcholinesterase, which is the enzyme responsible for degrading the neurotransmitter acetylcholine, leading to increased neurotransmission.