Final answer:
The relationship between a neuron's receptor site and a neurotransmitter is best described as a lock and key mechanism, allowing for specific and precise binding necessary for neurotransmission.
Step-by-step explanation:
The relationship between the receptor site on a neuron and the neurotransmitter can BEST be characterized as similar to the relationship between a lock and key. This analogy illustrates how specific neurotransmitters fit into specific receptors in a highly selective manner, much like how a key fits into its corresponding lock. In the brain, neurotransmitters such as acetylcholine are released into the synaptic cleft, where they then diffuse across to the postsynaptic neuron and bind to their respective receptor proteins, initiating a change in the postsynaptic membrane. This specific binding is a crucial part of neurotransmission, ensuring precise communication between neurons.