Receptor sites are specific locations on the surface of neurons that can receive chemical signals from neurotransmitters. These sites act like locks, and when the right key (neurotransmitter) fits into the receptor site, it triggers a response in the neuron.
A substance that binds to receptor sites but does not activate them, blocking or reducing the effects of neurotransmitters.
Lock and Key Model: This model explains how receptors and neurotransmitters interact - just like how only a specific key can fit into a lock, only specific neurotransmitters can bind to certain receptors.