Heres a good write up/explination of whats going on when hallucinogens or neurotransmitters enter the synapse.
The adjacent neuron’s postsynaptic membrane appears as an elaborate, thickened complex of interlinked proteins gathered on the surface of a dendrite, which is one of the many arborizing extensions of a neuron. Neurotransmitter receptor binding alters the electrical potential of the dendritic membrane, which processes spatially and temporally the incoming barrage of synaptic impulses to form a signal directed toward the cell body. The dendrite receives both excitatory and inhibitory synaptic inputs. Once a sufficient number of receptor binding events occurs and threshold is reached, the postsynaptic neuron then fires an action potential. That momentary electrochemical flux becomes one nod from one neuron among the brain’s hundred billion.1,2