All three parts of the figure show events at a monoamine synapse. In (a) the monoamine is contained in vesicles in the neuron, it emerges into the synaptic gap as the vesicles merge with the cell membrane, it binds to a proportion of the receptors on the neighbouring neuron, and move back into the original neuron through the reuptake channels, completing the cycle. In (b) there are fewer vesicles so less transmitter is released, resulting in reduced occupancy of the receptors. In (c) the reuptake channels are blocked by a reuptake inhibitor, so there is more monoamine bound to receptors on the neighbouring neuron.