Post-synaptic receptors complete the neurotransmission by associating with either ion-channels or G-proteins which either activate or inhibit second-messenger systems. The Gs-protein is shown stimulating adenylate cyclase (AC). The AC enzyme converts ATP (adenosine triphosphate) into cyclic AMP (cAMP= cyclic adenosine monophosphate). cAMP then can activate protein kinase A (pkA).
Activation of this enzyme is the beginning of a cascade of enzymatic activity that ends with effectors such as ion channels (as shown), various enzymes, contractile proteins and transport proteins.
Stimulation of the ligand-gated ion channel results in influx of the ion into the cell.
It is not difficult to imagine that the response times from the moment of receptor stimulation can vary from milliseconds (ion channel) to seconds (G protein).