What dertemines a neurotransmitter will have an inhibitory or excitatory effect?

The effect of a neurotransmitter, whether inhibitory or excitatory, is primarily determined by the type of receptor it binds to on the postsynaptic neuron. Excitatory neurotransmitters typically bind to receptors that allow the influx of positive ions (like sodium), leading to depolarization and increased likelihood of action potential generation. Inhibitory neurotransmitters, on the other hand, often bind to receptors that permit the influx of negative ions (like chloride) or the efflux of positive ions (like potassium), resulting in hyperpolarization and decreased likelihood of action potential generation. Thus, the specific receptor type and its associated ion channels dictate the overall effect on neuronal activity.