Neurotransmitters are not inherently excitatory or inhibitory; in fact, certain neurotransmitters are excitatory in some situations and inhibitory in others. The action of a neurotransmitter depends upon its postsynaptic receptor and the intracellular processes that are influenced by the receptor. As noted in Chapter 3, it is useful to distinguish between two general classes of receptors: those linked to or part of ion channels (ionophore-linked) and those coupled to second messenger-generating systems (second messenger-linked). These will be considered separately.