Neurons are often referred to as excitatory or inhibitory, but more accurately it is the combination of the type of neurotransmitter released from the presynaptic membrane and the type of receptor that it binds to at the postsynaptic membrane that determines if the target cell will be excited or inhibited.
Many neurotransmitters can bind to multiple types of receptors that may cause either excitation or inhibition.
Excitatory and inhibitory synapses on neurons may be distributed throughout the membrane, but in many neurons the dendrites are covered mostly by excitatory synapses, the soma is covered by more inhibitory synapses, and the axon terminals may be covered by both. Neurons like this receive most of their excitatory input through their dendrites, but the depolarizations produced by this may be blocked by inhibitory input to the soma. The amount of neurotransmitter released from individual axon terminals in response to action potentials may then be increased or decreased by excitatory or inhibitory input to those terminals.
Neurotransmitter receptors are divided into ionotropic and metabotropic types. Ionotropic neurotransmitter receptors are ligand-gated ion channels that open for certain ions when their neurotransmitter binds, and close when it is not bound. These receptors cause graded potentials when they are activated, which have a rapid effect on the potential of the nearby membrane that is brief and local. The target cell will usually be excited if the activated channel allows sodium or calcium to pass, or it will usually be inhibited if the activated channel allows chloride or potassium to pass.
Metabotropic neurotransmitter receptors are also activated when their neurotransmitter binds, but instead of allowing ions to pass through them directly, they act through second messengers that may change the activity of ion channels, other proteins, or genes.
When these receptors are activated, the response of the target cell occurs more slowly than with activation of ionotropic neurotransmitter receptors, but the effect may be larger and more widespread because of amplification through the second messenger system. The effect of activation of these receptors may involve brief excitation or inhibition of the target cell, or may cause longer-lasting changes to how the target cell behaves, either when it is at rest or when it responds to input.