Excitatory and Inhibitory Effects of Neurotransmitters

Neurotransmitters released by a presynaptic neuron can be excitatory or inhibitory.

Excitatory neurotransmitters, such as acetylcholine and glutamate increase the chance of generating an action potential in the postsynaptic cell. 

The binding of these neurotransmitters on the ion channel receptors of the postsynaptic cell triggers a rapid influx of sodium and a slow efflux of potassium ions. Resulting changes in the electrochemical gradient cause membranes to depolarize, generating an excitatory postsynaptic potential or EPSP. At the axon hillock, the summation of all EPSP then triggers an action potential that travels down the axon.

In contrast, inhibitory neurotransmitters, such as glycine and gamma-aminobutyric acid, decrease the likelihood of an action potential propagation.

The binding of inhibitory neurotransmitters to the postsynaptic cell’s ion channel receptors allows the efflux of potassium ions or influx of chloride ions—in both cases, making the cell cytoplasm more negative. Such hyperpolarizing potential changes are called inhibitory postsynaptic potential or IPSP. It decreases the likelihood of firing or inducing an action potential.