Medium spiny neurons (MSNs), also known as spiny projection neurons (SPNs), are a special type of GABAergic inhibitory cell representing 95% of neurons within the human striatum, a basal ganglia structure.

Medium spiny neurons have two primary phenotypes (characteristic types): D1-type MSNs of the direct pathway and D2-type MSNs of the indirect pathway.

Most striatal MSNs contain only D1-type or D2-type dopamine receptors, but a subpopulation of MSNs exhibit both phenotypes.