An interneuron (also called relay neuron, association neuron, connector neuron or local circuit neuron) is a neuron that forms a connection between other neurons. Interneurons are neither motor nor sensory. The term is also applied to brain and spinal cord neurons whose axons connect only with nearby neurons, to distinguish them from "projection" neurons, whose axons project to more distant regions of the brain or spinal cord.
When contrasted with the peripheral nervous system (PNS), the neurons of the central nervous system (CNS), including the brain, are all interneurons. However, in the CNS, the term interneuron is used for small, locally projecting neurons (in contrast to larger projection neurons with long-distance connections). CNS interneurons are typically inhibitory, and use the neurotransmitter GABA or glycine. However, excitatory interneurons using glutamate also exist, as do interneurons releasing neuromodulators like acetylcholine.
In 2008, a nomenclature for the features of GABAergic cortical interneurons was proposed, called Petilla terminology.