Interneurons, also called relay neurons, are multipolar nerve cells whose axons are limited to a single brain area. They connects afferent neurons and efferent neurons in neural pathways and integrative areas of the central nervous system. Principal neurons and their networks, such as the Betz cells, underlie local information processing/storage and represent the major sources of output from any brain region, whereas interneurons, by definition, have local axons that govern ensemble activity. While principal cells are mostly excitatory, using glutamate as a neurotransmitter, interneurons most often use gamma-aminobutyric acid (GABA) to inhibit their targets. Like motor neurons, interneuron cell bodies are always located in the central nervous system.
In the central nervous system, the term interneuron is used for small, locally projecting neurons, in contrast to larger projection neurons with long-distance connections. Central nervous system interneurons are typically inhibitory. Examples of interneurons include the sensory neuron and motor neuron also connecting to the brain through the association neurons. The 6-layered neocortex, as the center of the highest nervous functions such as conscious perception or cognition, has the largest number of interneuron types.