Interneurons are a type of neuron that connect other neurons in the central nervous system (CNS) . They act as a "middle-man" between sensory neurons, which receive signals from the peripheral nervous system, and motor neurons, which send signals to muscles and glands. Interneurons can be further broken down into two groups: local interneurons and relay interneurons. Local interneurons have short axons and form circuits with nearby neurons to analyze small pieces of information, while relay interneurons have long axons and connect circuits of neurons in one region of the brain with those in other regions.

Interneurons play a significant role in modifying other neurons through the release of neurotransmitters, as well as in neural oscillations, neurogenesis, and involuntary reflexes. They are primarily inhibitory and use the neurotransmitter GABA or glycine, although excitatory interneurons using glutamate in the CNS also exist, as do interneurons releasing neuromodulators like acetylcholine.

The interaction between interneurons allows the brain to perform complex functions such as learning and decision-making. They are involved in processing information, both in simple reflex circuits and in more complex circuits in the brain. Interneurons are the most numerous class of neurons in the CNS.