An interneuron (also called association neuron or bipolar neuron) is a sensory neuron in neural pathways. Like the motor neurons their cell bodies are always located in the central nervous system (CNS) which connects the afferent neuron and efferent neuron.
According to the peripheral nervous system (PNS) definition, the neurons of the central nervous system, 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. A human brain contains about 100 billion interneurons.
Examples of interneurons include the inhibitory interneurons in the neocortex which selectively inhibit sections of the thalamus based on synaptic input both from other parts of the neocortex and from the thalamus itself. This is theorized to help focus higher attention on relevant sensory input and help block out behaviorally irrelevant or unchanging input, such as the sensation of the backs of your thighs on a chair. The neurophysiological measure short-latency intracortical inhibition (SICI) is believed to be mediated by these inhibitory interneurons.citation needed
In 2008, a nomenclature of features of GABAergic cortical interneurons was proposed, called Petilla terminology.[1]
Spinal interneurons
1a Inhibitory Neuron: Found in Lamina VII. Responsible for inhibiting antagonist motor neuron. 1a spindle afferents activate 1a inhibitory neuron.
1b Inhibitory Neuron: Found in Lamina V, VI, VII. 1b afferent or Golgi tendon organ activates it.
