Interneuron

Overview
An interneuron (also called relay neuron, association neuron or bipolar neuron) is a term used to describe a neuron which has two different common meanings.

PNS
In the peripheral nervous system, an interneuron is a neuron that communicates only to other neurons. Interneurons are the neurons that provide connections between sensory and motor neurons, as well as between themselves. Contrast to sensory neurons or motor neurons, which respectively provide input from and output to the rest of the body.

Interneurons are found in the grey matter. One type of peripheral interneuron is the Renshaw cell.

CNS
According to the PNS definition, the neurons of the central nervous system, including the brain, are all interneurons. However, in the CNS, the term interneurons is also used for the general group of small, locally projecting neurons of the central nervous system. These neurons are typically inhibitory, and use the neurotransmitter GABA. However, excitatory interneurons also exist.

One example of interneurons are 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 behavioraly irrelevant or unchanging input, such as the sensation of the backs of your thighs on a chair. A human brain contains about 100 billion interneurons.