(also called association neuron
, local circuit neuron
or relay neuron
) is a neuron which connects afferent neurons
and efferent neurons
in neural pathways
. Like motor neurons
, interneuron cell bodies
are always located in the central nervous system
When contrasted with the peripheral nervous system
(PNS), 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
. 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.
In 2008, a nomenclature for the features of GABAergic cortical interneurons was proposed, called Petilla terminology.
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.
Cortical interneurons Parvalbumin-containing interneurons
Molecular layer interneurons (basket cells, stellate cells)
Golgi cells =0=