Interneuron

Interneuron
The mechanism of the reflex arc. (Relay neuron labeled at right center. Diagram discusses the PNS definition.)
MeSH	Interneurons
Dorlands/Elsevier	i_10/12455676

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 or glycine. However, excitatory interneurons using glutamate also exist as do interneurons releasing neuromodulators like acetylcholine. A human brain contains about 100 billion interneurons.

An 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. The neurophysiological measure short-latency intracortical inhibition (SICI) is believed to be mediated by these inhibitory interneurons.

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
• CCK-containing interneurons
• VIP-containing interneurons

Cerebellar interneurons

• Molecular layer interneurons (basket cells, stellate cells)
• Golgi cells
• Granule cells

External links

• Interneurons at eMedicine Dictionary

Anatomy of torso (primarily): the spinal cord
Spinal nerve	Dorsal (Root, Ganglion, Ramus) • Ventral (Root, Ramus) • Sympathetic trunk • rami communicantes (Gray, White)
Gray matter/Rexed laminae	Posterior horn (Column of Clarke, Substantia gelatinosa of Rolando, Nucleus proprius) • Lateral horn • Anterior horn • Central canal/Substantia gelatinosa centralis
White matter: somatic/ascending (blue)	Posterior/PCML: Gracilis • Cuneatus Lateral: Spinocerebellar (Dorsal, Ventral) • Spinothalamic (Lateral, Anterior) • Spinotectal • Posterolateral (Lissauer)
White matter: motor/descending (red)	Lateral: Corticospinal (Lateral) • Ep (Rubrospinal, Olivospinal) Anterior: Corticospinal (Anterior) • Ep (Vestibulospinal, Tectospinal, Reticulospinal)
Layers	Epidural space • Dura mater • Subdural space • Arachnoid mater • Subarachnoid space • Pia mater
Other structures	Denticulate ligaments • Conus medullaris • Cauda equina • Filum terminale • Cervical enlargement • Lumbar enlargement • Anterior median fissure

Histology: nervous tissue
Neurons (gray matter)	soma, axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament), dendrite (Nissl body, dendritic spine, apical dendrite, basal dendrite) types (bipolar, pseudounipolar, multipolar, pyramidal, Purkinje, granule)
Afferent nerve/Sensory nerve/Sensory neuron	GSA, GVA, SSA, SVA, fibers (Ia, Ib or Golgi, II or Aβ, III or Aδ or fast pain, IV or C or slow pain)
Efferent nerve/Motor nerve/Motor neuron	GSE, GVE, SVE, Upper motor neuron, Lower motor neuron (α motorneuron, γ motorneuron)
Synapses	neuropil, synaptic vesicle, neuromuscular junction, electrical synapse - Interneuron (Renshaw)
Sensory receptors	Free nerve ending, Meissner's corpuscle, Merkel nerve ending, Muscle spindle, Pacinian corpuscle, Ruffini ending, Olfactory receptor neuron, Photoreceptor cell, Hair cell, Taste bud
Glial cells	astrocyte, oligodendrocyte, ependymal cells, microglia, radial glia
Myelination (white matter)	Schwann cell, oligodendrocyte, nodes of Ranvier, internode, Schmidt-Lanterman incisures, neurolemma
Related connective tissues	epineurium, perineurium, endoneurium, nerve fascicle, meninges