Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are

neurons A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

that are not specifically

motor neuron A motor neuron (or motoneuron), also known as efferent neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly o ...

s or

sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduc ...

s. Interneurons are the central nodes of neural circuits, enabling communication between sensory or motor neurons and the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

(CNS). They play vital roles in

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

es,

neuronal oscillations Neural oscillations, or brainwaves, are rhythmic or repetitive patterns of neural activity in the central nervous system. Neural tissue can generate oscillatory activity in many ways, driven either by mechanisms within individual neurons or by ...

, and

neurogenesis Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). This occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells ( ...

in the adult mammalian brain. 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. Relay interneurons have long axons and connect circuits of neurons in one region of the brain with those in other regions. However, interneurons are generally considered to operate mainly within local brain areas. The interaction between interneurons allows the brain to perform complex functions such as

learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, Attitude (psychology), attitudes, and preferences. The ability to learn is possessed by humans, non-human animals, and ...

and

decision-making In psychology, decision-making (also spelled decision making and decisionmaking) is regarded as the Cognition, cognitive process resulting in the selection of a belief or a course of action among several possible alternative options. It could be ...

Structure

In the

human brain The human brain is the central organ (anatomy), organ of the nervous system, and with the spinal cord, comprises the central nervous system. It consists of the cerebrum, the brainstem and the cerebellum. The brain controls most of the activi ...

, approximately 20–30% of the neurons in the

neocortex The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, ...

are interneurons, and the remaining majority of neurons are pyramidal. Investigations into the molecular diversity of neurons is impeded by the inability to isolate cell populations born at different times for gene expression analysis. An effective means of identifyin
coetaneous
interneurons is neuronal birthdating. This can be achieved using nucleoside analogs such as EdU. In 2008, a nomenclature for the features of GABAergic cortical interneurons was proposed, called ''Petilla terminology''.

Spinal cord

* Ia inhibitory interneuron: Found in lamina VII. Responsible for inhibiting antagonist

. 1a spindle afferents activate 1a inhibitory neuron. * Anatomy and physiology of animals A reflex arc

Ib inhibitory interneuron: Found in lamina V, VI, VII. Afferent or

Golgi tendon organ The Golgi tendon organ (GTO) (also called Golgi organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a mus ...

activates it.

Cortex

Parvalbumin Parvalbumin (PV) is a calcium-binding protein with low molecular weight (typically 9–11 kDa). In humans, it is encoded by the ''PVALB'' gene. It is a member of the albumin family; it is named for its size (''parv-'', from Latin ' which means " ...

-expressing interneurons * CCK-expressing interneurons * VIP-expressing interneurons * SOM-expressing interneurons

Cerebellum

* Molecular layer ( basket cells,

stellate cell Stellate cells are neurons in the central nervous system, named for their star-like shape formed by dendritic processes radiating from the cell body. These cells play significant roles in various brain functions, including inhibition in the ce ...

s) *

Golgi cell In neuroscience, Golgi cells are the most abundant inhibitory interneurons found within the granular layer of the cerebellum. Golgi cells can be found in the granular layer at various layers. The Golgi cell is essential for controlling the activi ...

s *

Granule cell The name granule cell has been used for a number of different types of neurons whose only common feature is that they all have very small cell bodies. Granule cells are found within the granular layer of the cerebellum, the dentate gyrus of t ...

s *

Lugaro cell Lugaro cells are primary sensory interneurons of the cerebellum, that have an inhibitory function. They are fusiform, having a spindle shape that tapers at each end. They were first described by Ernesto Lugaro in the early 20th century. Lugaro cells ...

s *

Unipolar brush cell Unipolar brush cells (UBCs) are a class of excitatory glutamatergic interneuron found in the granular layer of the cerebellar cortex and also in the granule cell domain of the cochlear nucleus. Structure The UBC has a round or oval cell body ...

Striatum

-expressing interneurons *

Cholinergic Cholinergic agents are compounds which mimic the action of acetylcholine and/or butyrylcholine. In general, the word " choline" describes the various quaternary ammonium salts containing the ''N'',''N'',''N''-trimethylethanolammonium cation ...

interneurons *

Tyrosine hydroxylase Tyrosine hydroxylase or tyrosine 3-monooxygenase is the enzyme responsible for catalyzing the conversion of the amino acid L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA). It does so using molecular oxygen (O2), as well as iron (Fe2+) and ...

-expressing interneurons *Calretinin-expressing interneurons *Nitric oxide synthase-expressing interneurons

Function

Interneurons in the CNS are primarily

inhibitory An inhibitory postsynaptic potential (IPSP) is a kind of synaptic potential that makes a Chemical synapse, postsynaptic neuron less likely to generate an action potential.Purves et al. Neuroscience. 4th ed. Sunderland (MA): Sinauer Associates, Inc ...

, and use the neurotransmitter

GABA GABA (gamma-aminobutyric acid, γ-aminobutyric acid) is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system. GA ...

glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid. Glycine is one of the proteinogenic amino acids. It is encoded by all the codons starting with GG (G ...

. However, excitatory interneurons using

glutamate Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that ...

in the CNS also exist, as do interneurons releasing

neuromodulators Neuromodulation is the physiology, physiological process by which a given neuron uses one or more chemicals to regulate diverse populations of neurons. Neuromodulators typically bind to metabotropic receptor, metabotropic, G protein-coupled rece ...

acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

. In addition to these general functions, interneurons in the insect CNS play a number of specific roles in different parts of the nervous system, and also are either excitatory or inhibitory. For example, in the olfactory system, interneurons are responsible for integrating information from odorant receptors and sending signals to the mushroom bodies, which are involved in learning and memory. In the visual system, interneurons are responsible for processing motion information and sending signals to the optic lobes, which are involved in visual navigation. Interneurons are also important for coordinating complex behaviors, such as flight and locomotion. For example, interneurons in the thoracic ganglia are responsible for coordinating the activity of the leg muscles during walking and flying. Interneurons' main function is to provide a neural circuit, conducting flow of signals or information between sensory neurons and motor neurons.

References

{{Authority control Neurons