Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are neurons that are not specifically motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enabling communication between sensory or motor neurons and the central nervous system (CNS). They play vital roles in reflexes, neuronal oscillations, and neurogenesis 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. The interaction between interneurons allows the brain to perform complex functions such as learning and decision-making.
Structure
In the human brain, approximately 20–30% of the neurons in the neocortex 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 identifying coetaneous interneurons is neuronal birthdating. This can be achieved using nucleoside analogs such as EdU.
Spinal cord
- Ia inhibitory interneuron: Found in lamina VII. Responsible for inhibiting antagonist motor neuron. 1a spindle afferents activate 1a inhibitory neuron.
- thumb|A [[spinal interneuron (relay neuron) forms part of a reflex arc]]Ib inhibitory interneuron: Found in lamina V, VI, VII. Afferent or Golgi tendon organ activates it.
Cortex
- Parvalbumin-expressing interneurons
- CCK-expressing interneurons
- VIP-expressing interneurons
- SOM-expressing interneurons
Cerebellum
- Molecular layer (basket cells, stellate cells)
- Golgi cells
- Granule cells
- Lugaro cells
- Unipolar brush cells
Striatum
- Parvalbumin-expressing interneurons
- Cholinergic interneurons
- Tyrosine hydroxylase-expressing interneurons
- Calretinin-expressing interneurons
- Nitric oxide synthase-expressing interneurons 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.