Interneuron

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 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.

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 motor neuron. 1a spindle afferents activate 1a inhibitory neuron.
• 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