Interneuron: characteristics of this type of nerve cell

Interneurons are a type of nerve cell that connects motor neurons with sensory neurons.. Its axons and dendrites are projected in a single brain region, unlike what happens with the most cells of the nervous system, which usually have axonal projections in regions more distant. As we will see throughout the article, interneurons act as inhibitory neurons through the neurotransmitter GABA

Next, we will explain in more detail what these nerve cells consist of, what are their main characteristics and what functions they perform.

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Interneuron: definition and characteristics

An interneuron is a type of nerve cell that is generally located in integrative areas of the central nervous system, whose axons (and dendrites) are limited to a single brain area. This feature distinguishes them from the main cells, which often have axonal projections outside the area of ​​the brain where their cell bodies and dendrites are located.

The main neurons and their networks underlie the processing and storage of local information and represent the main sources of output of information from any brain region, while interneurons, by definition, have local axons that manage neuronal activity in their set.

While major cells are mostly excitatory, using glutamate as a neurotransmitter, interneurons often use gamma-aminobutyric acid (GABA) to inhibit their targets. Since GABA acts primarily through the opening of ion channels in the postsynaptic neuron, interneurons achieve their effects functionalities by hyperpolarization of large clusters of major cells (although, in some circumstances, they may also mediate depolarization).

Interneurons in the spinal cord can use glycine, along with GABA, to inhibit major cells, while interneurons in cortical areas or basal ganglia can release various neuropeptides (cholecystokinin, somatostatin, enkephalins, etc.) in addition to GABA. In some regions, such as the basal ganglia and cerebellum, the major neurons are also GABAergic.

Types

Most interneurons innervate different types of target cells (both main cells and interneurons) roughly in proportion to their appearance in the neuropil (the region between several cell bodies or neuron cell bodies of the gray matter of the brain and spinal cord), and by so much synapse predominantly on the most abundant cell type, which are the local major cells.

Below are the two main types of cortical interneurons: perisomatic and dendritic inhibitory cells.

1. Perisomatic inhibitory cells

The precise site of termination as well as the specific entry characteristics allow this cell group to be dissected into two main types of interneurons: axo-axonal or spider cells, which exclusively innervate the initial axon segments of the main cells and are produced both in the hippocampus and in the neocortex; and basket cells, which form multiple synaptic contacts in the somas and proximal dendrites of major cells.

Due to the strategic location of their axon terminals, it has been suggested that axo-axon cells simultaneously inhibit the production of large populations of major cells. However, recent evidence suggests that their postsynaptic GABAA receptor-mediated effect may be depolarizing and, as a consequence, may unload the entire population of pyramidal cells that they innervate, with the aim of synchronizing their production or reestablishing conductances in their trees dendritic.

Basket cells are present in many different areas of the brain, including the cerebral cortices and cerebellums.a (in the cerebellum, they inhibit Purkinje cells). In the neocortex and hippocampus, several subtypes of basket cells have been distinguished. The two main subtypes of hippocampal basket cells can be more easily distinguished based on their content of calcium and neuropeptide-binding proteins.

2. Dendritic inhibitory cells

This group of interneurons is the most diverse, both morphologically and functionally. Dendritic inhibitory cells are present in many different parts of the nervous system, including the cerebellum, the olfactory bulb, and all areas of the cerebral cortex. In fact, a wide variety of dendritic inhibitory interneurons have been described in the neocortex.

These types of interneurons include Martinotti cells, which target primarily the apical tuft region of pyramidal cells and contain the neuropeptide somatostatin; double bouquet cells; and bipolar cells, which target mainly the basal dendrites. However, the precise functions of these neocortical cell types have been difficult to identify.

Different types of dendritic interneurons have evolved to control the glutamatergic inputs of major cells from different sources. Notably, individual dendritic inhibitory cells of any type provide 2 to 20 synapses in a single target pyramidal cell, which are scattered throughout the dendritic tree.

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Cortical interneuron functions

What has so far been found is that interneurons regulate levels of physiological activity in the brain, avoiding runaway excitation in recurrent cortical networks. A similar role in stabilizing the dynamics of the cortical network has also been attributed to the inhibition of Renshaw cell-mediated feedback in marrow motor regions spinal.

There is evidence that lasting changes in the level of arousal are accompanied by a corresponding change in the general level of inhibition; however, transient imbalances between excitation and inhibition can also be induced. In the hippocampus and in the neocortex, changes in the level of interneuronal firing have been observed to accompany novel experiences relevant to behavior, and probably contribute to permitting the plastic changes induced by such stress events. learning.

Interneurons make a critical contribution to the generation of lattice oscillations and synchronize the activity of the main cells during oscillatory and transient brain states. Perisomatic interneurons in particular are considered essential for the generation of gamma rhythms. (involved in conscious perception), although the exact nature of their contribution could vary between different regions.

In addition to maintaining homeostasis and providing a time frame for cellular activity Mainly, interneurons are likely to play a more direct role in neuronal activity cortical. Interneurons that target specific dendritic regions can selectively block the exciting input from different sources, thus changing their relative contributions to the output of the cell. Dendritic inhibition can also control various forms of synaptic plasticity and at the cellular level through its interaction with active dendritic processes.

Feedback inhibition also introduces direct competition between members of a local major cell population, thus an increase in the activity of one cell tends to decrease the activity of other cells. Such competition can be a simple but effective means of noise suppression and, especially if complemented by local recurrent excitation, medium the selection between competing inputs, and can even implement complex activities such as working memory and decision-making in the neocortex.

Bibliographic references:

• DeFelipe, J. (2002). Cortical interneurons: from Cajal to 2001. In Progress in brain research (Vol. 136, pp. 215-238). Elsevier.
• Pi, H. J., Hangya, B., Kvitsiani, D., Sanders, J. I., Huang, Z. J., & Kepecs, A. (2013). Cortical interneurons that specialize in disinhibitory control. Nature, 503 (7477), 521.
• Wonders, C. P., & Anderson, S. TO. (2006). The origin and specification of cortical interneurons. Nature Reviews Neuroscience, 7 (9), 687.