Interneuron: characteristics of this type of nerve cell

What is an interneuron and what is its role in the nervous system?

Interneurons are a type of nerve cell that connects motor neurons with sensory neurons.. Their axons and dendrites project to a single brain region, unlike most cells in the nervous system, which tend to have axonal projections to more distant regions. 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 their main characteristics are and what functions they perform.

Interneuron: definition and characteristics

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

Principal neurons and their networks underlie local information processing and storage and represent the main sources of information output from any brain region, whereas interneurons, by definition, have local axons that manage neuronal activity as a whole.

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

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

Types

Most interneurons innervate different types of target cells (both principal cells and interneurons) roughly in proportion to their occurrence in the neuropil (the region between various cell bodies or somas of neurons in the gray matter of the brain and spinal cord), and therefore synapse predominantly on the most abundant cell type, which are the local principal cells..

The two main types of cortical interneurons are presented below: perisomatic and dendritic inhibitory cells.

1. Perisomatic inhibitory cells

The precise termination site as well as the specific input characteristics allow us to dissect this cell group into two main types of interneurons. two main types of interneuronsThe following are examples of axo-axonic or spider cells: axo-axonic cells, which exclusively innervate the axon initial segments of principal cells and occur in both the hippocampus and neocortex; and basket cells, which form multiple synaptic contacts in the somas and proximal dendrites of principal cells.

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

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

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 large variety of dendritic inhibitory interneurons have been described in the neocortex.

These types of interneurons include Martinotti cells, which primarily target the apical tuft region of pyramidal cells and contain the neuropeptide somatostatin; double bouquet cells; and bipolar cells, which primarily target 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 glutamatergic inputs to principal cells from different sources. Notably, individual dendritic inhibitory cells of any type provide 2 to 20 synapses on a single pyramidal target cell, which are scattered throughout the dendritic tree.

Functions of cortical interneurons.

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

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

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

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

Feedback inhibition also introduces direct competition between members of a local principal cell population, whereby 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, mediates selection among competing inputs, and can even implement complex activities such as working memory and decision making in the neocortex.