Interneuron: Characteristics Of This Type Of Nerve Cell
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 usually 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.
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 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 the processing and storage of local information and represent the main sources of information output from any brain region, while interneurons, by definition, have local axons that manage neuronal activity as a whole.
While chief cells are mostly excitatory, by using glutamate as a neurotransmitter, interneurons They 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 functional effects by hyperpolarizing large groups of principal cells (although, in some circumstances, they can also mediate depolarization).
Interneurons in the spinal cord can use glycine, along with GABA, to inhibit chief 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, principal neurons are also GABAergic.
Most interneurons innervate different types of target cells (both principal cells and interneurons) approximately in proportion to their appearance in the neuropil (the region between several cell bodies or somata of neurons in the gray matter of the brain and spinal cord). , and therefore They synapse predominantly on the most abundant cell type, which are the local chief cells
The two main types of cortical interneurons are presented below: 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 segments of the axon of the principal cells and occur in both the hippocampus and the neocortex; and basket cells, which form multiple synaptic contacts on the somata and proximal dendrites of principal cells.
Due to the strategic location of their axon terminals, it has been suggested that axo-axonal cells simultaneously inhibit the production of large populations of principal cells. However, recent evidence suggests that their effect mediated by the postsynaptic GABAA receptor can be depolarizing and, as a consequence, they can discharge the entire population of pyramidal cells that they innervate, with the aim of synchronizing their production or reestablishing conductances in their dendritic trees.
Basket cells are present in many different areas of the brain, including the cerebral cortices and cerebellum a (in the cerebellum, they inhibit Purkinje cells). In the neocortex and hippocampus, several subtypes of basket cells have been distinguished. The two major subtypes of hippocampal basket cells can be most easily distinguished based on their content of neuropeptide-binding proteins and calcium.
This group of interneurons It is the most diverse, both morphologically and functionally Inhibitory dendritic 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 inhibitory dendritic 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 target pyramidal cell, which are dispersed throughout the dendritic tree.
Functions of cortical interneurons
What has been confirmed so far is that interneurons regulate the levels of physiological activity in the brain, avoiding runaway excitation in recurrent cortical networks. A similar role in stabilizing cortical network dynamics has also been attributed to feedback inhibition mediated by Renshaw cells 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 likely contribute to enabling the plastic changes induced by such learning events.
The interneurons they make a critical contribution to the generation of network oscillations and synchronize the activity of principal cells 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 could vary between different regions.
In addition to maintaining homeostasis and providing a temporal framework for core cellular activity, interneurons likely play a more direct role in cortical neuronal activity. Interneurons targeting specific dendritic regions can selectively block excitatory input from different sources, thus changing their relative contributions to the cell’s output. 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 principal cell population, so 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 between competing inputs, and can even implement complex activities such as working memory and decision making in the brain. neocortex.
