One of the most common classifications of neurons is according to their morphology; more particularly, they are usually divided according to the number of dendrites and axons in their cell body.
In this article we will describe the characteristics of the main types of multipolar neurons, much more common than bipolar and pseudounipolar ones in the central nervous system of humans.
Characteristics of multipolar neurons
Multipolar neurons are mainly characterized by having a single axon together with multiple dendrites , extensions whose main function is the reception of synaptic impulses. This class of neuron specializes in the integration of information from other nerve cells.
This type of neuron is the most numerous in the central nervous system; its number is very high in the cerebral cortex, in the spinal cord and in the ganglia (sets of cell bodies) of the autonomic nervous system. Technically any neuron with one axon and at least two dendrites is considered a multipolar neuron.
As a rule, multipolar neurons have a soma with an approximately ovoid shape. Multiple dendrites emerge from this cell body and spread in all directions, forming tangled-looking branches. These dendritic trees give the neuron a larger area in which to receive nerve stimuli.
The axons of this type of neuron are usually very long, which facilitates the transmission of impulses along the central nervous system. They are often covered by Schwann cells, a type of neuroglia that forms the myelin sheath in this part of the nervous system; this substance allows efficient and rapid neuronal transmission.
Multipolar neurons can be divided into two subtypes: class A and class B . Type A neurons have densely branched dendritic trees and many dendritic spines. In contrast, both characteristics are much less marked in multipolar class B neurons, which also have a larger soma.
Multipolar neuron types
We will now describe three of the most relevant and numerous types of multipolar neurons in the human organism: Purkinje cells, pyramidal cells and Dogiel cells. Each of them has its own peculiarities, locations and functions.
1. Purkinje cells
Purkinje cells are located in the cerebellum, the back of the brain, which is responsible for coordinating and monitoring movement. The appearance of these neurons is very characteristic because of the density of their dendritic trees , which explains the strong role they play in receiving neural impulses.
2. Pyramidal cells
The upper pyramidal or motor neuron cells originate from the motor cortex. This type of multipolar neuron transmits action potentials through the corticospinal tract to the lower motor neurons in the spinal cord, which allow movement by synapting with the muscle cells .
In addition, pyramidal cells are fundamentally involved in cognition. This function is associated with the connections between pyramidal neurons and the prefrontal cortex of the brain. Their possible role in visual object recognition has also been hypothesized.
3. Dogiel cells
Dogiel cells are a type of multipolar neuron located in the prevertebral sympathetic nodes. They are part of the enteric nervous system, which regulates the functioning of the gastrointestinal tract.
Other types of neurons
Neurons can be classified according to different criteria. For example, if we divide them according to their function we find the sensory neurons, the motor neurons and the interneurons or association neurons. Likewise, we find the excitatory, inhibitory and modulating neurons if we pay attention to the type of synapses they carry out.
The term “multipolar” is part of the classification of neuron types according to their external morphology . More specifically, by dividing the neurons by the number of extensions (i.e. dendrites and axons) we can distinguish between multipolar, bipolar, pseudo-unipolar, unipolar and anaxonic neurons.
The cytoplasm of bipolar neurons has two extensions; one of them acts as a dendrite, receiving impulses from other neurons, and the second plays the role of an axon, sending them out. They act mainly as sensory neurons and are located in the spinal ganglia, the vestibuloclear nerve, the retina or the olfactory epithelium.
In these neurons both axon and dendrites originate from a single extension of the cell body. They do not exist in the human organism, although they do in other living beings.
Pseudounipolar neurons are a type of bipolar neuron whose axon splits in two to form the dendrites and the axon, so that appear unipolar, although they are not . Unlike true unipolar neurons, they are found in the body of humans.
We say that a neuron is anaxonic when it has no axon or when it cannot be distinguished from dendrites. Cells of this type act mainly as interneurons.
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