These kinds of nerve cells are very characteristic of the human brain, as well as versatile.
One of the most common classifications of neurons is the one carried out based on 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 neuron, much more common than bipolar and pseudounipolar neurons in the central nervous system of human beings.
Characteristics of multipolar neurons
Multipolar neurons are characterized mainly by presenting a single axon together with multiple dendrites, processes whose main function is the reception of synaptic impulses. This class of neuron specializes in integrating information from other nerve cells.
This type of neuron is the most numerous in the central nervous system; its quantity is very high in the cerebral cortex, in the spinal cord and in the ganglia (groups of cell bodies) of the autonomic nervous system. Technically any neuron with an axon and at least two dendrites is considered a multipolar neuron.
In general, multipolar neurons have a soma with an approximately ovoid shape. Multiple dendrites arise from this cell body that extend in all directions, forming tangled-looking branches. These dendritic trees make the neuron have a larger area to receive nerve stimuli.
The axons of this type of neuron are usually very long, which facilitates the transmission of impulses throughout the central nervous system. They are often covered by Schwann cells, a type of neuroglia that form the myelin sheaths in this part of the nervous system; said substance allows neuronal transmission to be efficient and fast.
The multipolar neurons can be divided into two subtypes: the Class A and Class B. Those of type A have very thickly branched dendritic trees and have many dendritic spines. In contrast, both characteristics are much less marked in class B multipolar neurons, which also have a larger soma.
Types of multipolar neuron
Below we will describe three of the most relevant and numerous types of multipolar neuron in the human body: 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, responsible for coordinating and supervising movements. The appearance of these neurons is very characteristic due to the density of their dendritic trees, which explains the strong role of reception of neural impulses that they carry out.
2. Pyramidal cells
Pyramidal cells or higher motor neurons originate in the motor cortex. This type of multipolar neuron transmits action potentials through the corticospinal tract to the lower motor neurons of the spinal cord, which allow movement by synapting with muscle cells.
Furthermore, pyramidal cells are fundamentally involved in cognition. This function is associated with the connections between pyramidal neurons and the prefrontal cortex of the brain. Its possible role in visual object recognition has also been hypothesized.
3. Dogiel cells
Dogiel cells are a type of multipolar neuron that is located in the prevertebral sympathetic ganglia. They are part of the enteric nervous system, which regulates the functioning of the gastrointestinal tract.
Other kinds of neuron
Neurons can be classified based on different criteria. For example, if we divide them according to their function, we find sensory neurons, motor neurons, and interneurons or association neurons. Likewise, we find excitatory, inhibitory and modulatory neurons if we look at 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, dividing neurons by the number of processes (ie, dendrites and axons) we can distinguish between multipolar, bipolar, pseudounipolar, unipolar, and anaxonic neurons.
The cytoplasm of bipolar neurons has two processes; one of them acts as a dendrite, receiving impulses from other neurons, and the second fulfills the role of an axon, sending them. They act mainly as sensory neurons and are located in the spinal ganglia, the vestibulocochlear nerve, the retina or the olfactory epithelium.
In these neurons, both the axon and the dendrites originate from a single extension of the cell body. They do not exist in the human organism, although they do exist in that of other living beings.
Pseudounipolar neurons are a type of bipolar neuron whose axon divides in two by forming the dendrites and the axon, so that they appear unipolar, although they are not. Unlike true unipolar neurons, these are found in the human body.
We say that a neuron is anaxonic when it does not present any axon or when it cannot be distinguished from the dendrites. Cells of this type act primarily as interneurons.
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