UNIPOLAR NEURON: PARTS, ANATOMY AND FUNCTIONS - GEOGRAFÍA - 2025

The unipolar neurons or monopolar neurons are a type of a single outgoing extension of the soma are characterized by. These neurons have a single cytoplasmic extension, which performs both the information input and information output functions.

Other types, bipolar and multipolar, are differentiated through their morphology. That is, the unipolar ones have a single input and output extension while the others have one output extension and one (bipolar) or several input extensions (multipolar).

Unipolar sensory neuron

Unipolar neurons can perform different functions. However, these tend to be sensory. That is, they constitute afferent cells (they transport the nerve impulse from the receptors or sensory organs to the central nervous system).

This article reviews the main characteristics of unipolar neurons. Likewise, its variants, its functions and its location in the brain are discussed.

Characteristics and parts of a unipolar neuron

Unipolar neuron diagram. Source: Jonathan Haas via Wikimedia Commons

Unipolar neurons are neurons that have a single projecting extension of the cell soma or nucleus. That is, these neurons simply contain a soma (cell body) and a prolongation that acts both as an axon and as a dendrite.

In this way, neurons have a single nerve terminal, which serves both to receive information from other neurons and / or cells, and to send information from the interior to other regions of the brain.

In some cases, the unipolar neuron can present the split prolongation. That is, one portion of the extension performs the function of conducting information and the other is responsible for capturing information from other neurons.

In this sense, the typical prolongation of unipolar neurons is characterized by acting both as an axon and as a dendrite. The other types, on the other hand, have an axon and one or more dendrites, therefore they contain more than one prolongation.

The axon is the unique, elongated structure of neurons that connects to the cell body. This element is responsible for conducting the nerve impulses generated in the nucleus of the neuron to the information output regions.

For their part, the dendrites are smaller extensions that are attached to the axon on the opposite side where the cell nucleus is located. These extensions are responsible for capturing and receiving the stimuli sent by the other neurons.

Thus, in general, unipolar neurons have a single extension that contains the typical axon shape. At the end of the extension, a bifurcation is detected that makes it possible to divide the input area and the information output area.

These types of neurons are originally bipolar embryonic structures. However, the axon and the dendrite end up unifying, thus generating a single nerve fiber.

Anatomy

Unipolar neurons present a series of anatomical and morphological properties that allow them to be differentiated from bipolar and multipolar neurons. The main ones are:

They have a single neurite

The neurite is any expansion of the soma of a neuron, it can adopt the shape and the functionality of a dendrite or an axon.

In the case of unipolar neurons, only one of these processes is found, unlike other types of cells that have two or more.

They have a rounded body

The global morphology of unipolar neurons is characterized by adopting a rounded shape. The other types of neurons tend to have a longer body.

They have only one extension

Apart from only having a single neurite, unipolar neurons are also characterized by containing a single prolongation.

This means that the nucleus of the neuron is located at one end, from which a single extension starts from one of its sides.

They have different segments

Another important morphological property of unipolar neurons is that they have different segments. These function as surface receptors and are responsible for capturing information from other cells.

They are found in the ganglia

Unipolar neurons are located mainly in the ganglia of invertebrates. They can also be located on the retina.

They transmit impulses without going through the soma

Finally, the last important anatomical property of this type of neurons is that they can transmit information without it having previously passed through the cell nucleus.

Types of unipolar neurons

1: unipolar neuron 2: bipolar neuron 3: multipolar neuron 4: pseudounipolar neuron. Source: Juoj8 / Public domain

Although unipolar neurons constitute a unique category, these types of cells present a variant.

Specifically, unipolar neurons can be unipolar neurons or pseudounipolar neurons.

Pseudounipolar neurons are found in the brains of mammals and are characterized by giving rise to two functional branches, one peripheral and one central, which constitute the dorsal roots of the spinal nerves.

In this sense, pseudounipolar neurons make up a type of sensory neuron of the peripheral nervous system. Its axon has a peripheral branch that receives information from the skin, joints, muscles, and other body regions.

Likewise, the axon contains a central branch that transmits information from the neuronal body to the spinal cord, a region of the nervous system where it forms synapses with other neurons.

Location and function

Neurons of a velvet worm nerve ganglion. Arrows mark the cell bodies of various peripherally located neurons. Source: Georg Mayer and Steffen Harzsch

Unipolar neurons are located mainly in the spinal ganglia, structures that are located in the spinal cord, near the spinal column.

The spinal ganglia are located at the level of the posterior roots of the spinal nerves that emerge from the spinal cord and appear as nodules of these nerves.

On the other hand, the spinal ganglia are characterized by constituting a kind of lump that is formed in the dorsal or posterior roots of the spinal nerves. In these regions the bodies of the unipolar neurons of the afferent pathway of the peripheral nervous system are housed.

Finally, the axons of the neurons of the ganglion of a dorsal root become afferent axons. These are responsible for transmitting nerve impulses from the periphery to the central nervous system. The latter is its main function.

References

1. Pinel, JPJ (2007) Biopsychology. Madrid: Pearson Education.
2. Purves, D.; Augustine, GJ; Fitzpatrick, D.; Hall; LaMantia, A.-S., McNamara, JO i Williams (2006).
3. Madrid: Editorial Médica Panamericana.
4. Rosenzweig, MR; Breedlove, SM; Watson, NV (2005) Psychobiology. An introduction to Behavioral, Cognitive and Clinical Neuroscience. Barcelona: Ariel.