Difference between Unipolar and Pseudounipolar Neuron

The fundamental unit of the nervous system is called a neuron. A neuron is an informational messenger that transmits electrical signals to other nerve cells. A neuronal cell body (also called soma) has a nucleus, organelles like endoplasmic reticulum, mitochondria, golgi apparatus and other cellular elements. A structure unique to neurons is a tree-like structure called dendrites that extend away from the soma, and are responsible for receiving the impulses at specialised junctions called synapses. A long extending nerve fibre called axon projects from soma that functions to send electrical impulses away from the nerve cell.

There are four basic types of neuron known:

  • Unipolar
  • Bipolar
  • Multipolar
  • Pseudounipolar

Refer: What Is a Neuron? – Definition, Structure, Parts and Function

Let us now differentiate between two of the four types, unipolar and pseudounipolar neurons in the table below.

Unipolar Neuron

Unipolar neurons can be defined by the presence of only one extension (also known as processes) from their nerve cell bodies or soma. The extension is called a neurite that further extends to form the dendritic branches and axon fibres.

The unipolar neurons are characteristically found in invertebrates, however they can be seen in some invertebrate brains as well. The length of the axonal processes is longer, and that of the dendritic branches varies.

Unipolar cells in insects are located at the periphery of the brain and are electrically inactive. In vertebrates it is found as unipolar brush cells in the cerebellum.

Pseudounipolar Neuron

Pseudounipolar neurons are the ones that develop as a bipolar neuron with two processes extending from the cell body, one to the spinal cord and the other to the peripheral targets. Eventually, the initial segment of the two extensions joins to form a T junction that looks like a unipolar neuron. Hence, a pseudounipolar neuron shares characteristics with both bipolar and unipolar neurons.

The axon is relatively short, and the neuronal cell body is usually devoid of dendrites. A pseudounipolar neuron can be found in both vertebrates and invertebrates.

The pseudounipolar neurons are sensory in nature and the axonal processes perform the function of receiving and transmitting information. They are located in the sensory ganglia of cranial nerves.

Unipolar vs Pseudounipolar Neuron

Unipolar Neuron

Pseudounipolar Neuron

Definition

A neuron that has one process extending from its soma is referred to as a unipolar neuron.

A neuron that develops as a bipolar neuron but eventually merges to form a unipolar neuron is referred to as pseudounipolar neuron.

Found in

It is majorly found in invertebrates.

It is found in both vertebrates and invertebrates.

Length of Axon

The axonal process is relatively longer.

The axonal process is relatively shorter.

Presence of Dendrites

The dendrites receive the incoming electrical impulses.

Dendrites are usually not present, the axonal processes receive and send the impulses.

Example

Located at the periphery of the brain in insects.

Found in the sensory ganglia of cranial nerves.

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Frequently Asked Questions on Difference between Unipolar and Pseudounipolar Neuron

Why are pseudounipolar neurons also called unipolar?

The pseudounipolar neurons develop as bipolar but eventually merge to form a unipolar neuron. That’s why they are sometimes referred to as a unipolar neuron.

What is the difference between bipolar and multipolar neurons?

A multiple neuron has one axonal process and many dendritic branches arising from its soma. On the other hand, a bipolar neuron has one axon and one dendritic branch arising from its soma.

What are synapses?

A synapse is a junction where the electrical impulse is received or delivered between two nerve cells.

What is an example of a multipolar neuron?

Multipolar neurons are commonly found in the cortex of the brain and spinal cord.

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