The Neuron – External Structure and Classification

The neuron, in my opinion, is the coolest cell type with the most mind-blowing structure you could find in any organism. Period.

The neuron’s main function is to transmit electrical signals (information) in one direction (dendrite to axon) hence, it is also said to be polarized, that is, information cannot travel backward.

In this article, we will talk about two basic concepts about the neuron:

External Structure of Neuron

A diagram of a multipolar neuron with labelled external structures including the soma, dendrite, and axon.
A diagram of the External Structure of Neuron

First, let’s take a look at the external structure of the most abundant type of neuron in the human body: the multipolar neuron.

Cell Body

The cell body (a.k.a. the soma, in neuroscience jargon) contains the nucleus and other smaller internal structures. It is responsible for most of the protein and energy production of the cell.

It is generally circular (round) but can also have a more “triangular” shape such as in the pyramidal cell.

Each soma receives electrical impulses from a number of dendrites.

Dendrites

Dendrites are like antennas that receive information from other neurons and transmit that information to the soma. Each dendrite that connects to the soma contains many different “branches.” This ensemble is called the “dendritic tree.”

Let’s dive into a little more details here:

Each point where one segment of the dendrite branches into two segments is called a bifurcation. (The first time the dendrite branches in two is called the first bifurcation, the second time it branches in two is called the second bifurcation, and so on.)

The dendrites have the amazing ability to receive incoming signals directly on their membrane, or on tiny little protrusions called dendritic spines. My professor liked to say that these spines look like, “forests of tiny lollipops.”

The Axon

Each neuron has one axon that will transmit the information to the following cell. The axon connects to the soma at the axon hillock, which is a very important structure as it is the final point where all the information from all the dendrites gets integrated into one clear signal, that will travel through the axon and to the next cell.

Often in a human body, a myelin sheath covers the axons. This helps increase the speed of propagation of electrical information. This myelin sheath has gaps which are called Nodes of Ranvier.

Axons can branch as well, and each axonal branch is called axon collateral.

The axon ends in small structures called synaptic terminals. The synaptic terminal will connect to another neuron’s dendrite (or dendritic spine) and transmit the information.

Now that we’ve mastered the basic external structure of neurons, let’s look at the different types of neurons that we can find in the human body.

Types of Neurons Based on Structure

The different types of Neurons are Multipolar, bipolar, pseudounipolar, and unipolar neurons.
Types of Neurons

Multipolar Neurons

These neurons contain a number of dendrites and one axon. They are the most common type of neurons and they can be found more or less anywhere in the nervous system.
For example:

  • Pyramidal neurons in the cerebral cortex
  • Purkinje neurons in the cerebellum
  • Motor neurons in the anterior horn of the spinal cord
The Motor Neuron, Pyramidal Neuron, and Purkinje cell are examples of Multipolar Neurons.

Bipolar Neurons

Bipolar neurons have only two processes that connect to the cell body: one dendrite and one axon. (This is easy to remember as, generally speaking, the prefix “bi” refers to the number two, such as in bilingual – two languages)

The Retinal and Olfactory Neurons are examples of Bipolar Neurons.

Bipolar neurons are only found in specific areas of the nervous system:

  • In the retina
  • In the nose (receptors of the olfactory epithelium)

Unipolar Neurons

These types of neurons are not found in humans. They are found in invertebrates such as insects. Their structure shows a single axon extending from the soma.

Differentiation between a unipolar and pseudounipolar neuron. The unipolar neuron has a single axon leaving the soma. While the pseudounipolar neuron also has a single process extending from the soma, the single axon splits into two axonal branches.
Differentiation between a unipolar and pseudounipolar neuron.

Pseudounipolar Neurons

Pseudounipolar neurons, on the other hand, also has a single axon leaving the soma (this gives us the “unipolar part”). What makes them different from unipolar neurons is that this single axon branches into two (which is why we add “pseudo” at the beginning… It doesn’t look unipolar). The two axonal branches on one side extend to the peripheral skin, joints, or muscles, and the other into the center (spinal cord).

These types of neurons DO NOT have dendrites. The axons take their role in receiving information.

Lastly, these neurons are found in humans and are exclusive to sensory neurons.

  • Pseudounipolar neurons can be found in the spinal ganglions.

Minimum Points to remember:

Neurons are the coolest type of cells.  They are made of:

  • A cell body-called the soma
  • Dendrites that receive information
  • An axon that transmits information to another cell.

Neurons are polarized in that the information can only travel in one direction: dendrite to the axon.

There are 3 different types of neurons based on their structure:

  1. Multipolar neurons: one axon, many dendrites
  2. Bipolar neurons: one axon, one dendrite
  3. Unipolar neurons: single axon
  4. Pseudounipolar neurons: One axon that branches in two.

That’s it for this introduction to the neuron’s structure.

If you want more articles and videos about the Nervous System, you can find them here. More resources are available to help make Biology fun. I invite you to absorb all the content you can find here at Interactive-Biology.com.


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