Neurons are often referred to as the basic units that, together, form the nervous system and the brain that is included in it, but the truth is that there is not just one class of these microscopic structures: there are many types of neurons with different shapes and functions.

The different kinds of neurons: a great diversity

The human body is made up of 37 trillion cells. Most of the cells of the nervous system are the glial cells , which are in fact the most abundant in our brain and which we tend to forget, but the rest of the diversity corresponds to the so-called neurons. These nerve cells that receive and emit electrical signals are interconnected forming communication networks that transmit signals through different areas of the nervous system via nerve impulses.

The human brain has approximately between 80 and 100 billion neurons . Neural networks are responsible for carrying out the complex functions of the nervous system, that is, these functions are not the result of the specific characteristics of each individual neuron. And, since there are so many things to do in the nervous system and the functioning of the different parts of the brain is so complex, these nerve cells also have to adapt to this multiplicity of tasks. How do they do it? By specializing and dividing into different types of neurons.

But before we start exploring the diversity of classes of neurons, let’s see what they have in common: their basic structure.

Structure of the neuron

When we think about the brain, the image of neurons usually comes to mind. But not all neurons are the same because there are different types. Now, usually its structure is composed of the following parts :

  • Soma : The soma, also called the pericarion , is the cell body of the neuron. It is where the nucleus is located, and from which two types of extensions are born
  • Dendrites : Dendrites are extensions that come from the soma and look like branches or spikes. They receive information from other cells.
  • Axon : The axon is an elongated structure starting from the soma. Its function is to conduct a nerve impulse from the soma to another neuron, muscle or gland in the body. Axons are usually covered with myelin, a substance that allows a faster circulation of the nerve impulse.

You can learn more about myelin in our article: “Myelin: definition, functions and characteristics”

One of the parts into which the axon is divided and which is responsible for transmitting the signal to other neurons is called the terminal button. The information that passes from one neuron to another is transmitted through the synapse, which is the link between the terminal buttons of the sending neuron and the dendrite of the receiving cell.

Types of neurons

There are different ways of classifying neurons, and they can be established based on different criteria.

1. According to the transmission of the nerve impulse

According to this classification, there are two types of neurons:

1.1. Presynaptic neuron

As already mentioned, the link between two neurons is the synapse. Well, the presynaptic neuron is the one that contains the neurotransmitter and releases it into the synaptic space to pass to another neuron .

1.2. Post-synaptic neuron

In the synaptic junction, this is the neuron that receives the neurotransmitter .

2. According to their function

Neurons can have different functions within our central nervous system, which is why they are classified in this way:

2.1. Sensory Neurons

They send information from the sensory receptors to the central nervous system (CNS) . For example, if someone puts a piece of ice in your hand, the sensory neurons send the message from your hand to their central nervous system that interprets the ice as cold.

2.2. Motor neurons

These types of neurons send information from the CNS to the skeletal muscles (somatic motor neurons), to effect movement, or to the smooth muscle or nodes of the CNS (visceral motor neurons).

2.3. Interneurons

An interneuron, also known as an integrative or association neuron, connects with other neurons but never with sensory receptors or muscle fibers . It is in charge of performing more complex functions and acts on reflexes.

3. According to the direction of the nerve impulse

Depending on the direction of the nerve impulse, there are two types of neurons:

Afferent neurons

This type of neuron is the sensory neuron. They receive this name because they transport the nerve impulse from the receptors or sense organs to the central nervous system .

3.2. Efferent neurons

These are the motor neurons. They are called efferent neurons because they transport nerve impulses out of the central nervous system to effectors such as muscles or glands .

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  • Learn more: “Afferent pathway and efferent pathway: nerve fiber types”

4.Depending on the type of synapse

Depending on the type of synapse we can find two types of neurons: excitatory and inhibitory neurons. About 80 percent of neurons are excitatory. Most neurons have thousands of synapses on their membrane, and hundreds of them are active simultaneously. Whether a synapse is excitatory or inhibitory depends on the type(s) of ions that are channeled into the postsynaptic streams, which in turn depend on the type of receptor and neurotransmitter involved in the synapse (e.g., glutamate or GABA)

4.1. Excitatory neurons

These are synapses in which the result of the synapses provokes an excitatory response , that is, it increases the possibility of producing an action potential.

4.2. Inhibitory neurons

They are those in which the result of these synapses provokes an inhibitory response , that is, that reduces the possibility of producing an action potential.

4.3. Modulating neurons

Some neurotransmitters may play a role in synaptic transmission other than excitatory and inhibitory, as they do not generate a transmitting signal but regulate it. These neurotransmitters are known as neuromodulators and their function is to modulate the cell’s response to a major neurotransmitter . They usually establish axo-axonal synapses and their main neurotransmitters are dopamine, serotonin and acetylcholine

5.According to the neurotransmitter

Depending on the neurotransmitter released by the neurons, they are called:

5.1. Serotoninergic Neurons

This type of neuron transmits the neurotransmitter called Serotonin (5-HT) which is related to, among other things, mood.

5.2. Dopaminergic Neurons

Dopaminergic neurons transmit Dopamine . A neurotransmitter related to addictive behavior.

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  • You may be interested in: “Dopamine: 7 essential functions of this neurotransmitter”

5.3. GABAergic Neurons

GABA is the main inhibitory neurotransmitter. GABAergic neurons transmit GABA.

5.4. Glutamatergic Neurons

This type of neuron transmits Glutamate . The main excitatory neurotransmitter.

  • You may be interested in: “Glutamate (neurotransmitter): definition and functions”

5.5. Cholinergic Neurons

These neurons transmit Acetylcholine . Among many other functions, acetylcholine plays an important role in short-term memory and learning.

5.6. Noradrenergic Neurons

These neurons are responsible for transmitting Noradrenaline (Norepinephrine) , a catecholamine with a dual function, as a hormone and a neurotransmitter.

5.7. Vasopressinergic Neurons

These neurons are responsible for transmitting Vasopressin , also called the chemical of monogamy or fidelity.

5.8. Oxytocinergic Neurons

They transmit Oxytocin, another neurochemical related to love . It is called the hugging hormone.

  • Learn more about oxytocin in our post: “The chemistry of love: a very powerful drug”

6.According to its external morphology

Depending on the number of extensions that the neurons have, they are classified as

6.1. Unipolar or Pseudounipolar Neurons

These are neurons that have a single two-way extension coming out of the soma, acting both as a dendrite and an axon (input and output). They are usually sensory neurons, that is, afferent .

6.2. Bipolar Neurons

They have two cytoplasmic extensions (prolongations) coming out of the soma. One acts as a dendrite (entrance) and the other acts as an axon (exit) . They are usually located in the retina, cochlea, vestibule and olfactory mucosa

6.3. Multipolar Neurons

They are the most abundant in our central nervous system. They have a large number of input extensions (dendrites) and only one output extension (axon) . They are found in the brain or spinal cord.

7.Other types of neurons

According to the location of the neurons and according to their shape, they are classified as

7.1. Mirror Neurons

These neurons were activated by performing an action and seeing another person perform an action. They are essential for learning and imitation.

  • Learn more: “Mirror Neurons and their Importance in Neurorehabilitation”

7.2. Pyramidal neurons

These are located in the cerebral cortex, the hippocampus, and the amygdalin body . They have a triangular shape, which is why they are called this.

7.3. Purkinje Neurons

They are found in the cerebellum , and are so named because their discoverer was Jan Evangelista PurkynÄ›. These neurons branch out to build an intricate dendritic tree and are lined up like dominoes facing each other.

7.4. Retinal Neurons

They are a type of receptive neuron that take signals from the retina in the eyes.

7.5. Olfactory neurons

These are neurons that send their dendrites to the olfactory epithelium , where they contain proteins (receptors) that receive information from the odorants. Their non-myelinated axons make synapses in the olfactory bulb of the brain.

7.6. Neurons in basket

These contain a single large apical dendritic tree , which branches out into a basket. Basket neurons are found in the hippocampus or cerebellum.

In conclusion

In our nervous system there is a great diversity of types of neurons that adapt and specialize according to their functions so that all mental and physiological processes can be developed in real time (at a dizzying speed) and without setbacks.

The brain is a very well-oiled machine precisely because both the classes of neurons and the parts of the brain perform very well the functions to which they adapt, although this can be a headache when studying and understanding them.

Bibliographic references:

  • Djurisic M, Antic S, Chen W, Zecevic D (2004). Voltage imaging from dendrites of mitral cells: EPSP attenuation and spike trigger zones. J Neurosci 24 (30): 6703-14.
  • Gurney, K. (1997). An Introduction to Neural Networks. London: Routledge.
  • Solé, Ricard V.; Manrubia, Susanna C. (1996). 15. Neurodynamics. Order and chaos in complex systems. Edicions UPC.