Types of neurons: characteristics and functions

What kinds of neurons do we have and what role do they play in our central nervous system?

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 kind of these microscopic structures: there are many types of neurons with different shapes and functions.

The different types of neurons: a great diversity

The human body is made up of 37 trillion cells. A large part of the cells of the nervous system are the glial cellsThe rest of the diversity is accounted for by the so-called neurons, which are in fact the most abundant in our brain and which we curiously tend to forget. These nerve cells, which receive and emit electrical signals, are interconnected to form 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.. Neuronal networks are responsible for carrying out the complex functions of the nervous system, i.e., these functions are not a consequence of the specific characteristics of each individual neuron. And, as 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 neuron types, let's look at what they have in common: their basic structure.

Neuron structure

When we think of the brain, the image of neurons usually comes to mind. But not all neurons are the same, as there are different types. However, neurons are not all the same, their structure is generally composed of the following parts:

• SomaThe soma, also called pericarionis the cell body of the neuron. It is where the nucleus is located, and from which two types of extensions are born
• DendritesDendrites are prolongations that come from the soma and look like branches or tips. They receive information from other cells.
• AxonAxon: The axon is an elongated structure that starts 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 junction 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 on the basis of 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 junction between two neurons is the synapse. The presynaptic neuron the presynaptic neuron is the one that contains the neurotransmitter and releases it into the synaptic space so that it can pass to another neuron..

1.2. Postsynaptic neuron

At 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, that 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 on your hand, the sensory neurons send the message from your hand to your central nervous system, which interprets the ice as cold.

2.2. Motor neurons

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

2.3. Interneurons

An interneuron, also known as an integrating or association neuron, connects with other neurons but never with sensory receptors or muscle fibers.. It is responsible for performing more complex functions and acts in reflex acts.

3. According to the direction of the nerve impulse

Depending on the direction of the nerve impulse, neurons can be of two types:

3.1. Afferent neurons.

This type of neurons are the sensory neurons. They are called afferent neurons because they carry the nerve impulse from the receptors or sensory 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..

• Learn more: "Afferent and efferent pathways: types of nerve fibers".

4. According to the type of synapse

Depending on the type of synapse, there are two types of neurons: excitatory neurons 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 fluxes, which in turn depend on the type of receptor and neurotransmitter involved in the synapse (e.g., glutamate or GABA).

Excitatory neurons

These are neurons in which the result of synapses elicits an excitatory response, i.e., increases the possibility of a response in the brain.i.e. increases the possibility of producing an action potential.

4.2. Inhibitory neurons

Inhibitory neurons are those in which the result of these synapses provokes an inhibitory response, i.e. it reduces the possibility of producing an action potential.i.e. it reduces the possibility of producing an action potential.

4.3. Modulatory neurons

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

5. According to the neurotransmitter

Depending on the neurotransmitter released by the neurons, they receive the following name:

5.1. Serotonergic neurons.

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

5.2. Dopaminergic neurons

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

5.3. GABAergic neurons

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

5.4. Glutamatergic neurons

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

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 dual functions.a catecholamine with dual function as a hormone and neurotransmitter.

5.7. Vasopressinergic neurons

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

5.8. Oxytocinergic neurons

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

• Learn more about oxytocin in our post: "The chemistry of love: a very potent drug".

6. According to their external morphology

According to the number of prolongations that the neurons have, they are classified as follows:

6.1. Unipolar or Pseudounipolar neurons.

These neurons have a single two-way prolongation that leaves the soma and acts both as a dendrite and as an axon (input and output). They are usually sensory neurons, i.e., afferent neurons..

6.2. Bipolar neurons

They have two cytoplasmic extensions (prolongations) that leave the soma. One acts as a dendrite (input) and the other acts as an axon (output).. 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 prolongations (dendrites) and only one output prolongation (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 follows:

7.1. Mirror neurons

These neurons are activated when performing an action and when seeing another person performing 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 amygdaloid body.. They have a triangular shape, that is why they receive this name.

7.3. Purkinje neurons

They are located in the cerebellumand are so named because their discoverer was Jan Evangelista Purkyně. These neurons branch in 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).where they contain proteins (receptors) that receive information from odorants. Their unmyelinated axons synapse in the olfactory bulb of the brain.

7.6. Basket neurons

These contain a single large apical dendritic arborwhich branches in the form of 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 develop in real time (at breakneck speed) and smoothly.

The encephalon 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 are adapted, although this can be a headache when it comes to studying and understanding them.

Bibliographical 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). Neurodynamics. Order and chaos in complex systems. Edicions UPC.