Types of neurons: characteristics and functions

It is common to refer to neurons as the basic units that, together, form the nervous system and the brain that is included in this, but the truth is that there is not only one class of these microscopic structures: there are many types of neurons with different forms and functions.

The different classes 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 cells, which in fact are the most abundant in our brain and that curiously 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 interconnect forming networks of communication that transmit signals through different areas of the nervous system through impulses nervous

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, that is, these functions are not a consequence of the specific characteristics of each neuron individual. And, as in the nervous system there are so many things to do and how the different functions work.

parts of the brain It is so complex, these nerve cells also have to adapt to this multiplicity of tasks. How do they do that? Specializing and dividing into different types of neurons.

But before we explore the diversity of neuron classes, let's see 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, usually its structure is composed of the following parts:

• Soma: The soma, also called perikaryon, 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 tips. They receive information from other cells.
• Axon: 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. The 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 divides and which is responsible for transmitting the signal to other neurons is called the terminal button. 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 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 junction between two neurons is the synapse. As 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. Postsynaptic neuron

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

2. According to its function

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

2.1. Sensory neurons

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

2.2. Motor neurons

These types of neurons send information from the CNS to skeletal muscles (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

These types of neurons are sensory neurons. They get this name because carry nerve impulses from receptors or sensory organs to the central nervous system.

3.2. Efferent neurons

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

• Know more: "Afferent pathway and efferent pathway: the types of nerve fibers"

4. According to 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 or types of ions that are channeled in the fluxes. postsynaptic, which in turn depend on the type of receptor and neurotransmitter involved in the synapse (for example, the glutamate or GABA).

4.1. Excitatory neurons

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

4.2. Inhibitory neurons

Are those in which the result of these synapses elicit an inhibitory response, that is, it reduces the possibility of producing an action potential.

4.3. Modulator neurons

Some neurotransmitters may play a role in synaptic transmission other than excitatory and inhibitory, since they do not generate a transmitter signal but rather regulate it. These neurotransmitters are known as neuromodulators and its 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 neurons, they receive the following name:

5.1. Serotonergic neurons

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

• Related article: "Serotonin: discover the effects of this hormone on your body and mind"

5.2. Dopaminergic neurons

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

• 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.

• Related article: "GABA (neurotransmitter): what it is and what role does it play in the brain"

5.4. Glutamatergic neurons

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

• You may be interested: "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 dual functions, 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. Oxytokinenergic 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 neurons have, they are classified into:

6.1. Unipolar or Pseudounipolar Neurons

They are neurons that have a single two-way extension that comes out of the soma, and that acts both as a dendrite and as an axon (entrance and exit). They are usually sensory neurons, that is, afferent.

6.2. Bipolar neurons

They have two cytoplasmic extensions (processes) that come out of the soma. One acts as a dendrite (input) and another 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 entry processes (dendrites) and a single exit process (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 into:

7.1. Mirror neurons

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

• Know more: "Mirror neurons and their importance in neurorehabilitation"

7.2. Pyramidal neurons

These are located in the cerebral cortex, the hippocampus, and the tonsillar body.. They have a triangular shape, that's why they receive this name.

7.3. Purkinje neurons

They are found in the cerebellum, and they are so called because their discoverer was Jan Evangelista Purkyně. 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 They take signals from the retina in the eyes.

7.5. Olfactory neurons

They are neurons that send their dendrites to the olfactory epithelium, where they contain proteins (receptors) that receive information from odorants. Their unmyelinated axons synapse in the olfactory bulb of the brain.

7.6. Neurons in basket or basket

These contain a single large apical dendritic tree, which branches out 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 to that all mental and physiological processes can be developed in real time (at breakneck 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 the functions to which they adapt very well, although this may be a headache when studying them and understand 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. UPC Editions.