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 it, but the truth is that there is not only 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. 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 we curiously 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 through nerve impulses.
The human brain has approximately between 80 and 100 billion neurons . Neural networks are responsible for performing the complex functions of the nervous system, ie, that these functions are not a consequence 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 different parts of the brain 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 start exploring the diversity of neuron classes, 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 comes to mind. But not all neurons are the same because there are different types. However, In general, its structure is composed of the following parts :
- Soma : The soma, also called pericarion, is the cell body of the neuron. It is where the nucleus is found, and from which two types of extensions are born
- Dendrites : The 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 drive a nerve impulse from the soma to another neuron, muscle or gland of the body. Axons are usually covered with myelin, a substance that allows a faster movement of the nerve impulse.
You can learn more about myelin in our article: "Myelin: definition, functions and characteristics"
One of the parts in which the axon is divided and that 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 emitting neuron and the dendrite of the recipient cell.
Types of neurons
There are different ways of classifying neurons, and they can be established based on different criteria.
1. According to nerve impulse transmission
According to this classification, there are two types of neurons:
1.1. Presynaptic neuron
As already said, the union between two neurons is the synapse. As well, the presynaptic neuron is the neurotransmitter contains and releases it to 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 its function
Neurons can have different functions within our central nervous system, that's 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, sensory neurons send the message from your hand to your central nervous system that interprets ice as cold.
2.2. Motor neurons
This type of neurons send information from the CNS to the skeletal muscles (somatic motoneurons), to effect movement, or to the smooth muscle or ganglia of the CNS (visceral motoneurons).
An interneuron, also known as an integrative 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 nervous impulse
Depending on the direction of the nerve impulse, neurons can be of two types:
3.1. Afferent neurons
This type of neurons are sensory neurons. They receive this name because they transport the nervous 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 like muscles or glands .
- Learn more: "Via afferent and via efferent: 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 into the postsynaptic flows, which in turn depend on the type of receptor and neurotransmitter involved in the synapse (for example, glutamate or GABA)
4.1. Exciting neurons
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 cause 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, because they do not generate a transmitting signal but rather regulate it. These neurotransmitters are known as neuromodulators and its function is to modulate the response of the cell to a main 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 that the neurons release, they receive the following name:
5.1. Serotoninergic neurons
This type of neurons They transmit the neurotransmitter called 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
Dopaminergic neurons transmit Dopamine . A neurotransmitter related to addictive behavior.
- You may be interested: "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 it plays in the brain"
5.4. Glutamatergic neurons
This type of neurons transmits glutamate . The main excitatory neurotransmitter.
- Maybe you're 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 double function, as hormone and neurotransmitter.
5.7. Vasopresynergic neurons
These neurons are responsible for transmitting Vasopressin , also called the chemical substance of monogamy or fidelity.
5.8. Oxytocinergic neurons
Transmit Oxytocin, another neurochemical related to love . It receives the name of hormone of the hugs.
- Learn more about oxytocin in our post: "The chemistry of love: a very powerful drug"
6. According to its external morphology
According to the number of extensions that the neurons have, they are classified as:
6.1. Unipolar or Pseudounipolar neurons
They are neurons that have a single extension of double meaning that leaves the soma, and that acts both as a dendrite and as an axon (input and output). They are usually sensory neurons, that is, afferent .
6.2. Bipolar neurons
They have two cytoplasmic extensions (extensions) that leave 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 input extensions (dendrites) and a single output (axon) . They are found in the brain or spinal cord.
7. Other types of neurons
Depending on the location of the neurons and according to their shape, they are classified as:
7.1. Mirror neurons
These neurons were activated when performing an action and 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 amygdala . They have a triangular shape, that's why they get this name.
7.3. Purkinje neurons
They are found in the cerebellum , and they are called that because their discoverer was Jan Evangelista Purkyně. These neurons branch out constructing an intricate dendritic tree and are aligned like domino pieces placed one opposite the 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 as a basket. Neurons in the basket are found in the hippocampus or cerebellum.
In our nervous system there is a great diversity of types of neurons that adapt and specialize according to their functions so that all the mental and physiological processes can be developed in real time (at a dizzying speed) and without setbacks.
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 adapt, although this can be a headache when it comes to studying and understanding them.
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- 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.