It’s common to refer to neurons as the basic units that together make up the nervous system and the brain that is included in it, but the truth is that there isn’t just one class of these microscopic structures: There are many. types of neurons with different shapes 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 are in fact the most abundant in our brains and which 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 interconnect to form communication networks that transmit signals through different areas of the nervous system via nerve impulses.
The human brain has about between 80 and 100 billion neurons. Neural networks are responsible for performing the complex functions of the nervous system, i.e. these functions are not a consequence of the specific characteristics of each individual neuron. And, since there is so much going on in the nervous system, and the workings of different parts of the brain are so complex, these nerve cells must also adapt to this multiplicity of tasks. How do they do? specialization and divide into different types of neurons.
But before we explore the diverse classes of neurons, let’s take a look at what they have in common: their basic structure.
When we think of the brain, the image of neurons usually 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 the pericarion, is the cell body of the neuron. This is where the kernel is and where 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 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 faster circulation of nerve impulses.
You can read more about myelin in our article: “Myelin: definition, functions and characteristics”
One of the parts in which the axon is divided 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 forms of classification of neurons, and they can be established on the basis of different criteria.
1. According to the transmission of nerve impulses
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. So good, the presynaptic neuron is contained in the neurotransmitter and releases it into the synaptic space to pass to another neuron.
1.2. Postsynaptic neuron
In the synaptic junction, it is the neuron that receives the neurotransmitter.
2. According to its function
Neurons can have different functions within our central nervous system, so they are classified as follows:
2.1. sensory neurons
They send information from 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 the hand to their central nervous system by interpreting the 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 smooth muscle or CNS ganglia (visceral motor neurons).
An interneuron, also called an integrating or associative neuron, it connects with other neurons but never with sensory receptors or muscle fibers. It is responsible for performing more complex functions and acts by 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 they carry nerve impulses from receptors or sensory organs to the central nervous system.
3.2. Efferent neurons
These are the motor neurons. They are called efferent neurons because transporting 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, we can find two types of neurons: excitatory and inhibitory neurons. About 80% of neurons are excitatory. Most neurons have thousands of synapses on their membranes, 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 postsynaptic flows, which in turn depend on the type of receptor and neurotransmitter involved in the synapse (for example, glutamate or GABA).
4.1. excitatory neurons
These are the ones in which the result of synapses provokes an excitatory responseIn other words, it increases the possibility of producing an action potential.
4.2. inhibitory neurons
These are the ones in which the result of these synapses triggers an inhibitory responseIn other words, it reduces the chances of producing an action potential.
4.3. modulatory neurons
Certain neurotransmitters may play a role in synaptic transmission other than excitatory and inhibitory, as they do not generate a transmission signal but 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 axoaxonal synapses and their main neurotransmitters are dopamine, serotonin, and acetylcholine.
5. According to the neurotransmitter
Depending on the neurotransmitter that neurons release, they are given the following name:
5.1. serotonergic neurons
This type of neurons transmit the neurotransmitter called serotonin (5-HT) which is linked, among other things, to mood.
5.2. dopaminergic neurons
Dopaminergic neurons transmit dopamine. A neurotransmitter linked 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.
5.4. glutamatergic neurons
This type of neuron 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 the transmission of norepinephrine (norepinephrine), A catecholamine with dual functions, as a hormone and neurotransmitter.
5.7. Vasopressinergic neurons
These neurons are responsible for transmitting vasopressin, As well called the chemical of monogamy or loyalty.
5.8. Oxytocinergic neurons
They transmit oxytocin, another neurochemical linked to love. It’s called the hug hormone.
- Learn more about oxytocin in our article: “The Chemistry of Love: A Very Potent Drug”
6. According to its external morphology
According to the number of extensions presented by neurons, they are classified into:
6.1. Unipolar or pseudounipolar neurons
These are neurons that have a single bidirectional extension that comes out of the soma and acts both as a dendrite and as an axon (entry and exit). They are usually sensory neurons, i.e. afferent.
6.2. bipolar neurons
They have two cytoplasmic extensions (extensions) 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 input extensions (dendrites) and a single output (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 the shape, they are classified into:
7.1. mirror neurons
These neurons were activated when performing an action and seeing another person take an action. They are essential for learning and imitation.
- Find out more: “Mirror neurons and their importance in neuroreeducation”
7.2. pyramidal neurons
These are located in the cerebral cortex, the hippocampus and the amygdala. They have a triangular shape, which is why they have 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 form a complex dendritic tree and are aligned like pieces of domino placed one in front of the other.
7.4. Retinal neurons
They are a type of receptive neuron that pick up 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 odorants. Its unmyelinated axons synapse into the olfactory bulb of the brain.
7.6. Neurons in a basket or basket
These contain a single large apical dendritic tree, Which branches into a basket. Basket neurons 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 mental and physiological processes can develop in real time (at breakneck speed) and without setbacks.
The brain is a very well-oiled machine precisely because classes of neurons and parts of the brain perform very well the functions to which they adapt, although this can be a headache when studied and understood.
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