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.

Types of neurons

The different kinds of neurons: a great diversity

The human body is made up of 37 billion 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 communication networks that transmit signals through different areas of the nervous system through nerve impulses.

The human brain has approximately 80 to 100 billion neurons. Neural networks are responsible for performing the complex functions of the nervous system, that is, 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 multitude of tasks. How do they do that? Specializing and dividing into different types of neurons.

But before we begin to explore the diversity of classes of neurons, 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 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 points. 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 body gland. Axons are usually covered with myelin, a substance that allows faster circulation of the nerve impulse.

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 sending 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 in the synapse. Well,  the presynaptic neuron is contained in the neurotransmitter and released into the synaptic space to pass to another neuron.

1.2. Postsynaptic 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, so they are classified in this way:

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 your hand to their central nervous system that interprets that the ice is cold.

2.2. Motor neurons

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

2.3. Interneurons

An interneuron, also known as 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 on 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 receive this name because they carry the nerve impulse from the receptors or sensory organs to the central nervous system

3.2. Efferent neurons

These are 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

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 the 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 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

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 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 different from excitatory and inhibitory since 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 the main neurotransmitter. They usually establish axon-axon 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 transmits the neurotransmitter called Serotonin (5-HT) that 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 double 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.

5.8. Oxytocinergic Neurons

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

6. According to its external morphology

Depending on the number of extensions that neurons have, these 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 at the same time as dendrite and as an axon (entry and exit). 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 (entrance) and another 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 extensions of the entry (dendrites) and a single exit (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 form, they are classified as:

7.1. Mirror neurons

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

7.2. Pyramidal neurons

These are located in the cerebral cortex, the hippocampus, and the tonsil body. They have a triangular shape, so they receive this name.

7.3. Purkinje neurons

They are in the cerebellum , and they are called that because their discoverer was Jan Evangelista Purkyně. These neurons branch out building an intricate dendritic tree and are aligned as domino pieces placed opposite each other.

7.4. Retinal neurons

They are a type of receptive neuron that takes 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. Its non-myelinated 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 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 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 may be a headache when studying and understanding them.

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