What functions do neurons and glial cells fulfill in our body?
What type of neurons and glial cells exist, what function they fulfill and what their structure is like are some of the questions that we try to answer below.
- Recommended article: “The 10 most important types of neurotransmitters (and their functions)” , by Bertrand Regader
Neurons: cells of the central nervous system
Neurons are the specialized cells of the central nervous system . These types of nerve cells resemble other cells, among other things, in that they have a membrane that surrounds them, contain genes, cytoplasm, mitochondria, and trigger basic cellular processes such as protein synthesis and energy production.
The characteristic feature of neurons is the excitability of their plasma membrane, which allows them not only to receive stimuli but also to conduct nerve impulses to other neurons through their axon or neurite.
structure of a neuron
Although there are many types of neurons, these nerve cells are basically made up of three components or parts:
- The cell body : it is the container of the nucleus of the neuron and it is here where the genetic information is stored.
- The axon : is the extension that functions as a cable that transmits electrical signals (action potentials) from the cell body to other neurons.
- Dendrites : are microscopic branches that capture electrical signals emitted by other neurons.
Classification of neurons
Neurons can be classified based on different criteria.
Below we will see a classification based on their structure, the function they perform , their discharge pattern, the action they exert on other neurons and the type of neurotransmitters they produce.
1. Because of its structure
Neurons can be classified according to their structure, that is, according to the shape, size or length of their dendrites.
1.1. unipolar neurons
These types of neurons are the most common in invertebrates, although they can also be found in the retina.
They are characterized by having a single two-way extension that comes out from the cell body and serves as both an axon and a dendrite. They are usually afferent sensory neurons.
1.2. bipolar neurons
These nerve cells have two branches that start from the soma or cell body: one acts as a dendrite or input channel, and the other as an axon or output channel.
They are common in the sensory pathways of sight, hearing, touch, and taste, as well as vestibular function.
1.3. pseudounipolar neurons
These neurons are distinguished from unipolar neurons in that their axon divides into two branches, one of which generally goes to the periphery and the other to the central nervous system.
They are important, for example, in the sense of touch. They are considered as a variant of bipolar neurons.
1.4. multipolar neurons
Most neurons belong to this group and are characterized by having a single, usually long, axon and many dendrites.
There are two classes: Golgi type I, with long axons, typical of the pyramidal cells of the cortex and the Purkinje cells of the cerebellum; and type Golgi II, with short axons, characteristic of granular cells.
2. Because of its function
In addition to their structure, neurons can be classified according to the role they play in the nervous system and their function in sending or receiving information, or as a link between some neurons and others.
2.1. sensory neurons
This type of afferent neurons are activated through sensory input that comes from sensory organs such as the skin, eyes, ears or nose.
They then send projections to the central nervous system to transmit the information to the brain or spinal cord.
2.2. motor neurons
The task of these neurons is to send signals from the brain and spinal cord to the muscles. They are, therefore, efferent neurons.
They can be somatic, which are responsible for sending information to the skeletal muscles to regulate movement; or visceral, sending information to the smooth muscles or ganglia of the central nervous system.
23. Interneurons
This type of nerve cells bridges between neurons, but never with sensory receptors or muscle fibers. They can have shorter or longer axons, depending on how far apart the neurons are from each other.
3. By action on other neurons
Neurons can influence other cells by releasing neurotransmitters and generating excitatory, inhibitory, or modulatory effects.
3.1. excitatory neurons
This type of neuron releases a substance called glutamate and provokes in the receptor of the neuron that receives it an increase in the possibility of producing an action potential (hence its excitatory power).
3.2. inhibitory neurons
Inhibitory neurons release a substance called GABA, a neurotransmitter with inhibitory effects that produces a reduction in the firing rate of the action potential of the neuron that captures it.
3.3. modulatory neurons
These types of neurons do not have a direct effect, but instead modify structural aspects of other nerve cells in the long term. They usually release neurotransmitters such as dopamine, serotonin and acetylcholine.
4. By your download pattern
We can also classify neurons based on their electrophysiological features and the firing pattern they generate.
4.1. Tonic or regular shots
These types of neurons are constantly active and release action potentials regularly.
4.2. Phasic or “bursting”
Phasic or bursting neurons are nerve cells noted for their explosive activation and burst discharge pattern.
4.3. of quick shots
They are a type of neurons that are characterized by their high firing rates; that is, they transmit discharge patterns with great frequency.
Globus pallidus neurons, retinal ganglion cells, or some cortical inhibitory interneurons are examples of fast-firing neurons.
5. By the production of neurotransmitters
Another way to classify neurons is based on the type of neurotransmitters they release.
5.1. cholinergic neurons
These neurons release the neurotransmitter acetylcholine into the synaptic cleft. This substance plays a significant role in short-term memory and learning,
5.2. glutamatergic neurons
These nerve cells release the substance glutamate, which, together with aspartate, represents the main excitatory neurotransmitters.
When blood flow to the brain is reduced, glutamate can be toxic by causing overactivation at synapses.
5.3. GABAergic neurons
These neurons release GABA, the main inhibitory neurotransmitter.
5.4. serotonergic neurons
They release serotonin or 5-HT and can act as both excitatory and inhibitory neurons. This neurotransmitter is related to the modulation of mood, perception or sexual appetite.
5.5. dopaminergic neurons
These neurons secrete dopamine, a neurotransmitter related to mood, the reward system, and behavior in general.
Other types of neurons
Although most neurons can be included in a classification like the one we have detailed above, there is no agreement among scientists when it comes to determining an exact number of them, but it is estimated that there could be more than 200 types .
In addition to those that have already been described, we would like to point out these others due to their importance:
6. Mirror neurons
These neurons are activated when an action is executed and when observing that same action being executed by another individual, generally of the same species. They play an important role in cognitive abilities linked to social life, empathy or imitation.
7. Olfactory neurons
In vertebrates, odors are initially perceived in the olfactory epithelium, which lines the nasal cavity and is packed with millions of olfactory cells or neurons.
These types of neurons are hair cells, that is, they are made up of tiny extensions of their membrane, which allow the contact surface between the nerve cell and the outside to be increased.
Glial cells: traveling companions of neurons
We have seen that neurons are a fundamental piece of the nervous system, but not everything is reduced to them. Along with neurons, we also find glial cells or neuroglia .
Glial cells are part of an essential support and care system for the proper functioning of the nervous system tissue. Unlike neurons, glial cells do not have axons, dendrites, or nerve channels. They are smaller than neurons and are about three times more numerous in the nervous system.
What are glial cells for?
Glial cells are found around neurons and perform essential functions such as providing structural and metabolic support to neurons. They are also related to brain development, as they have been shown to guide axons on their way to establishing long-distance connections.
The glia provide the axon with cell adhesion substances and trophic factors, which serve the nerve ending to increase its surface in specific directions, so that it can move towards its goal. These signals are critical for the creation of functional circuits that later organize complex sequences of reactions.
These types of cells are also able to control the composition of the extracellular medium. And how do they do it? Well, by taking care of processing metabolically active products and substances (such as ions, hormones, drugs, etc.) before they accumulate in that extracellular space,
Glial cell types
There are several types of glial cells in the central nervous system and the peripheral nervous system. The main ones are the following:
8. Astrocytes
They are the most abundant glial cells and are found in the brain and spinal cord. They are star-shaped and reside in the endothelial cells of the central nervous system that form the blood-brain barrier.
Astrocyte functions
Astrocytes supply neurons with nutrients, give them structural support, help them repair and regenerate, and act as a separation and insulation barrier between neurons, preventing the dispersal of neurotransmitters released at the terminal buttons of the neuron. They can also store and take up neurotransmitters.
9. Ependymal cells
Ependymal cells are specialized cells that line the ventricles of the brain and the central canal of the spinal cord. They are located in the choroid plexus of the meninges. These types of cells, like neuroglia, derive from a layer of embryonic tissue known as the neuroectoderm.
Ependymal cell functions
They mainly produce cerebrospinal fluid, responsible for acting as a shock absorber and protector from trauma to the central nervous system or for eliminating metabolites, among other functions. These cells play an important role in the transport of hormones in the brain.
10. Microglia
Microglia are tiny cells of the central nervous system that are dedicated to removing cellular debris and act as protectors against microorganisms such as bacteria, parasites or viruses, and are thought to be macrophages, a type of white blood cell that protects against foreign matter. They also help reduce inflammation by releasing substances called cytokines.
Microglia functions
The number of microglia cells is reduced under normal conditions, but when we suffer an injury or inflammation of our nervous tissue, these cells proliferate and act quickly (as happens with astrocytes), migrating to the affected area to phagocytize the remains cells, myelin fragments or damaged neurons.
In general, these cells act as phagocytic cells, protecting the brain from invading microorganisms.
11. Oligodendrocytes
Oligodendrocytes are structures of the central nervous system that cover some neuronal axons to form an insulating layer known as myelin. This myelin sheath is made up of lipids and proteins, and functions as an electrical insulator for the axons, promoting more efficient conduction of nerve impulses.
Connection between neurons and glial cells
That glial cells receive information from neurons has been known for a long time. However, recent research has revealed that glial cells also transmit information to neurons.
To carry out this transmission, the glia release a specific protein fragment that influences interneuronal communication , most likely through its attachment to the synaptic contacts that neurons use to communicate. The interruption of that flow of communication causes changes in the neural network, for example, during the learning process.
Scientists have discovered this underlying mechanism, which ranges from a molecular and cellular level to that of the neural network, which clearly has behavioral consequences. These results constitute an important advance in the understanding of the complex transmission pathways of brain signals.
Bibliographic references
- Dominik Sakry, Angela Neitz et al. Oligodendrocyte Precursor Cells Modulate the Neuronal Network by Activity-Dependent Ectodomain Cleavage of Glial NG2. PLoS Biology (2014).
- Guyton, AC (1994) Anatomy and physiology of the nervous system. Basic neuroscience. Madrid: Pan American Medical Editorial.
- Kolb, B. & Wishaw, I.Q. (2003). Fundamentals of Human Neuropsychology. (5th ed.). Freeman.
- Loring, D. W. (ed.) (1999). INS Dictionary of Neuropsychology. New York: Oxford University Press.
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.