We explain what a neuron is and what its main functions are. Also, the types that exist and what their structure is like.
What is a neuron?
It is known as a neuron (from the Greek neûron, “Cuerva” or “Nerve”) a a highly specialized type of cell, which makes up the nervous system, responsible for controlling the voluntary and involuntary functions of the body.
Neurons are characterized by their electrical excitability, which translates into the ability to conduct nerve impulses throughout the vast network of the nervous system, also transmitting them to other cells, such as muscle cells.
They are particularly abundant in the brain, reaching in the human being the figure of 86 x 109 cells, which can vary according to the animal species (fruit flies have 300,000, some nematode worms only 300).
The neurons of an adult individual, in addition, do not usually reproduce, but they are still created in the brain from stem cells and progenitor cells, in two locations of the brain only: the subgranular zone (ZSG) of the hippocampus and the subventriuclar zone (ZSV ), in a process called neurogenesis.
This does not mean that the entire neural network is replenished or restored, nor that it can cope on its own with diseases that deteriorate it, since the new neurons deal with very specific matters, such as smell.
Neurons they are not the only nerve cells, Nevertheless. Glial cells (astrocytes and Schwann cells) share the nervous system with them.
Neuron functions
Neurons fulfill the role of messengers and communicators of the organism. They are able to transmit nerve impulses to other cells in the body, like the muscular ones, and generate movement; to perceive and communicate external stimuli and turn them into an organized reaction, such as when faced with cold, heat, danger, etc .; or to keep a message running in a neural network, thus allowing the storage of information in memory.
This occurs thanks to the electrical transmission between these cells, through the use of sodium and potassium ions, among other chemical elements that pass from one cell to another. The speed of this transmission is such that it takes an impulse about 18.75 milliseconds to travel the distance from the toe to the brain in an adult human being.
Neuron types
There are many ways of classifying neurons. The main three are:
- According to its shape and size. Neurons can look like the following:
- Polyhedral. With a certain geometric shape.
- Fusiforms. Appearance similar to muscle cells, cylindrical.
- Starry. Star-shaped or spider-shaped, that is, with many limbs.
- Spherical. Round in shape.
- Pyramidal. In the shape of a pyramid.
- According to its function. Judging by the role they play in the nervous system, we can talk about:
- Motorboats. Those that are linked to movement and muscle coordination, both conscious and reflex.
- Sensory. Those linked to the perception of stimuli from outside the body through the senses.
- Interneuronal. Those that connect different types of neurons to each other and allow neural networks, thus giving rise to complex thinking, memory, etc.
- According to its polarity. Depending on the number and arrangement of your electrical terminations, they can be:
- Unipolar. Its axon is a single bifurcated extension.
- Bipolar With the nucleus in the center, they have a long axon and dendrite that tend to opposite ends.
- Multipolar. They have a long axon and multiple dendrites that allow many simultaneous connections.
- Monopolar. They have only one dendrite divided in two and directed to opposite ends, so they are considered false unipolar.
- Anaxonic. Extremely small, they do not distinguish their axons from their dendrites.
Structure of neurons
Neurons have a defined morphology and it is made up of four parts:
- Nucleus. Where the genetic information of the neuron is found, it usually occupies a central and highly visible position in it, especially in the youngest specimens.
- Pericarion. The space that surrounds the nucleus and makes up the cell body, in which the various organelles of the neuron are found, such as free ribosomes, the rough reticulum, the Golgi apparatus, etc.
- Dendrites. They are extensions of the cell’s cytoplasm, wrapped in a plasma membrane devoid of myelin, abundant in organelles and vesicles that allow interconnection and synapse.
- Axon. It is a tubular extension of the body of the neuron, covered with myelin and abundant in microtubules, which allows the passage of the electrical stimulus from one end of the cell to the other. At the end of the axon are a series of terminals that allow it to physically connect with other neurons and cells of another type.
Neurons and synapses
The synapse process occurs when neurons communicate with each other or with some other cell (such as the muscles to generate movement or the glands to secrete hormones), thus activating or deactivating certain processes in the body.
This occurs by transmitting a nerve impulse, that is, the segregation by the emitting cell of a chemical discharge in its membrane, which causes an electrical discharge perceived by the axon of the neuron. This, in turn, secretes chemical compounds called neurotransmitters, which are perceived by another intermediate neuron and thus a chain is continued that takes in total fragments of a second.