A specialized type of cells is known as neuron , which constitutes the nervous system, whose function is to control the voluntary and involuntary functions of the organism.
Neurons are distinguished by their electrical excitability, which translates into their ability to conduct nerve impulses throughout the extensive network of the nervous system, also transferring them to other cells such as molecular cells.
They are especially abundant in the brain , they can reach the figure of 86 x 109 cells, although this can vary with respect to the animal species. A clear example of this is fruit flies, they have 300,000, unlike certain nematode worms with just 300.
The neurons of an adult individual, moreover, do not tend to reproduce, but they continue to be created in the brain, starting from stem cells and progenitor cells, located only in two regions of the brain: the subgranular zone (ZSG) of the hippocampus and the subventricular zone (ZSV), in a process called neurogenesis.
This does not mean that the entire neural network is restored or restored, nor that it can attack the diseases that deteriorate it by itself, since the new neurons have very specific functions, such as smell.
Apart from neurons, there are multiple nerve cells. So, they share with them the nervous system of glial cells (shwann cells and astrocytes)
Neurons carry out the messenger and communicator functions of the organism . They have the ability to transmit nerve impulses to other cells in the body, such as molecular cells, and produce movement; to perceive and communicate external stimuli and transform them into an organized reaction, such as in the face of heat, cold, danger, etc .; or to preserve a message that travels in a neural network, thus allowing the storage of information in memory.
This procedure occurs thanks to the electrical transmission between these cells, through the use of sodium and potassium ions, among many other chemical elements that pass from one cell to another. The speed at which this transmission travels is such that it takes about 18.75 milliseconds for an impulse to travel the distance from the toe to the brain, specifically in an adult human.
Examples of neuron types
There are several ways to classify a neuron. The main three are:
According to their shape and size: neurons can have the following appearance:
- Fusiform: cell-like in appearance, cylindrical.
- Polyhedral: with a certain geometric shape.
- Starry: in the form of a star or spider, that is, with many extremities.
- Pyramidal: they have a pyramid shape.
- Spherical: round in appearance.
According to their function: judging by the function they perform in the nervous system , we can speak:
- Motor: those that are related to movement and muscle coordination, both as conscious and reflex.
- Interneuronal: those that link different types of neurons to each other and allow neural networks, thus giving way to complex thinking, memory, etc.
- Sensory: those that are related to the perception of stimuli from outside the body through the senses.
According to their polarity: according to the number and arrangement of their electrical terminations they can be:
- Unipolar: its axon is a single bifurcated extension.
- Bipolar: with the nucleus in the center, it contains a long axon and dendrite that tend to opposite ends.
- Multipolar: contains a long axon and multiple dendrites that allow many simultaneous connections.
- Monopolar: they contain only one dendrite divided in two and that goes to opposite extremes, so they are classified as false unipolar.
- Anaxonic: extremely small, they do not distinguish their axons from the dendrites.
Neurons and synapses
The synapse process takes place when neurons communicate with each other or with some other cell (like muscles produce gland movements to secrete hormones), thus activating or deactivating certain processes that take place in the body.
This occurs through the transmission of a nerve impulse , that is, the secretion by the emitting cell of a chemical discharge that is generated in its membrane, which produces an electrical discharge perceived by the axon of a neuron. This, in turn, secretes chemical compounds called neurotransmitters, which are perceived by another intermediate neuron and in this way a chain continues that takes fragments of seconds in total.