Neuron

Neuron . Neurons are cells of the nervous system whose main characteristic is the electrical excitability of their plasma membrane ; They are specialized in the reception of stimuli and conduction of nervous impulses. They are highly differentiated.

Summary

[ disguise ]

• 1 Historical summary
• 2 Structure
• 3 Function of neurons
• 4 Types of neurons
  • 1 Its function
  • 2 According to polarity
  • 3 The characteristics of neurites
  • 4 According to the chemical mediator
• 5 Synapses
  • 1 Artificial neural networks
• 6 Sources

Historical summary

At the beginning of the 20th century , Santiago Ramón y Cajal placed neurons as functional elements of the nervous system for the first time. Cajal proposed that they acted as discrete entities that, intercommunicating, established a kind of network through specialized connections or spaces. This idea is recognized as the neuron doctrine, one of the central elements of modern neuroscience. It opposes that defended by Camillo Golgi , who advocated the continuity of the neural network and denied that they were discrete interconnected entities.

In order to observe the histology of the nervous system under the microscope, Cajal used Silver stains (with Silver Salts ) of histological sections for optical microscopy, developed by Golgi and improved by himself. This technique allowed very precise cellular analysis, even of tissue as dense as brain tissue. The neuron is the structural and functional unit of the nervous system . It receives stimuli from the environment, converts them into nervous impulses and transmits them to another neuron, a muscle or glandular cell where they will produce a response.

Structure

Cyton , Soma or Cell Body : Refers to the body of the Cell . Nucleus : Contains the information that directs the neuron in its general function. Cytoplasm : Where structures that are important for the functioning of the neuron are found. Dendrites : They are short extensions that originate in the soma or cell body, whose function is to receive impulses from other neurons and send them to the soma of the neuron . Axon: It is a single and long extension that can measure up to one meter in length and whose function is to remove the impulse from the neuronal soma and conduct it to another place in the system or to a receiving organ, for example a muscle. Plasma Membrane or Plasmalema : This limits the neuron and is especially important for its role in the reception and transmission of nerve impulses.

Neuron function

Neurons receive and transport nerve impulses , which are electrochemical in nature. The neuron at rest is polarized, and has a potential of -70mV. While a neuron that receives a stimulus is depolarized and reaches a potential of +30mV. The stimulus lets in Sodium , and to stabilize it lets out Potassium . Then the Na-K pumps act, using energy to remove the Na and let in the K. Refractory Period, corresponds to the moment in which the neuron cannot be stimulated again. Excitation Threshold is the amount of stimulus necessary to depolarize the neuron. The impulse is independent of the stimulus. When two impulses meet, they cancel each other out. The impulse is “saltatory”, between the Nodes of Ranvier .

Types of neurons

Neurons can be classified according to:

Its function

Sensory neurons: They conduct impulses from receptors (for example the skin) to the brain and spinal cord, these impulses are informative ( Vision , Sound , Touch , Pain , etc.) their somata or cell bodies form a large part of the root posterior of the spinal cord and cranial ganglia . They are bipolar. Motor neurons: They conduct impulses from the brain and spinal cord to the receptors (for example, the muscles and exocrine glands ), that is, in the opposite direction to the sensory neurons. It is the motor component of the spinal and cranial nerves.

These nerve cells are multipolar. Interneurons : They are multipolar nerve cells whose body and processes are located exclusively in the central nervous system, specifically in the brain, and do not have direct contact with peripheral structures (receptors and transmitters). There is an important group of interneurons whose axons end in motor neurons, in the brainstem and in the spinal cord, they are called high motor neurons, these are responsible for the modification, coordination, integration, facilitation and inhibition that must occur between the input sensory and motor output. Unipolar neurons : It is another type of interneurons that generally connect with bipolar or multipolar neurons.

According to the polarity

According to the number and anatomy of their extensions, neurons are classified as: Unipolar: they are those from which only one extension arises that bifurcates and behaves functionally like an Axon except for its branched ends in which the peripheral branch receives signals and functions. as dendrites and transmit the impulse without it passing through the neuronal soma. They are typical of invertebrate ganglia and the Retina . Bipolar: they have an elongated cell body and a dendrite starts from one end and the axon from the other (there can only be one per neuron).

The nucleus of this type of neuron is located in the center of it, so it can send signals to both poles of it. Examples of these neurons are found in the bipolar cells of the retina (cones and rods), the cochlear and vestibular ganglia, these ganglia are specialized for the reception of auditory waves and balance. Multipolar: they have a large number of dendrites that arise from the cell body. These types of cells are the classic neuron with small extensions (dendrites) and a long extension or axon. They represent the majority of neurons. Within multipolar ones, we distinguish between those that are Golgi I type, with a long axon, and those that are Golgi II type, with a short axon.

Projection neurons are of the first type, and local neurons or interneurons of the second. Pseudounipolar: these are those in which the cell body has a single dendrite or neurite, which divides a short distance from the cell body into two branches, which is why they are also called pseudounipolar (pseudos in Greek means “false”), one that It goes to a peripheral structure and another that enters the central nervous system. Examples of this form of neuron are found in the posterior root ganglion. Anaxonic: they are small. The dendrites of the axons are not distinguished. They are found in the brain and special sense organs.

The characteristics of neurites

According to the nature of the axon and dendrites, we classify neurons into: Very long axon or Golgi type I. The axon branches away from the perikaryon. With axons up to 1 m. Short axon or Golgi type II. The axon branches next to the cell soma. No defined axon. Like the amacrine cells of the retina. Isodendrites. With rectilinear dendrites that branch so that the daughter branches are longer than the mother ones. Idiodendritic. With dendrites organized depending on the neuronal type; for example, like the Purkinje cells of the cerebellum. Allodendritic. Intermediate between the two previous types.

According to the chemical mediator

Cholinergic. They release Acetylcholine . Noradrenergic. They release norepinephrine . Dopaminergic. They release dopamine . Serotonergic. They release Serotonin . Gabaergic. They release GABA, that is, γ-aminobutyric acid.

Synapse

Neurons are organized into networks and systems. Contact between them is made through highly specialized functional contacts called synapses. Most synapses are of the Chemical type , that is, they use molecules called neurotransmitters to communicate with each other. There are several types of synapses between neurons: Axosomatic: The axon is inserted into the neuronal body Axodendritic: Axon with Dendrites . Axoaxonic: Axon in axon.

Artificial neural networks

Knowledge of biological neural networks has given rise to a design used in artificial intelligence. These networks work because each neuron receives a series of inputs through interconnections and emits an output. This output is given by three functions: a propagation function that generally consists of the sum of each input multiplied by the weight of its interconnection; an activation function, which modifies the previous one and may not exist, in this case the output being the same propagation function; and a transfer function, which is applied to the value returned by the activation function. It is used to limit the output of the neuron and is generally given by the interpretation that we want to give to said outputs.