The ability of cells to respond to stimuli from the outside world is the main criterion of a living organism. The structural elements of the nervous tissue - the neurons of mammals and humans - are able to transform stimuli (light, smell, sound waves) into the process of excitation. Its end result is an adequate reaction of the body in response to various environmental influences. In this article, we will study the function of the neurons of the brain and the peripheral parts of the nervous system, and also consider the classification of neurons in connection with the peculiarities of their functioning in living organisms.
Formation of nerve tissue
Before studying the functions of a neuron, let's look at how neurocyte cells are formed. At the neurula stage, the neural tube is laid in the embryo. It is formed from the ectodermalleaf with a thickening - the neural plate. The expanded end of the tube will later form five parts in the form of brain bubbles. They form parts of the brain. The main part of the neural tube in the process of embryonic development forms the spinal cord, from which 31 pairs of nerves depart.
The neurons of the brain combine to form nuclei. 12 pairs of cranial nerves emerge from them. In the human body, the nervous system is differentiated into the central section - the brain and spinal cord, consisting of neurocyte cells, and the supporting tissue - neuroglia. The peripheral section consists of the somatic and vegetative parts. Their nerve endings innervate all organs and tissues of the body.
Neurons are structural units of the nervous system
They have different sizes, shapes and properties. The functions of a neuron are diverse: participation in the formation of reflex arcs, perception of irritation from the external environment, transmission of the resulting excitation to other cells. A neuron has several branches. The long one is an axon, the short ones branch and are called dendrites.
Cytological studies have revealed in the body of a nerve cell a nucleus with one or two nucleoli, a well-formed endoplasmic reticulum, many mitochondria and a powerful protein-synthesizing apparatus. It is represented by ribosomes and RNA and mRNA molecules. These substances form a specific structure of neurocytes - Nissl's substance. The peculiarity of nerve cells - a large number of processes contributes to the fact that the main function of the neuron is the transmission of nerveimpulses. It is provided by both the dendrites and the axon. The former perceive signals and transmit them to the body of the neurocyte, and the axon, the only very long process, conducts excitation to other nerve cells. Continuing to find the answer to the question: what function do neurons perform, let us turn to the structure of such a substance as neuroglia.
Nervous tissue structures
Neurocytes are surrounded by a special substance that has supporting and protective properties. It also has a characteristic ability to divide. This connection is called neuroglia.
This structure is in close connection with nerve cells. Since the main functions of a neuron are the generation and conduction of nerve impulses, glial cells are influenced by the excitation process and change their electrical characteristics. In addition to trophic and protective functions, glia provides metabolic reactions in neurocytes and contributes to the plasticity of the nervous tissue.
Mechanism of conducting excitation in neurons
Each nerve cell forms several thousand contacts with other neurocytes. Electrical impulses, which are the basis of excitation processes, are transmitted from the body of the neuron along the axon, and it contacts other structural elements of the nervous tissue or enters directly into the working organ, for example, into the muscle. To establish what function neurons perform, it is necessary to study the mechanism of excitation transmission. It is carried out by axons. In motor nerves, they are covered with a myelin sheath and are called pulpy. In the vegetativenervous system are unmyelinated processes. Through them, excitation should enter the neighboring neurocyte.
What is a synapse
The place where two cells meet is called a synapse. The transfer of excitation in it occurs either with the help of chemicals - mediators, or by passing ions from one neuron to another, that is, by electrical impulses.
Due to the formation of synapses, neurons create a mesh structure of the stem part of the brain and spinal cord. It is called the reticular formation, starts from the lower part of the medulla oblongata and captures the nuclei of the brain stem, or brain neurons. The mesh structure maintains the active state of the cerebral cortex and directs the reflex acts of the spinal cord.
Artificial Intelligence
The idea of synaptic connections between neurons of the central nervous system and the study of the functions of reticular information is currently embodied by science in the form of an artificial neural network. In it, the outputs of one artificial nerve cell are connected to the inputs of another by special connections that duplicate real synapses in their functions. The activation function of a neuron of an artificial neurocomputer is the summation of all input signals entering the artificial nerve cell, converted into a nonlinear function of the linear component. It is also called the actuation function (transfer). When creating artificial intelligence, the most widely used are linear, semi-linear and stepwise activation functions.neuron.
Afferent neurocytes
They are also called sensitive and have short processes that enter the cells of the skin and all internal organs (receptors). Perceiving the irritation of the external environment, the receptors transform them into the process of excitation. Depending on the type of stimulus, nerve endings are divided into: thermoreceptors, mechanoreceptors, nociceptors. Thus, the functions of a sensitive neuron are the perception of stimuli, their discrimination, the generation of excitation and its transmission to the central nervous system. Sensory neurons enter the dorsal horns of the spinal cord. Their bodies are located in nodes (ganglia) located outside the central nervous system. This is how the ganglia of the cranial and spinal nerves are formed. Afferent neurons have a large number of dendrites; together with the axon and body, they are an essential component of all reflex arcs. Therefore, the functions of a sensitive neuron consist both in the transfer of the process of excitation to the brain and spinal cord, and in participation in the formation of reflexes.
Features of the interneuron
Continuing to study the properties of the structural elements of the nervous tissue, we will find out what function interneurons perform. This type of nerve cells receives bioelectrical impulses from the sensory neurocyte and transmits them:
a) other interneurons;
b) motor neurocytes.
Most interneurons have axons, the terminal sections of which are terminals, connected with neurocytes of one center.
The intercalary neuron, whose functions are the integration of excitation and its distribution further to the parts of the central nervous system, are an essential component of most unconditioned reflex and conditioned reflex nerve arcs. Excitatory interneurons promote signal transmission between functional groups of neurocytes. Inhibitory intercalary nerve cells receive excitation from their own center through feedback. This contributes to the fact that the intercalary neuron, whose functions are the transmission and long-term preservation of nerve impulses, ensures the activation of sensory spinal nerves.
Motor neuron function
Motoneuron is the final structural unit of the reflex arc. It has a large body enclosed in the anterior horns of the spinal cord. Those nerve cells that innervate skeletal muscles have the names of these motor elements. Other efferent neurocytes enter the secreting cells of the glands and cause the release of appropriate substances: secrets, hormones. In involuntary, that is, unconditioned reflex acts (swallowing, salivation, defecation), efferent neurons depart from the spinal cord or from the brain stem. To perform complex actions and movements, the body uses two types of centrifugal neurocytes: central motor and peripheral motor. The body of the central motor neuron is located in the cerebral cortex, near the Roland sulcus.
The bodies of peripheral motor neurocytes that innervate the muscles of the limbs, trunk, neck,located in the anterior horns of the spinal cord, and their long processes - axons - come out of the anterior roots. They form motor fibers of 31 pairs of spinal nerves. Peripheral motor neurocytes innervating the muscles of the face, pharynx, larynx, and tongue are located in the nuclei of the vagus, hypoglossal, and glossopharyngeal cranial nerves. Therefore, the main function of the motor neuron is the unhindered conduction of excitation to the muscles, secreting cells and other working organs.
Metabolism in neurocytes
The main functions of a neuron - the formation of a bioelectric action potential and its transmission to other nerve cells, muscles, secreting cells - are due to the structural features of the neurocyte, as well as specific metabolic reactions. Cytological studies have shown that neurons contain a large number of mitochondria that synthesize ATP molecules, a developed granular reticulum with many ribosomal particles. They actively synthesize cellular proteins. The membrane of the nerve cell and its processes - the axon and dendrites - performs the function of selective transport of molecules and ions. Metabolic reactions in neurocytes proceed with the participation of various enzymes and are characterized by high intensity.
Excitation transmission in synapses
Considering the mechanism of conducting excitation in neurons, we got acquainted with synapses - formations that occur at the point of contact of two neurocytes. Excitation in the first nerve cell causes the formation of molecules of chemical substances - mediators - in the collaterals of its axon. These includeamino acids, acetylcholine, norepinephrine. Released from the vesicles of synoptic endings in the synoptic cleft, it can affect both its own postsynaptic membrane and affect the membranes of neighboring neurons.
Neurotransmitter molecules serve as an irritant for another nerve cell, causing changes in charges in its membrane - an action potential. Thus, the excitation quickly spreads along the nerve fibers and reaches the parts of the central nervous system or enters the muscles and glands, causing their adequate action.
Neuron plasticity
Scientists have found that in the process of embryogenesis, namely at the stage of neurulation, a very large number of primary neurons develop from the ectoderm. About 65% of them die before the birth of a person. During ontogenesis, some brain cells continue to be eliminated. This is a natural programmed process. Neurocytes, unlike epithelial or connective cells, are incapable of division and regeneration, since the genes responsible for these processes are inactivated in human chromosomes. Nevertheless, the brain and mental performance can be maintained for many years without significantly declining. This is explained by the fact that the functions of the neuron, lost in the process of ontogenesis, are taken over by other nerve cells. They have to increase their metabolism and create new additional nerve connections that compensate for the lost functions. This phenomenon is called neurocyte plasticity.
Whatreflected in neurons
At the end of the 20th century, a group of Italian neurophysiologists established an interesting fact: a mirror reflection of consciousness is possible in nerve cells. This means that a phantom of the consciousness of the people with whom we communicate is being formed in the cerebral cortex. The neurons included in the mirror system act as resonators for the mental activity of the surrounding people. Therefore, a person is able to predict the intentions of the interlocutor. The structure of such neurocytes also provides a special psychological phenomenon called empathy. It is characterized by the ability to penetrate the world of another person's emotions and empathize with their feelings.
