The ability of cells to respond to stimuli of the outside world is the main criterion for a living organism. The structural elements of the nervous tissue - mammalian and human neurons - are able to transform stimuli (light, smell, sound waves) into an excitation process. Its final result is an adequate reaction of the body in response to various environmental influences. In this article we will study what function neurons of the brain and peripheral parts of the nervous system perform, and also consider the classification of neurons in connection with the features of their functioning in living organisms.
Neural tissue formation
Before studying the functions of a neuron, let's look at how neurocyte cells are formed. At the stage of neurula, a neural tube is laid in the embryo. It is formed from the ectodermal leaf, which has a thickening - the neural plate. The expanded end of the tube will subsequently form five parts in the form of brain bubbles. Of these, parts of the brain are formed . The main part of the neural tube in the process of embryonic development forms the spinal cord, from which 31 pairs of nerves depart.
Brain neurons combine to form nuclei. Of these, 12 pairs of cranial nerves come out. In the human body, the nervous system differentiates into the central section - the brain and spinal cord, which consists of neurocyte cells, and the supporting tissue - neuroglia. The peripheral part consists of a somatic and vegetative part. Their nerve endings innervate all organs and tissues of the body.
Neurons - structural units of the nervous system
They have various sizes, shapes and properties. The functions of the neuron are diverse: participation in the formation of reflex arcs, the perception of irritation from the external environment, the transmission of the resulting excitation to other cells. Several processes depart from the neuron. Long - axon, short branches and are called dendrites.
Cytological studies revealed a nucleus with one or two nucleoli in the body of a nerve cell, 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 - the Nissl substance. A feature of nerve cells - a large number of processes contributes to the fact that the main function of a neuron is the transmission of nerve impulses. It is provided by both dendrites and axon. The former perceive signals and transmit them to the body of the neurocyte, while the axon is the only very long process that stimulates other nerve cells. Continuing to find the answer to the question: what function do neurons perform, let's turn to the structure of such a substance as neuroglia.
Nerve tissue structures
Neurocytes are surrounded by a special substance that has inherent support and protective properties. He also has a characteristic ability to divide. This compound is called neuroglia.
This structure is in close connection with nerve cells. Since the main function of a neuron is the generation and conduction of nerve impulses, glial cells are affected by the excitation process and change their electrical characteristics. In addition to trophic and protective functions, glia provides metabolic reactions in neurocytes and promotes plasticity of the nervous tissue.
The mechanism of 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 neuron body along the axon, and it is in contact with 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 transmission of excitation. It is carried out by axons. In the motor nerves, they are covered with a myelin sheath and are called meaty. In the autonomic nervous system are myelin-free processes. According to them, the excitation should enter the neighboring neurocyte.
What is synapse
The contact point of two cells 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 pulses.
Due to the formation of synapses, neurons create the network structure of the stem part of the brain and the 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 neurons of the brain. 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 with special connections that duplicate real synapses with 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 response function (transfer). When creating artificial intelligence, linear, semilinear, and step-wise activation functions of a neuron were most widely used.
Afferent neurocytes
They are also called sensitive and have short processes that enter the skin cells and all internal organs (receptors). Perceiving stimulation of the external environment, receptors transform them into a 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, generation of excitation and its transmission to the central nervous system. Sensory neurons enter the posterior horns of the spinal cord. Their bodies are located in nodes (ganglia) located outside the central nervous system. This is how 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 are 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 find out what function is performed by intercalary neurons. This type of nerve cell receives bioelectric impulses from a 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.
An intercalary neuron whose functions are the integration of excitation and its further distribution into the departments of the central nervous system are an essential component of most unconditioned reflex and conditioned reflex nerve arches. Excitatory interneurons facilitate signal transmission between functional groups of neurocytes. Insertion inhibitory nerve cells receive excitation from their own feedback center. 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. He has a large body enclosed in the front horns of the spinal cord. Those nerve cells that innervate skeletal muscles have the names of these motor elements. Other efferent neurocytes enter the secretory cells of the glands and cause the release of the corresponding substances: secrets, hormones. In involuntary, that is, unconditionally reflex acts (swallowing, salivation, defecation), efferent neurons depart from the spinal cord or from the stem part of the brain. 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, close to the roland groove.
The bodies of peripheral motor neurocytes innervating the muscles of the limbs, trunk, and neck are located in the front horns of the spinal cord, and their long processes - axons - come out of the front roots. They form motor fibers of 31 pairs of spinal nerves. Peripheral motor neurocytes innervating the muscles of the face, pharynx, larynx, tongue are located in the nuclei of the vagus, sublingual 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, and secreting cells — are determined by the structural features of the neurocyte, as well as specific metabolic reactions. Cytological studies have proved that neurons contain a large number of mitochondria synthesizing 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 occur with the participation of various enzymes and are characterized by high intensity.
Transmission of excitation in synapses
Considering the mechanism of excitation in neurons, we got acquainted with synapses - formations that occur at the site of contact of two neurocytes. Excitations in the first nerve cell are caused by the formation of chemical molecules - mediators in the collaterals of its axon. These include amino acids, acetylcholine, norepinephrine. Standing out from the bubbles of synoptic endings in the synoptic cleft, it can affect both its own postsynaptic membrane and affect the membranes of neighboring neurons.
Molecules of neurotransmitters 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.
Plasticity of neurons
Scientists have found that in the process of embryogenesis, namely in the stage of neurulation, a very large number of primary neurons develop from the ectoderm. About 65% of them die before the time of birth. During ontogenesis, some brain cells continue to eliminate. 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 persist for many years, without significantly decreasing. This is due to the fact that neuron functions lost during ontogenesis are taken over by other nerve cells. They have to strengthen their metabolism and create new additional nerve connections that compensate for lost functions. This phenomenon is called neurocyte plasticity.
What is reflected 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 in the cortex of the brain is formed a phantom of the consciousness of the people with whom we communicate. The neurons entering the mirror system act as resonators of the mental activity of the people around them. 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 into the world of emotions of another person and empathize with his feelings.