The structure of the nervous system

The nervous system performs the following functions: control of the activity of various systems and apparatuses that make up the integral organism, coordination of the processes occurring in it, establishment of the organism's interrelations with the external environment. The great physiologist I.P. Pavlov wrote: "The activity of the nervous system is directed, on the one hand, at unification, integration of the work of all parts of the organism, on the other - at the organism's connection with the environment, at balancing the organism's system with external conditions."

Nerves penetrate all organs and tissues, form numerous branches with receptor (sensory) and effector (motor, secretory) endings, and together with the central sections (brain and spinal cord) ensure the connection of all parts of the body into a single whole. The nervous system regulates the functions of movement, digestion, respiration, excretion, blood circulation, immune (protective) and metabolic (metabolism) processes, etc.

The activity of the nervous system, according to I.M. Sechenov, is reflexive in nature.

A reflex (from Latin reflexus - reflected) is a response of the body to a particular stimulus (external or internal impact), which occurs with the participation of the central nervous system (CNS). The human body, living in the external environment surrounding it, interacts with it. The environment affects the body, and the body, in turn, reacts accordingly to these influences. The processes occurring in the body itself also cause a response. Thus, the nervous system ensures the interconnection and unity of the body and the environment.

The structural and functional unit of the nervous system is the neuron (nerve cell, neurocyte). The neuron consists of a body and processes. The processes that conduct a nerve impulse to the body of the nerve cell are called dendrites. From the body of the neuron, the nerve impulse is directed to another nerve cell or to the working tissue along a process called an axon or neurite. The nerve cell is dynamically polarized, i.e. it is capable of conducting a nerve impulse in only one direction - from the dendrite through the cell body to the axon (neurite).

Neurons in the nervous system, when they come into contact with each other, form chains along which nerve impulses are transmitted (moved). The transmission of a nerve impulse from one neuron to another occurs at the points of their contacts and is ensured by a special type of formation called interneuronal synapses. A distinction is made between axosomatic synapses, when the endings of the axon of one neuron form contacts with the body of the next, and axodendritic synapses, when the axon comes into contact with the dendrites of another neuron. The contact type of relations in a synapse under various physiological conditions can obviously either be “created” or “destroyed”, ensuring a selective reaction to any irritation. In addition, the contact structure of neuron chains creates the possibility of conducting a nerve impulse in a certain direction. Due to the presence of contacts in some synapses and disconnection in others, the conduction of an impulse can occur purposefully.

In the neural chain, different neurons have different functions. In this regard, three main types of neurons are distinguished according to their morphofunctional characteristics.

Sensory, receptor, or afferent (bringing) neurons. The bodies of these nerve cells always lie outside the brain or spinal cord - in the nodes (ganglia) of the peripheral nervous system. One of the processes, extending from the body of the nerve cell, goes to the periphery to one or another organ and ends there in one or another sensory ending - a receptor. Receptors are capable of transforming the energy of external influence (irritation) into a nerve impulse. The second process is directed to the central nervous system, spinal cord or to the brain stem as part of the posterior roots of the spinal nerves or the corresponding cranial nerves.

The following types of receptors are distinguished depending on their location:

1. exteroceptors perceive irritation from the external environment. These receptors are located in the outer coverings of the body, in the skin and mucous membranes, in the sense organs;
2. interoceptors are stimulated mainly by changes in the chemical composition of the internal environment of the body and the pressure in tissues and organs;
3. Proprioceptors perceive irritation in muscles, tendons, ligaments, fascia, and joint capsules.

I.P. Pavlov attributed reception, i.e. the perception of irritation and the beginning of the spread of the nerve impulse along the nerve conductors to the centers, to the beginning of the analysis process.

Locking, intercalary, associative, or conductor neuron. This neuron transmits excitation from the afferent (sensory) neuron to the efferent neurons. The essence of the process is to transmit the signal received by the afferent neuron to the efferent neuron for execution in the form of a response. I.P. Pavlov defined this action as the "phenomenon of neural closure". Locking (intercalary) neurons are located within the CNS.

Effector, efferent (motor or secretory) neuron. The bodies of these neurons are located in the central nervous system (or on the periphery - in the sympathetic, parasympathetic nodes of the vegetative part of the nervous system). The axons (neurites) of these cells continue in the form of nerve fibers to the working organs (voluntary - skeletal and involuntary - smooth muscles, glands), cells and various tissues.

After these general remarks, let us consider in more detail the reflex arc and the reflex act as the basic principle of the activity of the nervous system.

A reflex arc is a chain of nerve cells that includes afferent (sensory) and effector (motor or secretory) neurons, along which a nerve impulse moves from its place of origin (from the receptor) to the working organ (effector). Most reflexes are carried out with the participation of reflex arcs, which are formed by neurons of the lower parts of the central nervous system - neurons of the spinal cord and brainstem.

The simplest reflex arc consists of only two neurons - afferent and effector (efferent). The body of the first neuron (receptor, afferent), as noted, is outside the CNS. Usually this is a pseudounipolar (unipolar) neuron, the body of which is located in the spinal ganglion or sensory ganglion of one of the cranial nerves. The peripheral process of this cell follows as part of the spinal nerves or cranial nerves with sensory fibers and their branches and ends in a receptor that perceives external (from the external environment) or internal (in organs, tissues) irritation. This irritation in the nerve ending is transformed into a nerve impulse, which reaches the body of the nerve cell. Then the impulse along the central process (axon) as part of the spinal nerves is directed to the spinal cord or along the corresponding cranial nerves - to the brain. In the gray matter of the spinal cord or in the motor nucleus of the brain, this process of the sensory cell forms a synapse with the body of the second neuron (efferent, effector). In the interneuronal synapse, with the help of mediators, the transmission of nerve excitation from the sensory (afferent) neuron to the motor (efferent) neuron occurs, the process of which exits the spinal cord as part of the anterior roots of the spinal nerves or motor nerve fibers of the cranial nerves and is directed to the working organ, causing muscle contraction.

As a rule, a reflex arc does not consist of two neurons, but is much more complex. Between two neurons - receptor (afferent) and efferent - there is one or more closure (intercalary, conductive) neurons. In this case, excitation from the receptor neuron is transmitted along its central process not directly to the effector nerve cell, but to one or more intercalary neurons. The role of intercalary neurons in the spinal cord is performed by cells located in the gray matter of the posterior columns. Some of these cells have an axon (neurite), which is directed to the motor cells of the anterior horns of the spinal cord at the same level and closes the reflex arc at the level of a given segment of the spinal cord. Axons of other cells in the spinal cord can preliminarily divide in a T-shape into descending and ascending branches, which are directed to the motor nerve cells of the anterior horns of neighboring, higher- or lower-lying segments. Along the route, each ascending or descending branch can give off collaterals to the motor cells of these and other neighboring segments of the spinal cord. In this regard, it becomes clear that irritation of even the smallest number of receptors can be transmitted not only to the nerve cells of a certain segment of the spinal cord, but also spread to the cells of several neighboring segments. As a result, the response is a contraction of not one muscle or even one group of muscles, but several groups at once. Thus, in response to irritation, a complex reflex movement occurs. This is one of the body's reactions (reflex) in response to external or internal irritation.

I.M. Sechenov in his work "Reflexes of the Brain" put forward the idea of causality (determinism), noting that each phenomenon in the body has its cause, and the reflex effect is a response to this cause. These ideas were further creatively developed in the works of S.P. Botkin and I.P. Pavlov, who are the founders of the doctrine of nervism. I.P. Pavlov's great merit is that he extended the doctrine of the reflex to the entire nervous system, from the lower sections to its highest sections, and experimentally proved the reflex nature of all forms of vital activity of the body without exception. According to I.P. Pavlov, a simple form of activity of the nervous system, which is constant, innate, species-specific and for the formation of structural prerequisites of which social conditions are not required, should be designated as an unconditioned reflex.

In addition, there are temporary connections with the environment acquired during an individual's life. The ability to acquire temporary connections allows the organism to establish the most diverse and complex relationships with the external environment. I.P. Pavlov called this form of reflex activity conditioned reflex (as opposed to unconditioned reflex). The place where conditioned reflexes are closed is the cerebral cortex. The brain and its cortex are the basis of higher nervous activity.

P.K. Anokhin and his school experimentally confirmed the existence of the so-called feedback of the working organ with the nerve centers - "feedback afferentation". At the moment when efferent impulses from the centers of the nervous system reach the executive organs, a response reaction (movement or secretion) is generated in them. This working effect irritates the receptors of the executive organ. The impulses resulting from these processes are sent back along the afferent pathways to the centers of the spinal cord or brain in the form of information about the performance of a certain action by the organ at any given moment. In this way, it is possible to accurately record the correctness of the execution of commands with the help of nerve impulses coming to the working organs from the nerve centers, and their constant correction. The existence of two-way signaling along closed circular or ring reflex nerve chains of "feedback afferentation" allows for constant, continuous, moment-by-moment corrections of any reactions of the organism to any changes in the conditions of the internal and external environment. Without feedback mechanisms, the adaptation of living organisms to the environment is unthinkable. Thus, the old ideas that the basis of the nervous system’s activity is an “open” (unclosed) reflex arc have been replaced by the idea of a closed, circular chain of reflexes.

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