Structure of the nervous system

The nervous system performs the following functions: control of the activity of various systems and apparatuses, which constitute an integral organism, coordination of the processes occurring in it, establishment of interrelations of the organism with the external environment. The great physiologist Ivan Pavlov wrote: "The activity of the nervous system is directed, on the one hand, to the unification, integration of the work of all parts of the organism, on the other - to the connection of the organism with the environment, to the balancing of the body system with external conditions."

Nerves penetrate into all organs and tissues, form numerous branches, which have receptor (sensory) and effector (motor, secretory) endings, and together with the central sections (the 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, discharge, circulation, immune (protective) and metabolic (metabolism) processes, etc.

The activity of the nervous system, according to IM Sechenov, has a reflex character.

Reflex (from Latin reflexus - reflected) is the response of the body to a particular stimulus (external or internal effect), which occurs with the participation of the central nervous system (CNS). The human organism that lives in its external environment interacts with it. The environment affects the body, and the body, in turn, responds appropriately to these influences. The processes that take place in the body also cause a reaction. Thus, the nervous system ensures the interconnection and unity of the organism and environment.

The structural and functional unit of the nervous system is the neuron (nerve cell, neurocyte). The neuron consists of the body and the 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 the appendage, which is called the axon, or neurite. The nerve cell is dynamically polarized, i.e. Is able to conduct a nerve impulse only in one direction - from the dendrite through the body of the cell to the axon (neurite).

Neurons in the nervous system, coming into contact with each other, form chains through which nerve impulses are transmitted (moving). Transmission of a nerve impulse from one neuron to another occurs at the sites of their contacts and is provided by a special kind of formation, called interneuronal synapses. Distinct synapses are axosomatic when the endings of the axon of one neuron form contacts with the next body, and axodendritic when the axon comes into contact with the dendrites of another neuron. The contact type of the relationship in the synapse under different physiological states can obviously either be "created" or "destroyed", providing a selective response to any irritation. In addition, contact construction of chains of neurons creates an opportunity for carrying out a nerve impulse in a certain direction. Due to the presence of contacts in some synapses and disconnection in others, the impulse can be carried out purposefully.

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

Sensitive, receptor, or afferent (bringing), neurons. The bodies of these nerve cells lie always outside the brain or spinal cord - in the nodes (ganglia) of the peripheral nervous system. One of the processes that extends from the body of the nerve cell, follows the periphery of this or that organ and ends there with one or another sensitive receptor - receptor. Receptors are able to transform the energy of the external stimulus into a nerve impulse. The second process is directed to the central nervous system, the spinal cord or to the stem part of the brain in the posterior roots of the spinal nerves or the corresponding cranial nerves.

There are following types of receptors depending on localization:

1. exteroceptors perceive irritation from the external environment. These receptors are located in the outer veils of the body, in the skin and mucous membranes, in the sensory organs;
2. interoceptors get irritation mainly with changes in the chemical composition of the internal environment of the body and pressure in tissues and organs;
3. proprioceptors perceive irritations in muscles, tendons, ligaments, fasciae, joint capsules.

Reception, i. Perception of irritation and the beginning of the spread of the nerve impulse along the nervous conductors to the centers, IP Pavlov attributed to the beginning of the analysis process.

Closing, intercalary, associative, or conductor, neuron. This neuron transfers excitation from the afferent (sensitive) neuron to efferent ones. The essence of the process consists in the transmission of the signal received by the afferent neuron to the efferent neuron for execution in the form of a response. IP Pavlov defined this action as "a phenomenon of nerve closure". Closure (intercalary) neurons lie 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). Axons (neurites) of these cells continue as nerve fibers to the working organs (arbitrary - skeletal and involuntary - smooth muscles, glands), cells and various tissues.

After these general observations, we will consider in more detail the reflex arc and the reflex act as the basic principle of the activity of the nervous system.

The reflex arc is a chain of nerve cells, including afferent (sensory) and effector (motor, or secretory) neurons, through which the nerve impulse moves from the place of its 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 CNS - 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 central nervous system. Usually it is a pseudo-unipolar (unipolar) neuron whose body is located in the spinal node or the sensitive node of one of the cranial nerves. The peripheral outgrowth of this cell follows in the spinal nerves or having sensitive fibers of cranial nerves and their branches and ends with a receptor that perceives external (from the external environment) or internal (in organs, tissues) irritation. This irritation in the nerve endings is transformed into a nerve impulse that reaches the body of the nerve cell. Then the impulse along the central process (axon) in the spinal cord 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 sensitive cell forms a synapse with the body of the second neuron (efferent, effector). In the interneuron synapse, mediators transmit nervous excitation from the sensory (afferent) neuron to the motor (efferent) neuron, the process of which exits from the spinal cord in 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, the reflex arc consists not of two neurons, but is much more complicated. Between the two neurons - the receptor (afferent) and the effector (efferent) - there is one or more closure (intercalary, conductive) neurons. In this case, excitation from the receptor neuron from its central process is transmitted 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 lying 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 of the same level and closes the reflex arc at the level of this segment of the spinal cord. Axons of other cells can be preliminarily divided into the descending and ascending branches in the spinal cord, which are sent to the motor nerve cells of the anterior horns of neighboring, higher or lower segments. On the way, each ascending or descending branch can give collaterals to the motor cells of these and other neighboring segments of the spinal cord. In this connection, it becomes clear that the irritation of even the smallest number of receptors can be transmitted not only to the nerve cells of a particular segment of the spinal cord, but also spread to the cells of several neighboring segments. As a result, the response is a reduction in not just one muscle or even one muscle group, but several groups at once. So, in response to irritation, a complex reflex movement arises. This is one of the reactions of the body (reflex) in response to external or internal stimulation.

IM Sechenov in his work "Reflexes of the brain" put forward the idea of causality (determinism), noting that every phenomenon in the body has its own cause, and the reflex effect is the answer to this cause. These ideas received further creative development in the works of SP Botkin and IP Pavlov, who are the founders of the doctrine of nervism. Pavlov's tremendous service consists in spreading the doctrine of the reflex to the entire nervous system, beginning from the lower divisions and ending with its highest departments, and experimentally proved the reflex nature of all forms of vital activity of the organism without exception. According to IP Pavlov, a simple form of activity of the nervous system, which is constant, innate, specific and for the formation of structural preconditions of which social conditions are not required, should be designated as an unconditioned reflex.

In addition, there are temporary connections with the environment that are acquired during an individual's life. The possibility of acquiring temporary connections allows the body to establish a multifarious and complex relationship with the external environment. This form of reflex activity IP Pavlov called conditioned reflex (in contrast to unconditioned-nonreflective). The site of the closure of conditioned reflexes is the cortex of the cerebral hemispheres. The brain and its cortex are the basis of higher nervous activity.

PK Anokhin and his school experimentally confirmed the presence of the so-called feedback of the working organ with nerve centers - "reverse afferentation." At the moment when the efferent impulses from the centers of the nervous system reach the executive organs, they produce a response (movement or secretion). This working effect irritates the receptors of the executive organ. The impulses resulting from these processes along the afferent paths are directed back to the centers of the spinal cord or brain in the form of information about the organ's performance of a certain action at any given moment. Thus, it is possible to accurately account for 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 on closed circular or ring reflex nervous chains of "reverse afferentation" allows for permanent, continuous, remedial correction of any body reactions to any changes in the conditions of the internal and external environment. Without mechanisms of feedback, adaptation of living organisms to the environment is inconceivable. So, to replace the old ideas that the "open" (non-closed) reflex arc lies at the basis of the activity of the nervous system, an idea has come of a closed, ring, chain of reflexes.

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