Nervous system – a set of organs formed by the nervous tissue that control all physiological functions and metabolism of the body and communicate with the environment.
Numerous functions of a complex animal organism are performed by various specialized organs: digestive, respiratory, excretory, circulatory organs, organs of movement, and others. The existence of an organism as a whole is impossible without interaction, coordination of various functions among themselves. Changes in the environment surrounding the body, continuous changes in its internal environment require appropriate regulation of the intensity and quality of functions of all organs; the individual cells that make up this or that organ could not act simultaneously and in concert, without receiving nerve impulses.
Only the simplest animal organisms perceive changes in environmental conditions (irritations) directly and respond to them with direct reactions of the whole body. In a complex organism of higher animals and humans, the function of perception of changes in the external and internal environment and the transmission of the response to the executive organs are carried out by special organs – the Nervous system .
The main property of nervous tissue elements is excitability and the ability to transmit excitations in the form of so-called nerve impulses to a distance. Nerve tissue consists of nerve cells – neurons , which have two types of processes: mostly short and branched – dendrites , and long – neuritis.
• Dendrites perceive irritation and transmit excitation (nerve impulse) to the nerve cell; neuritis are nervous imiuls of cells.
• Neuritis consists of a more or less long (sometimes up to 1 m or more) nerve fiber covered with an insulating sheath.
• The central part of neuritis – the axial cylinder, consists of a special protein and contains the thinnest filaments – neurofibrils, which are a transmitting substance through which a nerve impulse travels at an enormous speed (27 m per second).
The perception of environmental changes occurs in special organs – receptors, in which the ends of the nerve fiber branch. Receptors are formed directly by the terminal branches of the dendrites themselves – nerve sensitive endings located among the cells of organs (in the skin, in the mucous membranes, walls of the blood vessels), or represent complex formations that make up a significant part of the sensory organs in which the dendrites branch (for example, the retina of the eye , Corti’s organ of the ear, taste buds, tactile bodies). In both cases, any influence of the environment, all kinds of irritations – physical (light, sound, temperature, pressure, touch), chemical (from substances that are in a gaseous state in air or dissolved in liquids of the body’s internal media, in perceived food) – processed into a nerve impulse.
Some neurons with their processes (dendrites and neuritis) are directly connected with the perceiving and executive organs, while others are only transferring between other neurons . The connection between neurons is carried out by contacting the branches of neurites with their dendrites. The organs of the nervous system that carry out its functions are the brain and spinal cord and nerves. The brain performs centralized control of the body and its connection with the external environment; nerves are conductors through which nerve impulses — signals, flow to the brain from the periphery of the body (i.e., from the surface of the body, sensory organs, internal organs), and response, executive, nerve impulses flow from the brain to the periphery. The vast majority of nerve cells (except for cells of the sensitive ganglia, see below) are concentrated in the gray matter of the brain and spinal cord.
The processes of neurons exit the central nervous system and connect to the nerves. The peripheral nervous system consists of 12 pairs of cranial nerves , 31 pairs of spinal nerves, as well as nodes and plexuses of the autonomic nervous system. Transfer processes of the neurons , and central processes of peripheral neurons are found in the central nervous system, forming a white substance of the brain.
Nerve fibers that transmit impulses from receptors to the central nervous system are called centripetal, or sensitive . Their neurons are not located directly in the central nervous system, but in the so-called spinal nodes, outside it.
Nerve fibers that transmit impulses from the central nervous system to organs are called centrifugal.
The nerves connecting the central nervous system with organs, tissues, carry both centripetal and centrifugal fibers. Thus, through the peripheral nervous system a two-way connection of central N. is carried out with. with organs and tissues. In central N. of page signals are received about all changes in both the external and internal environment; it processes these signals and transmits response pulses to the periphery.
The set of perceiving, intermediate and transmitting neurons is called a reflex arc.
The reflex arc can be located at different levels of the central nervous system: in the spinal cord, in the oblong, middle, intermediate and, finally, in the cortex of the cerebral hemispheres. Depending on the nature of the changes that caused the reflex, the latter may be more or more complex.
Distinguish between constant unconditioned reflexes and unstable conditioned reflexes.
In conditioned reflexes, impulses are transmitted along the same reflex arc to the same organs, for example. touching food to the mucous membrane of the mouth causes salivation, light irritation – narrowing of the pupils, etc. Such reflexes are carried out through parts of the nervous system – the spinal, oblong, midbrain. With conditioned reflexes, the body’s reaction depends on the processing of centripetal impulses in the higher parts of the central nervous system – in the cerebral cortex.
The nervous system is divided into somatic (from lat. Soma – body) and vegetative (from lat. Vegeta – plant).
1 The somatic nervous system communicates with the external environment mainly through the senses; its executive part is represented mainly by motor nerves of skeletal muscle. With the help of the somatic nervous system, voluntary, conscious activity with its basis is carried out and conditioned reflexes are performed.
2 The autonomic nervous system regulates the activity of all internal organs, which occurs unconsciously, automatically, using unconditioned reflexes. However, this does not mean that the functions of the somatic and autonomic nervous systems are strictly separated; on the contrary, they are closely interconnected and both are under the control of the higher part of the central nervous system – the cerebral cortex, which adapts the function of internal organs to external conditions and vice versa – adapts external conditions to the functions of internal organs.
The nervous system is not the only mechanism of communication between organs; this connection is also carried out with the help of hormones secreted by the organs of internal secretion, which in their activity are also under the control of the central nervous system, which, in this way, carries out neurohumoral regulation of the body. The nervous system, in addition to regulating the functions of all organs, also regulates the metabolism of tissues, as a result of which deprivation of their innervation causes tissue disorders – the so-called. trofich . disorders ( e.g. ulceration).
The nervous system has an auxiliary apparatus. Each nerve fiber is surrounded by a special membrane, the role of which is to isolate it from other fibers: this is the so-called. pulpy shell, giving white color.
In the central nervous system, nerve cells are embedded in a special supporting tissue – neuroglia , which plays a protective and trophic role (through which they are nourished). Nerve cells are extremely sensitive to a lack of oxygen and nutrients; the central nervous system is plentifully supplied with blood: the slightest violation of the blood flow disrupts its activity. Changes in the state of the body, all kinds of intoxications (alcohol, nicotine, etc.) primarily affect nerve cells.