The human organism is a living system with multiple levels of organization. At each level, different processes occur that are appropriate to it. Cells are the carriers of all vital chemical processes in the organism, and their life and activity are based on the complex biological activity of molecules. This is the fundamental living system on which the hierarchical organization of more complex living systems depends, that is, the functioning and maintenance of the structure of living beings. Each cell type is specially adapted to the function it needs to perform. Groups of cells of the same type build different tissues of the organism, and several different tissue types together form an organ. Groups of organs linked by a common function in the overall activity of the organism, organ systems, are responsible for carrying out more complex life functions. For any higher level of organization to function properly, all its components must coordinate and integrate their activities. Functional systems are characteristic units with specific biological purposes, whose operation draws on the individual functions of organs from the organ systems. Different functional systems are responsible for reproduction, nutrition, respiration, circulation of body fluids, excretion of harmful substances, information, and integration of the organism.
The human nervous system primarily controls rapid bodily activities, including muscle contractions and quick changes in the function of internal organs. Directly or indirectly, it also takes part in regulating hormone secretion. For the nervous system to function, stimulus receptors must exist which, under the influence of a certain, specific energy, bring the body into an appropriate state of excitation (mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, etc.). Neural regulation of bodily functions follows this scheme: receptor – nervous system – body tissues and organs. In response to information received about the conditions of the external environment and the body’s internal milieu, the organism reacts by activating muscles and glands.
The human nervous system is divided into the cerebrospinal and the autonomic (vegetative) systems.
The cerebrospinal part of the nervous system consists of the central nervous system (the nerves located in the brain and in the spinal cord) and the peripheral nervous system (all the nerves that emerge from the brain and from the spinal cord). The components of the cerebrospinal nervous system regulate voluntary activities of the muscles of the head and limbs, as well as movement in space; for this reason, this part of the nervous system is also referred to as the somatic system.
Although connected to the central nervous system, the autonomic part of the nervous system operates independently, that is, without the involvement of will or consciousness. Its task is to control the activity of the internal organs that maintain the most vital, i.e., vegetative functions. The autonomic nervous system is divided into the sympathetic and parasympathetic divisions.
The sympathetic nervous system is composed of two connected chains of paired ganglia along the sides of the spinal cord and ganglia located among the visceral organs.
The parasympathetic system includes the tenth cranial nerve and the complex of sacral spinal nerves, and its ganglia are located within the internal organs.
Some centers of the autonomic part of the nervous system are located in the spinal cord and the medulla oblongata. The principal centers are situated in the diencephalon, which enables them to coordinate and complement the activity of the autonomic nervous system with the central nervous system.
The spinal cord has a dual function: reflex (somatic and vegetative reflexes) and conduction (sensory impulses to the brain and motor impulses to effectors). The medulla oblongata connects the brain with the spinal cord.
The system of chemical regulation of the organism’s functioning, which controls and adjusts the speed and intensity of chemical reactions in all, or in specific, body cells, is carried out by means of hormones. These biologically active substances (proteins, amino acids, steroids, etc.) are secretions of independent glandular cells or of their tissue and organ formations, the glands of internal secretion (endocrine glands). Hormones from these glands enter directly into the bloodstream and the lymph and thus reach all the cells of the body. Hormonal regulation of bodily functions proceeds according to this scheme: receptor – nervous system – glands of internal secretion – hormones – blood – body tissues and organs.