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By nervous system is meant a morpho-functional and structural unit characterized by a biological tissue highly specialized in receiving, transmitting, controlling and processing internal and external stimuli of the body, by means of bioelectric signals in metazoans, ultimately allowing a living organism to relate to its environment; the nervous system underlies the muscular, sensory, psychic and intellectual functions of animals, including humans: examples of such functions are respiration, locomotion, vision, pain, memory, knowledge and consciousness.
Like other systems in the body, the nervous system is composed of organs, principally the brain, spinal cord, nerves, and ganglia. These, in turn, consist of various tissues, including nerve, blood, and connective tissue. Together these carry out the complex activities of the nervous system.
Millions of sensory receptors detect changes, called stimuli, which occur inside and outside the body. They monitor such things as temperature, light, and sound from the external environment. Inside the body, the internal environment, receptors detect variations in pressure, pH, carbon dioxide concentration, and the levels of various electrolytes. All of this gathered information is called sensory input.
Sensory input is converted into electrical signals called nerve impulses that are transmitted to the brain. There the signals are brought together to create sensations, to produce thoughts, or to add to memory; Decisions are made each moment based on the sensory input. This is integration.
Based on the sensory input and integration, the nervous system responds by sending signals to muscles, causing them to contract, or to glands, causing them to produce secretions. Muscles and glands are called effectors because they cause an effect in response to directions from the nervous system. This is the motor output or motor function.
The nervous system or apparatus is a set of cells or complex of organs that, in multicellular animals, is responsible for receiving external and internal stimuli to the organism, to process them and transform them into coordinated impulses. The latter act to give an adequate response to environmental or internal situations in order to maintain the organism in the best conditions of life. The nervous system connects the various parts of the organism, coordinating both the activities that oversee the vegetative life, and, in higher animals, those that affect the life of relationships, as well as those that provide automatic responses to stimuli (nervous reflexes), and those that, in humans, regulate the psychic activity (affective states, intelligence, ideation, etc.).
In vertebrates it is composed of organic tissue, highly differentiated (nervous tissue), which has the specific ability to produce, receive and propagate, with maximum speed and minimum energy consumption, nerve impulses (nervous excitability). Nervous tissue is basically made up of nerve cells or neurons. In addition to them, the nervous system contains blood vessels, supporting connective tissue (only in the peripheral nervous system) and cells with functions different from those of the nerve elements, called neuroglia cells or glia. All of these elements are derived from the outer leaflet of the embryo or ectoderm (see also neurulation).
Nerve cells have a characteristic morphology that allows the establishment of very close reciprocal relationships (synapses) specialized in the transmission of impulses from one element to another of the system. Each nerve cell consists of a cell body or pyrenophore and long extensions that belong to two types: the dendrites, which together with the cell body receive stimuli from the external environment or from other neurons, and the axon that usually conducts the impulses away from the cell body.
The neuroglia is a substance of ectodermal and mesenchymal origin, consisting of intertwined cells and fibers; it is unrelated to the functions of stimulus conduction and is found in the interstices between neurons and nerve fibers. It practically replaces connective tissue (absent in the cerebrospinal axis, except around blood vessels) and thus has trophomechanical and supportive functions. In addition, neuroglia cells are involved in the repair of the central nervous system, and are responsible for the formation of the myelin sheath in nerve fibers.
There are six main types of neuroglia: ependyma cells, astrocytes, oligodendrocytes, satellite cells, Schwann cells, and microglia cells. All neuroglia cells, with the exception of microglia, are derived from the ectoderm. Microglia are of mesodermal origin. The ependyma is the epithelium that lines the cavities of the brain and spinal cord.
Astrocytes are scattered among nerve cells and their extensions are found in the central nervous system. Oligodendrocytes are small cells equipped with a modest number of thin extensions and surround neurons in the central nervous system.
The myelin sheath of nerve fibers in the white matter of the central nervous system originates from oligodendrocyte cells. Microglia consist of small cellular elements capable of amoeboid movements and phagocytic activity toward products of neuronal breakdown. The nervous system can be divided into two main categories: cerebro-spinal nervous system and autonomic nervous system.
The cerebro-spinal nervous system is, in turn, composed of the central and peripheral nervous systems. The central nervous system (consisting of the encephalon, enclosed in the cranial box, and the spinal cord contained in the vertebral canal) is responsible for the reception of incoming stimulation and the subsequent processing of centrifugal impulses, as well as psychic activities in humans.
The peripheral nervous system (consisting of cranial and spinal nerves and their extensions) is responsible for the connection, through the nerves, between the receptors and the central nervous organs and between these and the peripheral organs executors or effectors. In addition, the sense organs, or receptors, have the purpose of relating the organism with its internal organs and the outside world, to continuously collect stimulation, such organs as sight, hearing, smell, taste and touch, are also those of sensitivity, balance, etc..
According to the nature and origin of the stimulus that they collect, the sense organs are divided into exteroceptors, enteroceptors and proprioceptors. The organization of the nervous tissue in the sense organs and peripheral system is relatively simple, while that of the central nervous system – in relation to its more complex functions – is much more complex.
The cell bodies and their axons occupy different areas of the central nervous system. The parts in which cell bodies, dendrites and the initial tract of axons (not coated with myelin) are located form the so-called gray matter, which occupies the peripheral part or cortex of the cerebral hemispheres and cerebellum and the central region of the spinal cord.
The axons of neurons coated with myelin (nerve fibers) constitute the white substance, so called because of the white color of the myelin. The white matter occupies the central portion of the cerebral hemispheres and cerebellum, and the peripheral portion of the spinal cord. The autonomic or vegetative nervous system controls visceral motility (smooth muscle and cardiac muscle) and glandular secretion. It is divided into the sympathetic or orthosympathetic nervous system and the parasympathetic nervous system.
Organization of the nervous system
Although terminology seems to indicate otherwise, there is really only one nervous system in the body. Although each subdivision of the system is also called a “nervous system,” all of these smaller systems belong to the single, highly integrated nervous system. Each subdivision has structural and functional characteristics that distinguish it from the others. The nervous system as a whole is divided into two subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
The central nervous system
The brain and spinal cord are the organs of the central nervous system. Because they are so vitally important, the brain and spinal cord, located in the dorsal body cavity, are encased in bone for protection. The brain is in the cranial vault, and the spinal cord is in the vertebral canal of the vertebral column. Although considered to be two separate organs, the brain and spinal cord are continuous at the foramen magnum.
The peripheral nervous system
The organs of the peripheral nervous system are the nerves and ganglia. Nerves are bundles of nerve fibers, much like muscles are bundles of muscle fibers. Cranial nerves and spinal nerves extend from the CNS to peripheral organs such as muscles and glands. Ganglia are collections, or small knots, of nerve cell bodies outside the CNS.
The peripheral nervous system is further subdivided into an afferent (sensory) division and an efferent (motor) division. The afferent or sensory division transmits impulses from peripheral organs to the CNS. The efferent or motor division transmits impulses from the CNS out to the peripheral organs to cause an effect or action.
Finally, the efferent or motor division is again subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system, also called the somatomotor or somatic efferent nervous system, supplies motor impulses to the skeletal muscles. Because these nerves permit conscious control of the skeletal muscles, it is sometimes called the voluntary nervous system. The autonomic nervous system, also called the visceral efferent nervous system, supplies motor impulses to cardiac muscle, to smooth muscle, and to glandular epithelium. It is further subdivided into sympathetic and parasympathetic divisions. Because the autonomic nervous system regulates involuntary or automatic functions, it is called the involuntary nervous system.