Innervation

INNERVATION, the supply of organs and tissues with nerves. Centripetal, or afferent, nerves, by which irritation is brought to the central nervous system, and centrifugal, or efferent, nerves, by which impulses are transmitted from the centers to the periphery, are distinguished. Only the centrifugal nerves of an organ have a direct relation to its work; the centripetal nerves coming from the given apparatus do not necessarily participate in its functioning. In the case where the work of an organ is stimulated or regulated by a reflex path, the participation of centripetal nerves is necessary. It should be emphasized that the number of centripetal nerves whose irritation can cause a reflex impulse in one centrifugal nerve is very great. Already within the limits of one spinal cord, the number of incoming afferent nerves to a given segment significantly exceeds the number of efferent nerves leaving it (Sherrington's funnel). In the presence of the cerebral cortex, the irritation of any afferent nerve can, in the order of a conditioned reflex, cause an impulse in any efferent nerve and consequently any activity of the organism. There is no known activity of the organism that proceeds completely independently of nervous influences. In a number of cases, the work of the effector apparatus occurs exclusively under the influence of nervous impulses. Such is, for example, the activity of the entire skeletal musculature, which is conditioned exclusively by reflex irritation or direct irritation of the nervous centers. In these cases, cutting the centrifugal nerve causes a complete loss of function of the given apparatus. In other cases, the work of an organ is caused by nervous impulses (reflexes) and by the direct action of certain irritants on the tissue of the given organ. Such is, for example, the work of the gastric glands, pancreas. Finally, there are cases when nervous impulses exert only a regulating influence on the work of an organ (a typical example is cardiac activity). In some cases, innervation has a relatively secondary significance for the work of an organ (for example, the secretion of urine by the kidneys) or an unclear significance (for example, the secretion of bile by the liver). Only very few processes apparently are not subject to direct nervous influence (for example, the diffusion of gases through the wall of the alveoli). At present it is proved that the processes of metabolism in the tissues also depend on nervous influences. From what has been said it is clear that for the normal work of an organ its connection with the centers through centrifugal nerves is necessary. The latter are divided into somatic, directly going from the anterior horns of the spinal cord to the innervated apparatus (muscles), and vegetative, passing through ganglia (see Autonomic nervous system). Apparently the majority, if not all, of the apparatuses of the organism possesses double innervation—vegetative and somatic [muscles (Boucke, Orbely)] or sympathetic and parasympathetic innervation (for example, the heart, intestine, stomach). The majority of data forces us to recognize that between the nerve and the innervated apparatus a special formation is included, playing an important role in the processes of transmission of excitation. According to some authors (Langley), this formation (substance /S) is not identical with the nerve ending. However, the question of the existence of a special intermediate link between the nerve and the innervated apparatus cannot finally be resolved (Lapicque). Histological aspect of the question—see Nerve endings. As a rule, not only those parts of the central nervous system from which the nerves innervating the corresponding organs originate are related to the work of organs. The higher parts of the brain are always related to the work of all organs. When speaking of the center of some activity (for example, respiratory center), it should be borne in mind that the discussion cannot be about a narrowly limited anatomical area. Alongside the main center (for a number of vegetative functions) located in the medulla oblongata, there are always subordinate ones in the spinal cord. Even after the complete exclusion of centers, certain primitive innervation mechanisms gradually recover at the expense of nerve ganglia and those nerve cells that are located in the organ itself (the said refers only to the area of innervation by the autonomic nervous system).- Regarding the intimate mechanism of innervation processes and the mechanism of transmission of excitation from the nerve to the innervated apparatus, there are no exact and complete data. Experiments by Levi (Loewy) showed that upon irritation of the cardiac nerves some chemical substance is produced, producing the same action as the irritation of the nerves themselves. Samoilov expressed an analogous view regarding the mechanism of transmission of irritation from the nerve to the muscle. From this point of view, the transmission of excitation is reduced, as it were, to the secretion by the nerve ending of a certain chemical agent possessing a specific action. In recent years it has been proved that the transmission of irritation from the nerve to the muscle is connected with the splitting of creatine phosphoric acid into its components.- On theories of conduction of excitation along the nerve and theories of central innervation processes—see Nervous system, Ionic theory of excitation. Innervation of individual organs—see corresponding organs and Autonomic nervous system.

^г- Konradi.