Neurohumoral Regulation
in animals and man, the joint influence of the nervous system and humoral factors on the regulation, coordination, and integration of bodily functions and processes. Humoral factors, which include metabolites, hormones, and mediator substances (chemical transmitters), are biologically active substances that are contained in the blood, lymph, and tissue fluid.
Neurohumoral regulation plays an important role in helping the body adapt to changes in the external environment; it also has a homeostatic role, that is, neurohumoral regulation helps to keep the composition and properties of the body’s internal environment relatively constant. In lower organisms, the connection between different types of cells and organs is realized by chemical substances. As organs and tissues differentiated and grew more complex over the course of evolution, these substances acquired specific physiological functions; they became mediators, neurohormones, and hormones.
Humoral regulation and its subtype, hormonal regulation, merged with nervous regulation. In addition to directly influencing cells, tissues, and organs, the numerous biologically active metabolic products that are elaborated in response to a nerve impulse can cause a chemical reflex by acting as stimulants to the endings of sensory nerves (seeCHEMORECEPTOR). These metabolic products also act as the humoral link in reflex arcs, that is, they transmit information to the brain and spinal cord, which subsequently produce a wave of nerve impulses that spread out from the central nervous system to the effector organs.
The activity of the brain and spinal cord depends not only on nerve signals but also on nutrition, metabolism, and the chemical composition and physicochemical and biological properties of the tissue fluid that surrounds the nerve cells. It is in these capacities that the closest interrelationship of neural and humoral processes occurs. For example, CO2 stimulates the cells of the respiratory center, and stimulation of certain nerve formations leads to the secretion of mediator substances into the synapses (such as acetylcholine, noradrenaline, and serotonin). Should the mediators enter the bloodstream, they participate in the humoral regulation of bodily functions; they can therefore be called neurohormones.
Participation of hormones in neurohumoral regulation makes it possible to speak of a single neurohumoral-hormonal mechanism of regulation of bodily functions. The juxtaposition of different types of bodily regulation, for example, reflex versus humoral-hormonal regulation, is not consistent with the modern physiological viewpoint. The biosynthesis and action of many biologically active substances can occur in response to a conditioned reflex. This is regarded by a number of investigators as an indication of the participation of the cerebral cortex in neurohumoral regulation.
Chain reactions by which the body adapts to such strong stimuli as physical and mental tension, pain, disease, or trauma—all of which produce a state of stress—serve as examples in which the action of a humoral mechanism of regulation follows that of a neural mechanism in stepwise fashion. The chain reaction begins when excitation arises in the cerebral cortex and is transmitted through subcortical elements to the hypothalamus, where the higher centers of neurohumoral regulation are located.
Under the influence of nerve signals, the cells and nerve endings of the hypothalamus release noradrenaline in bound form. Noradrenaline, by acting on elements of the reticular formation of the brainstem that are sensitive to it, promotes excitation in the central and peripheral divisions of the sympathetic nervous system. The impulses that enter the adrenal gland along sympathetic nerves intensify the formation of adrenaline in the adrenal medulla. Upon entering the blood, the adrenaline passes into the hypothalamus, where it produces excitation of the adrenergic nerve elements—neural structures that are specifically sensitive to adrenaline. The excitation of the adrenergic elements in the hypothalamus stimulates the secretion of releasing factors, under whose influence adrenocorticotropic hormone (ACTH) is synthesized and released from the hypophysis. The presence of ACTH in the blood is the signal for the formation of the adrenocortical hormones—corticosteroids that produce a many-linked chain of neural and humoral reactions in the body and hence actively participate in the body’s adaptation to stress. (SeeADAPTATION SYNDROME.)
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