Neuroregulation

the coordinating influence of the nervous system on cells, tissues, and organs. Neuroregulatory mechanisms adjust the activities of the body in accordance with the needs of the organism and with the changes that occur in the environment.

Neuroregulation is one of the principal mechanisms for the self-regulation of bodily functions. In multicellular organisms, such vital processes as growth, development, and reactions to external influences are well coordinated. In all organisms, this coordination is maintained by a number of regulatory mechanisms; in animals, the neuroregulatory mechanisms have become especially important. Neuroregulation is responsible for the initiation, cessation, intensification, and weakening of activities in the cells and organs. The functional and biochemical properties of cells and organs can be altered, as well as the structural details. Multicellular organisms without nervous systems lack neural regulation, for example, plants, animal embryos, and sponges; in such cases, the orderliness of functions is ensured by ionic, metabolic, or some other variety of cellular interaction.

The activity of some cells can be regulated by the metabolic products of other cells. (SeeHUMORAL REGULATION.) An excited state on the surface membrane of certain cells can sometimes spread from cell to cell. The cardiac excitation wave is an example of this type of neural-like excitation (the process resembles the conduction of nerve impulses in that both types of conduction involve an ionic mechanism). On the basis of intercellular hormonal regulation and the conductive excitability of surface membranes, over the course of evolution neuroregulation and hormonal regulation became the two principle coordinating mechanisms in the animal body. Two kinds of intervening substances are distinguished accordingly—mediator substances (also called transmitter substances) and hormones.

A hormone spreads through the body by way of the bloodstream. As a result, the effects of hormonal regulation are widespread and are achieved slowly. In contrast, neuroregulation can be rapid and localized, both because a mediator substance is released from the nerve endings directly onto the innervated cells and because the release of the mediator substance is triggered by a rapidly spreading signal, a nerve impulse. In some cases, no sharp distinction can be drawn between hormonal regulation and neuroregulation: some nerve endings release active substances into the blood. (See.) Since the swiftness and specificity of neuroregulation are especially important in the control of movement, neuroregulation is well developed in organisms that are capable of higher locomotion. Having become the dominant regulatory mechanism over the course of evolution, neuroregulation in higher animals controls not only the motor functions but all the other systems of the body as well. Both effector and receptor, or sensory, organs and cells, as well as all the autonomic functions, are under neural control.

Neuroregulation also affects the tissues that respond to the metabolic needs of the body, for example, fatty tissue. A cell must be sensitive to a mediator substance in order that the mediator have an effect—the cell must have the corresponding receptors. In the skeletel musculature of vertebrates, for example, cholinergic receptors are situated on the surface of every muscle fiber; these receptors are capable of interacting with acetylcholine, the mediator substance released by motor nerve endings. The reaction between the mediator substance and the receptor alters the ionic permeability of the surface membrane of the innervated cell. This changes the membrane potential and the ionic composition of the cytoplasm, after which the activity of that cell is intensified or diminished. (SeeMEMBRANE THEORY OF EXCITATION.)

It would appear in some cases that a mediator substance can exert a direct influence, not mediated by ions, on the metabolic processes in the cell (this mechanism is known as the “enzymo-chemical” hypothesis of nerve excitation, proposed by Kh. S. Koshtoiants in 1950). Less is known about the role of mediator substances in the regeneration of tissue and in the realization of the influence of the nervous system on the growth and differentiation of organs and tissues. How mediator substances effect the trophic function of the nervous system, that is, the maintenance of a certain functional and biochemical level in the innervated cells, is also incompletely understood. The proteins and the other substances that are released from the nerve endings simultaneously with the mediator substance may possibly be a factor in these forms of neuroregulation. (See.)