Mediators

Mediators - active chemicals contributing to the transfer of excitation in the synapse (see). The mediators in the form of small bubbles (vesicles) accumulate on the presynaptic membrane. Under the influence of nerve impulses vesicles burst and their contents are flowing into the synaptic gap. Acting on the postsynaptic membrane, mediators cause depolarization (see Excitation). The most studied and widely used in the body neurotransmitters are acetylcholine (see) and norepinephrine. In line with this, all the nerves that transmit excitation by different bodies are divided into cholinergic, where the mediators synaptic transmission is acetylcholine, and adrenergic in which a mediator is norepinephrine. To cholinergic include fiber somatic nervous system, transfer of excitation in skeletal muscle, preganglionarnah fibers of the sympathetic and parasympathetic systems and postganglionarnyh parasympathetic fibers. Postganglionic fibers are predominantly adrenergic. In the Central nervous system are synapses, using as mediator as acetylcholine, noradrenaline, and serotonin, gamma-aminobutyric acid, L-glutamate and some other amino acids.

The mediators (from lat. mediator the mediator) - substances, through which the transfer of excitation from the nerve to the authorities and from one neuron to another.
The systematic study of chemical mediators of the nervous effects (nerve impulses) began with the classic experiments levy (O. Loewi).
Subsequent studies have confirmed the results of the experiments levy on the heart and showed that not only in the heart, but also in other organs parasympathetic nerves exert their influence through the mediator acetylcholine (see), and sympathetic - of neurotransmitter norepinephrine. It was further established that the somatic nervous system transmits its impulses skeletal muscles with participation of a mediator acetylcholine.
Through M is also the transmission of nerve impulses from one neuron to another in the peripheral gangliev and Central nervous system
Dale (N. Dale), based on the chemical nature M, divides the nervous system in holinergicakih (with M. acetylcholine) and adrenergic (M noradrenaline). To cholinergic are postganglionarnyh parasympathetic nerves, preganglionarnah parasympathetic and sympathetic nerves and motor nerves skeletal musculature; to adrenergic - most postganglionic nerve. Sympathetic vasodilator nerves, and the nerves of the sweat glands, it seems, belong to cholinergic. In the Central nervous system detected as cholinergic, and adrenergic neurons.
Continue intensively studied issues: limited to whether the nervous system in its activities only two chemical intermediaries acetylcholine and norepinephrine; what mediators due to the development of the braking process. In respect of the peripheral part of the sympathetic nervous system there is evidence that the braking effect on the activities of the bodies is carried out by means of adrenaline (see), and incentive - norepinephrine. Flory (that is, Florey) extracted from the Central nervous system of mammals inhibitory substance called them the factor J, which probably contains the brake mediator. Factor J detected in the gray matter of the brain, centres associated with correlation and integration of motor functions. It is identical aminohydrocinnamic acid. When the application of factor J to the spinal cord develops braking reflex reactions, especially blocked the tendon reflexes.
In some synapses in invertebrates inhibitory role of the mediator plays a gamma-aminobutyric acid.
Some authors tend to attribute mediator function of serotonin. The concentration of serotonin high in the hypothalamus, the midbrain and in the gray matter of the spinal cord, below-in the big hemispheres, cerebellum, dorsal and ventral roots. The distribution of serotonin in nervous system coincides with the distribution of noradrenaline and adrenaline.
However, the presence of serotonin in nervous system, deprived of nerve cells, suggests that this substance has no relation to the mediator function.
The mediators are synthesized mainly in the body of the neuron, while many authors acknowledge the possibility of additional M. synthesis and axonal endings. Synthesized in the body nerve cells Meters transported by AXONE to its end, where M fulfill its main function of the excitation transfer on effector body. Together with the mediator on AXONE transported and enzymes, providing its synthesis (for example, cholineacetylase synthesizing acetylcholine). Liberated into the presynaptic nerve endings, M. diffuses through the synaptic space to postsynaptic membrane on the surface of which it binds to specific chemoreception substance that has either stimulating (depolarizing)or inhibitory (hyperpolarize) effect on the membrane of the postsynaptic cells (see Synapse). Here the mediator is destroyed under the influence of enzymes. Acetylcholine is split cholinesterase, noradrenaline and adrenaline - mainly by monoamine oxidase.
Thus, these enzymes regulate the duration of Meters and the extent of its spread to nearby structures.
Cm. also Arousal, Neurohumoral regulation.