The vegetative (or autonomous, independent of the will of a person) nervous system provides regulation of the functions of internal organs and glands of external secretion, the tone of blood vessels. It is represented by central neurons located in the spinal cord and brain, ganglia (peripheral accumulation of nerve cells) and terminal synapses, in which excitation is transmitted to the cells of the innervated organ (tissue). Depending on the localization of the centers in the brain and the location of the ganglia, the nature of the effect on the internal organs and the mediator released in the terminal synapses, the autonomic nervous system is subdivided into parasympathetic (a mediator, usually acetylcholine) and sympathetic (mediator – norepinephrine). The main mediator of both sympathetic and parasympathetic ganglia is acetylcholine. Embryogenetically, the sympathetic ganglia neurons are related to the accumulations of chromaffin cells in the adrenal medulla and carotid sinus zones, receiving cholinergic innervation. Medicines, depending on the effect on synapses with norepinephrine or acetylcholine mediation, are subdivided into adrenergic and cholinergic drugs.
The main mediator of the adrenergic system is norepinephrine. Released under the influence of a nerve impulse into the synaptic cleft, it interacts with adrenergic receptors, and then is destroyed by catechol-O-methyltransferase (COMT) or undergoes reuptake; in the cytoplasm of the nerve endings, norepinephrine is deaminated by monoamine oxidase (MAO). In the chromaffin cells of the adrenal glands, norepinephrine is methylated and converted into adrenaline, a hormone that mainly affects metabolic processes, but retains the ability to excite adrenergic systems.
Based on the different sensitivity to chemical compounds, there are two types of adrenergic receptors – alpha- and beta-adrenergic receptors, each of which has at least two subtypes – alpha1, alpha2 and beta1, beta2 receptor subtypes. Alpha1-adrenergic receptors are localized in postsynaptic membranes (vessels, radial muscle of the iris, gastrointestinal tract, smooth muscles of the distal bronchi), alpha2-adrenergic receptors – pre- (CNS) and extrasynaptically in the vascular wall. Presynaptic alpha2-adrenergic receptors regulate the release of norepinephrine by participating in a negative feedback system. In turn, beta1-adrenergic receptors are predominantly located in the heart, and beta2-adrenergic receptors – in the bronchi, vessels and uterus. Presynaptic beta2-adrenergic receptors implement a positive feedback, stimulating the release of norepinephrine. Adrenergic substances can selectively act on different types of adrenergic receptors: excite them (adrenomimetics – agonists) or inhibit (adrenergic blockers or adrenolytics – antagonists).
The main mediator of the parasympathetic nervous system is acetylcholine. Receptors sensitive to acetylcholine are divided into m- (excited by muscarin) and n- (excited by small doses of nicotine). In turn, m-cholinergic receptors, depending on localization, are subdivided into m1 (central nervous system, peripheral nervous system), m2 (heart) and m3 (smooth muscles, exocrine glands). Nicotinic cholinergic receptors are divided into nn (neuronal) – receptors of the central nervous system, autonomic ganglia, adrenal medulla and nm (muscle) – receptors of the striated muscles. Substances that stimulate one or another type of cholinergic receptors are called cholinomimetics. They fully (m, n-cholinomimetics) or partially (m- or n-cholinomimetics) reproduce the effects of acetylcholine. Drugs that interfere with the interaction of acetylcholine with receptors are called anticholinergics or anticholinergics.
In medical practice, adrenergic drugs are used mainly for cardiovascular (hypo- and hypertension, coronary artery disease, arrhythmia, etc.), bronchopulmonary and allergic disorders, cholinergic drugs are used in the treatment of diseases associated with impaired smooth muscle tone of internal organs (various types of colic or atony of the gastrointestinal tract and bladder, gastric ulcer, bronchial asthma, etc.).