Drugs affecting the ANS

The autonomic nervous system, along with the endocrine system, coordinates the regulation and integration of bodily functions. The endocrine system sends signals to target tissues by varying the levels of blood-borne hormones. In contrast, the nervous system exerts its influence by the rapid transmission of electrical impulses over nerve fibers that terminate at effector cells, which specifically respond to the release of neuromediator substances. Drugs that produce their primary therapeutic effect by mimicking or altering the functions of the autonomic nervous system are called autonomic drugs and are discussed in the following four chapters. These autonomic agents act either by stimulating portions of the autonomic nervous system or by blocking the action of the autonomic nerves. This chapter outlines the fundamental physiology of the autonomic nervous system, and it describes the role of neurotransmitters in the communication between extracellular events and chemical changes within the cell.

Drugs affecting the autonomic nervous system are divided into two groups according to the type of neuron involved in their mechanism of action. The cholinergic drugs, which are described in this and the following chapter, act on receptors that are activated by acetylcholine. The second group act on receptors that are stimulated by norepinephrine or epinephrine. Cholinergic and adrenergic drugs both act by either stimulating or blocking receptors of the autonomic nervous system. 

The cholinergic antagonists (also called cholinergic blockers, parasympatholytics or anticholinergic drugs) bind to cholinoceptors, but they do not trigger the usual receptor-mediated intracellular effects. The most useful of these agents selectively block muscarinic synapses of the parasympathetic nerves. The effects of parasympathetic innervation are thus interrupted, and the actions of sympathetic stimulation are left unopposed. A second group of drugs, the ganglionic blockers, show a preference for the nicotinic receptors of the sympathetic and parasympathetic ganglia. Clinically, they are the least important of the anticholinergic drugs. A third family of compounds, the neuromuscular blocking agents, interfere with transmission of efferent impulses to skeletal muscles. These agent are used as adjuvants in anesthesia during surgery.

The adrenergic drugs affect receptors that are stimulated by norepinephrine or epinephrine. Some adrenergic drugs act directly on the adrenergic receptor (adrenoceptor) by activating it and are said to be sympathomimetic.  block the action of the neurotransmitters at the receptors (sympatholytics), whereas still other drugs affect adrenergic function by interrupting the release of norepinephrine from adrenergic neurons. This chapter describes agents that either directly or indirectly stimulate adrenoceptors .

The adrenergic antagonists (also called blockers or sympatholytic agents) bind to adrenoceptors but do not trigger the usual receptor-mediated intracellular effects. These drugs act by either reversibly or irreversibly attaching to the receptor, thus preventing its activation by endogenous catecholamines. Like the agonists, the adrenergic antagonists are classified according to their relative affinities for α or β receptors in the peripheral nervous system. [Note: Antagonists that block dopamine receptors are most important in the central nervous system (CNS) and are therefore considered in that section ).] 


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