Chapter 3 Autonomic Pharmacology

The autonomic nervous system (ANS) is so named because it is autonomous; it functions independently of the conscious or somatic nervous system. For example, you do not need to consciously tell your heart to beat faster when you exercise or your digestive tract to increase activity after eating. However, the ANS can be influenced by conscious thought; a classic example was demonstrated by the experiment on Pavlov’s dog, which salivated at the sound of a bell because the bell had been rung before every meal, so the dog had learned to associate the bell with meals.

To understand autonomic function, and by extension to understand how to manipulate the ANS, you will need to understand how the two types of the ANS coexist and function, how each system exerts its effects, and finally what pharmacologic mechanisms exist to increase or decrease each component of the ANS. Memorization of the receptors, their distribution, and their effects is mandatory for achieving this goal and will enable you to accurately predict effects and side effects of drugs (Table 3-1).

TABLE 3-1 Autonomic Receptors: Function and Distribution

Receptor	Function	Distribution
Sympathetic
α₁	Constriction of smooth muscles	Blood vessels and piloerectors in skin (vasoconstriction and goose bumps) Sphincters (bladder, gastrointestinal [GI]) Uterus and prostate (contraction) Eye (contraction of the radial muscle = pupillary dilation/mydriasis)
α₂	Inhibition of sympathetic autonomic ganglia (decreases the sympathetic nervous system [SNS])	Presynaptic ganglionic neurons GI tract (less important pharmacologically, included for completeness)
β₁	Increase cardiac performance and liberation of energy	Heart, the most important for β₁; (increased heart rate, contractility, conduction speed) Fat cells (release fat for energy via lipolysis) Kidney (release renin to conserve water)
β₂	Relaxation of smooth muscles and liberation of energy	Lungs (bronchodilation) Blood vessels in muscles (vasodilation) Uterus (uterine relaxation) GI (intestinal relaxation) Bladder (bladder relaxation) Liver (to liberate glucose via glycogenolysis)
Parasympathetic
N (Nicotinic)	”Nerve to nerve” and ”nerve to muscle” communication	Sympathetic and parasympathetic ganglia Neuromuscular junction (NMJ)
M (Muscarinic)	To oppose most sympathetic actions at the level of the organs	Lung (bronchoconstriction) Heart (slower rate, decreased conduction, decreased contractility) Sphincters of GI and bladder (relax) Bladder (constriction) GI (intestinal contraction) Eye (contraction of the circular muscle = pupillary constriction or meiosis) Eye (contraction of the ciliary muscle = focus for near vision) Glands: lacrimal, salivary, bronchial (secretion)
Special Notes
There is no parasympathetic innervation of blood vessels. Sweat glands are innervated by sympathetic nerves, but paradoxically use M receptors. Sexual arousal is parasympathetic, but orgasm is sympathetic.