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]) β₁ Increase cardiac performance and liberation of energy β₂ Relaxation of smooth muscles and liberation of energy Parasympathetic N (Nicotinic) ”Nerve to nerve” and ”nerve to muscle” communication M (Muscarinic) To oppose most sympathetic actions at the level of the organs Special Notes

The ANS has two parts:

As a primer to help you remember the fight-or-flight response, consider a caveman who requires an intact SNS to stay alive. While he is fighting with (or maybe running away from) a saber-toothed tiger, what physiologic effects would promote his survival?

The PNS, to keep everything in balance, would therefore oppose all these effects. If you remember the flight-or-fight response and think of the opposite of the fight-or-flight response, you will remember the majority of the important ANS functions.