Human Physiology, 14th edition (2016)

The Autonomic Nervous System 255

CLINICAL APPLICATION

Beta blockers are drugs that act as beta-adrenergic recep-

tor antagonists. Propranolol, for example, is nonselective for

b 1 - and b 2 -adrenergic receptors, while atenolol is relatively

selective for the b 1 -adrenergic receptors in the heart and so

is used more often to lower cardiac rate and blood pressure

in patients with hypertension (high blood pressure). How-

ever, because the smooth muscle of the bronchioles (air-

ways) in the lungs has b 2 -adrenergic receptors, a drug that

blocks these could reduce the bronchodilation effect of epi-

nephrine. This promotes bronchoconstriction in susceptible

people, making breathing difficult. Asthmatics used to inhale

epinephrine as a beta agonist to stimulate the b 2 -adrenergic

receptors and promote bronchodilation, but this also stim-

ulated the b 1 -adrenergic receptors in the heart and raised

blood pressure. Now people with asthma use inhalers con-

taining more selective b 2 -adrenergic receptors agonists,

such as salbutamol (Albuterol), terbutaline, and others.

The alpha agonist drugs phenylephrine and pseu-

doephedrine are used in cold medicines to stimulate the

a 1 -adrenergic receptors in the nasal mucosa, promoting

vasoconstriction that relieves nasal congestion but can

raise blood pressure. Clonidine is an a 2 -adrenergic recep-

tor agonist that acts in the CNS to reduce the activity of the

sympathoadrenal system, thereby promoting vasodilation

and lowering the blood pressure.

Figure 9.10 Receptors

involved in autonomic

regulation. Acetylcholine is

released by all preganglionic

neurons and stimulates

postganglionic neurons by means

of nicotinic ACh receptors.

Postganglionic parasympathetic

axons regulate their target

organs through muscarinic

ACh receptors. Postganglionic

sympathetic axons provide

adrenergic regulation of their

target organs by binding of

norepinephrine to a 1 -, b 1 -, and

b 2 -adrenergic receptors.

Postganglionic

neurons

Preganglionic

neurons

ACh

ACh Norepinephrine

Nicotinic

ACh receptors

Parasympathetic division Sympathetic division

Dilation of

bronchioles (of lung)

and blood vessels

Stimulates

muscarinic ACh

receptors

Parasympathetic

nerve effects

Stimulates

α 1 -adrenergic

receptors

Vasoconstriction in

viscera and skin

Stimulates

β 1 -adrenergic

receptors

Increased

heart rate and

contractility

Stimulates

β 2 -adrenergic

receptors

Clinical Investigation CLUES

Sofia used atenolol among other drugs for her hyperten-

sion, took a cold medicine that contained pseudoephed-

rine, and used a puffer for asthma.

• Which receptors do these drugs stimulate or block,
  and what are their effects?


• What might have provoked Sofia’s asthma, and how
  would the inhaler help?