Autonomic Nervous System

The autonomic nervous system—also known as the visceral system—involun-tarily regulates smooth muscles and glands including the heart, respiratory system, GI tract, peristalsis (digestion), bladder, and eyes.

The autonomic nervous system has two sets of nerves. These are the sensory neurons (afferent) and the motor neurons (efferent). Sensory neurons send impulses to the central nervous system, which are transmitted to the brain where they are interpreted. The brain then sends a response to the motor neuron's brain through the spinal cord that directs specific organ cells to respond to the sensory neuron's impulse.

Previously in this chapter you learned that the autonomic nervous system has two branches. These are the sympathetic branch and parasympathetic branch. Both branches act on the same organ cells but in an opposite way. The sympathetic branch stimulates a response and the parasympathetic branch depresses a response by the organ cell. Together, they keep the organ in balance (homeostasis).

The sympathetic branch stimulates a response using norepinephrine, a neuro-transmitter. Medications that mimic the effect of norepinephrine are called adrenergic drugs or sympathomimetics (mimic sympathetic nervous system actions) (see chart). These drugs are also known as adrenergic agonists because they start a response at the adrenergic receptor sites. There are four types of adrenergic receptors. These are alpha1, alpha2, beta1, and beta2. (see chart)

The parasympathetic branch depresses a response using adrenergic blockers—also known as sympatholytics. Lytic means to stop effect. Adrenergic blockers prevent the norepinephrine response at the adrenergic receptor sites.

The parasympathetic branch is sometimes referred to as the cholinergic system because an acetylcholine neurotransmitter is used to innervate muscle cells at the end of the neuron. Acetylcholine stimulates receptor cells to produce a

Receptor Physiologic responses


Increases force of contraction of heart. Vasoconstriction: increases blood pressure. Mydriasis: dilates pupils of the eyes. Glandular (salivary): decreases secretions. Bladder & prostate: capsule increases contraction and ejaculation.


Inhibits the release of norepinephrine, dilates blood vessels, and produces hypotension; decreases gastrointestinal motility and tone.


Increases heart rate and force of contraction; increases rennin secretion, which increases blood pressure


Dilates bronchioles; promotes GI and uterine relaxation; promotes increase in blood sugar through glycogenolysis in the liver; increases blood flow in the skeletal muscles.

response. However, the enzyme acetylcholinesterase can inactivate the acetylcholine before it reaches the receptor cell.

Drugs that mimic acetylcholine are cholinergic agonists because they initiate a response. These are also known as cholinergic drugs or parasympathomimetics (see chart).

Drugs that block the effect of acetylcholine are called anticholinergic, or parasympatholytics. They are also known as cholinergic antagonists because they inhibit the effect of acetylcholine on the organ.

There are two types of cholinergic receptors. These are nicotinic or muscarinic. Nicotinic receptors are stimulated by alkaloids nicotine. Muscarinic receptors are stimulated by muscarine.

Sympathetic Stimulants

Parasympathetic Stimulants

Sympathomimetics (adrenergics,


adrenomimetics, or adrenergic agonists)

Parasympathomimetics (cholinergics, or

Increase blood pressure

cholinergic agonists)

Increase pulse rate

Decrease blood pressure

Relax bronchioles

Decrease pulse rate

Dilate pupils of eyes

Constrict bronchioles

Uterine relaxation

Constrict pupils of eyes

Increase blood sugar

Increase urinary contraction

Increase peristalsis


Cholinesterase Inhibitors (anticholinesterase)

Increase muscle tone

Sympathetic Depressants

Parasympathetic Depressants

Sympatholytics (adrenergic blockers, adrenolytics, or adrenergic antagonists)

Decrease blood pressure

Decrease pulse rate

Constrict bronchioles

Parasympatholytics (anticholinergics, cholinergic antagonists, or antispasmodics)

Increase pulse rate

Decrease mucus secretions

Decrease gastrointestinal motility

Increase urinary retention

Dilate pupils of eyes

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