Human Physiology, 14th edition (2016)

The Autonomic Nervous System 251

The sympathetic and parasympathetic divisions of the auto-
nomic system affect the visceral organs in different ways.
Mass activation of the sympathetic system prepares the body
for intense physical activity in emergencies; the heart rate
increases, blood glucose rises, and blood is diverted to the
skeletal muscles (away from the visceral organs and skin). The
theme of the sympathetic system has been aptly summarized in
a phrase: “fight or flight.”
The fight-or-flight concept can give the erroneous
impression that the sympathetic division is activated only in

organs contain postganglionic fibers. An overall view of the
autonomic nervous system, with a comparison of the sym-
pathetic and parasympathetic divisions, can be obtained by
reviewing figure 9.5.

| CHECKPOINT

3. Describe the sympathetic pathway from the CNS to
   the (a) heart; and (b) adrenal gland.


4. Explain the functional relationship between the
   sympathetic division and the adrenal glands.


5. Describe the parasympathetic pathway to the eye
   and to the heart, identifying the neurons involved.

9.3 FUNCTIONS OF THE

AUTONOMIC NERVOUS SYSTEM

The sympathetic division of the autonomic system acti-

vates the body to “fight or flight,” largely through the

release of norepinephrine from postganglionic neurons

and the secretion of epinephrine from the adrenal medulla.

The parasympathetic division often produces antagonistic

effects through the release of acetylcholine from its post-

ganglionic neurons.

LEARNING OUTCOMES

After studying this section, you should be able to:

6. Identify the neurotransmitters of the sympathetic
   and parasympathetic divisions, and the hormone
   released by the adrenal medulla.


7. Describe the effects of adrenergic stimulation on
   different organs, and identify the types of adrenergic
   receptors involved.


8. Describe the effects of parasympathetic nerve
   regulation, and explain how atropine and related
   drugs affect this regulation.


9. Describe and give examples of antagonistic,
   cooperative, and complementary actions of the
   sympathetic and parasympathetic divisions of the
   autonomic system.

emergencies. But unlike somatic motor neurons, sympathetic

neurons display tonic (continuous) activity. As a result, sym-

pathetic nerves tonically regulate the heart, blood vessels, and

other organs. Also, whereas the fight-or-flight reaction stimu-

lates the mass activation of sympathetic nerve activity, there is

also a more tailored, moment-to-moment regulation of the car-

diovascular system and kidneys by the sympathetic division.

Unlike the fight-or-flight theme of the sympathetic divi-

sion, there is no universally recognized phrase to describe the

actions of the parasympathetic division. However, because

many of its actions are opposite to those of the sympathetic

division, the theme of the parasympathetic division might be

described as rest and digest, or repast and repose.

The effects of parasympathetic nerve stimulation are in

many ways opposite to those produced by sympathetic stimu-

lation. The parasympathetic system, however, is not normally

activated as a whole. Stimulation of separate parasympathetic

nerves can result in slowing of the heart, dilation of visceral

blood vessels, and increased activity of the digestive tract. Vis-

ceral organs respond differently to sympathetic and parasym-

pathetic nerve activity because the postganglionic axons of

these two divisions release different neurotransmitters.

CLINICAL APPLICATION

Sympathomimetic drugs are those that mimic the effects

of sympathoadrenal stimulation. Such drugs include the

naturally occurring catecholamines (epinephrine, norepi-

nephrine, and dopamine) and their analogs, as well as drugs

that promote the release of epinephrine and norepinephrine,

block their reuptake from the synaptic cleft, and block their

degradation (such as the MAO inhibitor drugs discussed

in chapter 7). Many of these drugs have medical uses, but

some abused drugs are sympathomimetics. Amphetamine

and its derivatives (methamphetamine and mephedrone)

promote the presynaptic release of norepinephrine; cocaine

blocks its reuptake into presynaptic terminals. Sympathomi-

metic toxicity can cause tachycardia, diaphoresis (profuse

perspiration), and hypertension, and is frequently a cause of

cardiac arrest and death from abused drugs.

Adrenergic and Cholinergic

Synaptic Transmission

Acetylcholine (ACh) is the neurotransmitter of all preganglionic

axons (both sympathetic and parasympathetic). Acetylcholine

is also the transmitter released by most parasympathetic post-

ganglionic axons at their synapses with effector cells ( fig. 9.7 ).

Transmission at these synapses is thus said to be cholinergic.

The neurotransmitter released by most postganglionic

sympathetic nerve fibers is norepinephrine(noradrenaline).

Transmission at these synapses is thus said to be adrenergic.

There are a few exceptions, however. Some sympathetic fibers

that innervate blood vessels in skeletal muscles, as well as sym-

pathetic fibers to sweat glands, release ACh (are cholinergic).