Chapter 14
The Autonomic Nervous System
533
14
spinal cord; terminal ganglia; collateral ganglia; innervates
adrenal medulla.
5.
How does a visceral reflex differ from a somatic reflex?
For answers, see Appendix H.
ANS Physiology
Define cholinergic and adrenergic fibers, and list the
different types of their receptors.
Describe the clinical importance of drugs that mimic or
inhibit adrenergic or cholinergic effects.
State the effects of the parasympathetic and sympathetic
divisions on the following organs: heart, blood vessels,
gastrointestinal tract, lungs, adrenal medulla, and external
genitalia.
Describe autonomic nervous system controls.
Neurotransmitters and Receptors
Te major neurotransmitters released by ANS neurons are
ace-
tylcholine
(
ACh
) and
norepinephrine
(
NE
). ACh, the same neu-
rotransmitter secreted by somatic motor neurons, is released
by (1) all ANS preganglionic axons and (2) all parasympathetic
postganglionic axons at synapses with their effectors. Fibers that
release ACh are called
cholinergic fibers
(ko
0
lin-er
9
jik).
Spinal cord
Dorsal root ganglion
Autonomic ganglion
Stimulus
Response
Visceral sensory
neuron
Integration center
• May be preganglionic
neuron (as shown)
• May be a dorsal horn
interneuron
• May be within walls
of gastrointestinal
tract
Receptor in viscera
2
3
1
5
Visceral effector
Motor neuron
(two-neuron chain)
• Preganglionic neuron
• Postganglionic neuron
4
Figure 14.7
Visceral reflexes.
Visceral reflex arcs have the same
five elements as somatic reflex arcs. The visceral afferent (sensory)
fibers are found both in spinal nerves (as depicted here) and in
autonomic nerves.
also the distal half of the large intestine. For the most part, sym-
pathetic fibers
inhibit
the activity of muscles and glands in the
abdominopelvic visceral organs.
Pathways with Synapses in the Adrenal Medulla
Some fibers traveling in the thoracic splanchnic nerves pass
through the celiac ganglion without synapsing and terminate by
synapsing with the hormone-producing medullary cells of the ad-
renal gland. When stimulated by preganglionic fibers, the med-
ullary cells secrete
norepinephrine
and
epinephrine
(also called
noradrenaline
and
adrenaline
, respectively) into the blood, produc-
ing the excitatory effects we have all felt as a “surge of adrenaline.”
Embryologically, sympathetic ganglia and the adrenal medulla
arise from the same tissue. For this reason, the adrenal medulla
is sometimes viewed as a “misplaced” sympathetic ganglion, and
its hormone-releasing cells, although lacking nerve processes, are
considered equivalent to postganglionic sympathetic neurons.
Visceral Reflexes
Because most anatomists consider the ANS to be a visceral mo-
tor system, the presence of sensory fibers (mostly visceral pain af-
ferents) is o±en overlooked. However,
visceral sensory neurons
,
which send information concerning chemical changes, stretch, and
irritation of the viscera, are the first link in autonomic reflexes.
Visceral reflex arcs
have essentially the same components
as somatic reflex arcs—receptor, sensory neuron, integration
center, motor neuron, and effector. However, a key difference
is that a visceral reflex arc has
two
neurons in its motor compo-
nent (
Figure 14.7
; compare with Figure 13.15). ²wo examples
of visceral reflexes are the reflexes that empty the rectum and
bladder, which we discuss in Chapters 23 and 25.
Nearly all the sympathetic and parasympathetic fibers we
have described so far are accompanied by afferent fibers con-
ducting sensory impulses from glands or muscles. Tis means
that peripheral processes of visceral sensory neurons are found
in cranial nerves VII, IX, and X, splanchnic nerves, and the
sympathetic trunk, as well as in spinal nerves.
Like sensory neurons serving somatic structures (skeletal mus-
cles and skin), the cell bodies of visceral sensory neurons are lo-
cated either in sensory ganglia of cranial nerves or in dorsal root
ganglia of the spinal cord. Visceral sensory neurons are also found
in sympathetic ganglia where preganglionic neurons synapse.
Furthermore, complete three-neuron reflex arcs (with sensory
neurons, interneurons, and motor neurons) exist entirely within
the walls of the gastrointestinal tract. Neurons composing these
reflex arcs make up the
enteric nervous system
, which plays an im-
portant role in controlling gastrointestinal tract activity. We will
discuss the enteric nervous system in more detail in Chapter 23.
Visceral sensory neurons are also involved in the phenom-
enon of referred pain that we described in Chapter 13 (p. 490).
Check Your Understanding
4.
State whether each of the following is a characteristic of
the sympathetic or parasympathetic nervous system: short
preganglionic fibers; origin from thoracolumbar region of
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