The Autonomic Nervous System
spinal cord; terminal ganglia; collateral ganglia; innervates
How does a visceral reﬂex differ from a somatic reﬂex?
For answers, see Appendix H.
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
Describe autonomic nervous system controls.
Neurotransmitters and Receptors
Te major neurotransmitters released by ANS neurons are
). 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
Dorsal root ganglion
• May be preganglionic
neuron (as shown)
• May be a dorsal horn
• May be within walls
Receptor in viscera
• Preganglionic neuron
• Postganglionic neuron
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
also the distal half of the large intestine. For the most part, sym-
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
, 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.
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
neurons in its motor compo-
; 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
State whether each of the following is a characteristic of
the sympathetic or parasympathetic nervous system: short
preganglionic ﬁbers; origin from thoracolumbar region of