Neuronal Signaling and the Structure of the Nervous System
183
usually activated reciprocally; that is, as the activity of one
division increases, the activity of the other decreases. Think
of this like a person driving a car with one foot on the brake
and the other on the accelerator. Either depressing the brake
(parasympathetic) or relaxing the accelerator (sympathetic)
will slow the car. Dual innervation by neurons that cause
opposite responses provides a very fi ne degree of control over
the effector organ.
A useful generalization is that the sympathetic system
increases its activity under conditions of physical or psycholog-
ical stress. Indeed, a generalized activation of the sympathetic
system is called the
ght-or-fl
ight response,
describing the
situation of an animal forced to either challenge an attacker or
run from it. All resources for physical exertion are activated:
heart rate and blood pressure increase; blood fl ow increases
to the skeletal muscles, heart, and brain; the liver releases glu-
cose; and the pupils dilate. Simultaneously, the activity of the
gastrointestinal tract and blood fl ow to the skin are inhibited
by sympathetic fi ring. In contrast, when the parasympathetic
system is activated, a person is in a
rest-or-digest
state in
which homeostatic functions are predominant.
The two divisions of the autonomic nervous system rarely
operate independently, and autonomic responses generally rep-
resent the regulated interplay of both divisions. Autonomic
Smooth
or cardiac
muscles,
glands,
or GI
neurons
Skeletal
muscles
AUTONOMIC NS
Parasympathetic
division
SOMATIC NS
CNS
ACh
N-AChR
N-AChR
N-AChR
M-AChR
Ganglion
NE
ACh
Adrenal
medulla
Epi
Adrenergic
receptors
via bloodstream
ACh
Ganglion
CNS
Sympathetic
division
Skeletal
muscles
Smooth
or cardiac
muscles,
glands,
or GI
neurons.
Table 6–10
Locations of Receptors for
Acetylcholine, Norepinephrine, and
Epinephrine
I.
Receptors for acetylcholine
a. Nicotinic receptors
On postganglionic neurons in the autonomic ganglia
At neuromuscular junctions of skeletal muscle
On some central nervous system neurons
b. Muscarinic receptors
On smooth muscle
On cardiac muscle
On gland cells
On some central nervous system neurons
On some neurons of autonomic ganglia (although the
great majority of receptors at this site are nicotinic)
II.
Receptors for norepinephrine and epinephrine
On smooth muscle
On cardiac muscle
On gland cells
On some central nervous system neurons
Figure 6–46
Transmitters used in the various components of the peripheral efferent nervous system. Notice that the fi rst neuron exiting the central nervous
system—whether in the somatic or the autonomic nervous system—releases acetylcholine. In a very few cases, postganglionic sympathetic
neurons may release a transmitter other than norepinephrine. (ACh, acetylcholine; NE, norepinephrine; Epi, epinephrine; N-AChR, nicotinic
acetylcholine receptor; M-AChR, muscarinic acetylcholine receptor.)
Figure 6–46
physiological
inquiry
How would the effects differ between a drug that blocks muscarinic acetylcholine receptors versus one that blocks nicotinic acetylcholine
receptors?
Answer can be found at end of chapter.
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