534
UNIT 3
Regulation and Integration of the Body
14
All
postganglionic neurons, both sympathetic and para-
sympathetic
Te hormone-producing cells of the adrenal medulla
When ACh binds to nicotinic receptors anywhere, the effect is
always
stimulatory. Just as at the sarcolemma of skeletal mus-
cle (examined in Chapter 9), ACh binding to any nicotinic
receptor directly opens ion channels, depolarizing the post-
synaptic cell.
Muscarinic Receptors
Muscarinic receptors occur on all ef-
fector cells stimulated by postganglionic cholinergic fibers—
that is, all parasympathetic target organs and a few sympathetic
targets, such as eccrine sweat glands. When ACh binds to mus-
carinic receptors, the effect can be either inhibitory or stimu-
latory, depending on the subclass of muscarinic receptor on
the target organ. For example, ACh binding to cardiac muscle
receptors slows heart activity, whereas ACh binding to recep-
tors on smooth muscle of the gastrointestinal tract increases its
motility.
Adrenergic Receptors
Tere are also two major classes of adrenergic (NE-binding)
receptors:
alpha (
a
)
and
beta (β)
. Tese receptors are further
divided into subclasses (
a
1
and
a
2
; β
1
, β
2
, and β
3
). Organs that
In contrast, most sympathetic postganglionic axons release
NE and are called
adrenergic fibers
(ad
0
ren-er
9
jik). An excep-
tion is sympathetic postganglionic fibers that secrete ACh onto
sweat glands.
Unfortunately for memorization purposes, the effects of
ACh and NE on their effectors are not consistently excitatory or
inhibitory. Why not? Because the action of any neurotransmit-
ter depends on the receptor to which it binds. Each autonomic
neurotransmitter binds with two or more kinds of receptors,
allowing it to exert different effects (activation or inhibition)
at different body targets.
Table 14.2
summarizes the receptor
types that we introduce next.
Cholinergic Receptors
Te two types of cholinergic (ACh-binding) receptors are
named for drugs that bind to them and mimic acetylcholine’s
effects.
Nicotinic receptors
(nik
0
o-tin
9
ik) respond to nicotine.
Muscarinic receptors
, the other set of ACh receptors, can be
activated by the mushroom poison
muscarine
(mus
9
kah-rin).
All ACh receptors are either nicotinic or muscarinic.
Nicotinic Receptors
Nicotinic receptors are found on:
Te sarcolemma of skeletal muscle cells at neuromuscular
junctions (which, as you will recall, are somatic and not au-
tonomic targets)
Table 14.2
Cholinergic and Adrenergic Receptors
NEUROTRANSMITTER
RECEPTOR TYPE
MAJOR LOCATIONS*
EFFECT OF BINDING
Acetylcholine (ACh)
Cholinergic
 
 
 
Nicotinic
All postganglionic neurons; adrenal
medullary cells (also neuromuscular
junctions of skeletal muscle)
Excitation
 
Muscarinic
All parasympathetic target organs
Excitation in most cases; inhibition of
cardiac muscle
 
 
Limited sympathetic targets (e.g., eccrine
sweat glands
)
Activation
Norepinephrine (NE) (and
epinephrine released by
adrenal medulla)
Adrenergic
 
 
 
b
1
Heart predominantly, but also kidneys and
adipose tissue
Increases heart rate and force of contraction;
stimulates kidneys to release renin
 
b
2
Lungs and most other sympathetic target
organs; abundant on blood vessels serving
the heart, liver, and skeletal muscle
Effects mostly inhibitory; dilates blood
vessels and bronchioles; relaxes smooth
muscle walls of digestive and urinary
visceral organs; relaxes uterus
 
b
3
Adipose tissue
Stimulates lipolysis by fat cells
 
a
1
Most importantly blood vessels serving the
skin, mucosae, abdominal viscera, kidneys,
and salivary glands; also, virtually all
sympathetic target organs except heart
Constricts blood vessels and visceral organ
sphincters; dilates pupils of the eyes
 
a
2
Membrane of adrenergic axon terminals;
pancreas; blood platelets
Inhibits NE release from adrenergic
terminals; inhibits insulin secretion by
pancreas; promotes blood clotting
* Note that all of these receptor subtypes are also found in the CNS.
Sympathetic cholinergic vasodilator fibers are found in other animals, but do not appear to be present in humans.
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