Human Physiology, 14th edition (2016)

(Tina Sui) #1
The Autonomic Nervous System 257

Figure 9.11 Comparison of nicotinic and muscarinic acetylcholine receptors. Nicotinic receptors are ligand-gated, meaning
that the ion channel (which runs through the receptor) is opened by binding to the neurotransmitter molecule (the ligand). The muscarinic ACh
receptors are G-protein-coupled receptors, meaning that the binding of ACh to its receptor indirectly opens or closes ion channels through
the action of G-proteins.


Nicotinic ACh
receptors

Postsynaptic membrane of


  • All autonomic ganglia

  • All neuromuscular junctions

  • Some CNS pathways
    Na+
    ACh ACh ACh


K+
K+ K+

Depolarization

Excitation

Ligand-gated channels
(ion channels are part
of receptor)

Muscarinic ACh
receptors


  • Produces parasympathetic nerve effects in
    the heart, smooth muscles, and glands

  • G-protein-coupled receptors (receptors
    influence ion channels by means of G-proteins)


Hyperpolarization Depolarization

(K+ channels
opened)

(K+ channels
closed)

Inhibition
Produces slower
heart rate

Excitation
Causes smooth muscles of the
digestive tract to contract

Na+ or Ca2+

γ
αβ
γ
αβ

Receptor Tissue Response Mechanisms
Nicotinic Skeletal muscle Depolarization, producing action potentials and
muscle contraction

ACh opens cation channel in receptor

Nicotinic Autonomic
ganglia

Depolarization, causing activation of postganglionic
neurons

ACh opens cation channel in receptor

Muscarinic
( M 3 ,  M 5 )

Smooth muscle,
glands

Depolarization and contraction of smooth muscle,
secretion of glands

ACh activates G-protein coupled receptor,
opening Ca^2 1 channels and increasing
cytosolic Ca^2 1
Muscarinic
( M 2 )

Heart Hyperpolarization, slowing rate of spontaneous
depolarization

ACh activates G-protein coupled receptor,
opening channels for K^1

Table 9.6 | Cholinergic Receptors and Responses to Acetylcholine


Source: Simplified from table 6-2, p. 119, of Goodman and Gilman’s The Pharmacological Basis of Therapeutics. Ninth edition. J.E. Hardman et al., eds. 1996 and
table 6-3, p. 156, of the Eleventh edition, 2006. McGraw-Hill.


Other Autonomic Neurotransmitters

Certain postganglionic autonomic axons produce their effects
through mechanisms that do not involve either norepinephrine
or acetylcholine. This can be demonstrated experimentally by
the inability of drugs that block adrenergic and cholinergic
effects from inhibiting the actions of those autonomic axons.
These axons, consequently, have been termed “nonadrenergic,

Clinical Investigation CLUES


Sofia had her pupils dilated for an eye exam that morning.


  • What drug was used to dilate her pupils, and how
    did that drug work?

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