186 Chapter 7
G-Protein-Coupled Channels
There is another group of chemically regulated ion channels
that, like the ligand-gated channels, are opened by the bind-
ing of a neurotransmitter to its receptor protein. However, this
group of channels differs from ligand-gated channels in that
Figure 7.26 Nicotinic acetylcholine (ACh) receptors also function as ion channels. The nicotinic acetylcholine
receptor contains a channel that is closed ( a ) until the receptor binds to ACh. ( b ) Na^1 and K^1 diffuse simultaneously, and in
opposite directions, through the open ion channel. The effect of the inward diffusion of Na^1 predominates, resulting in an excitatory
postsynaptic potential (EPSP).
- Channel closed until
neurotransmitter
binds to it - Open channel
permits diffusion of
specific ions
(b)
Acetylcholine
Cytoplasm
Plasma
membrane
Ion
channel
Nicotinic ACh
receptors
(a)
Extracellular Fluid
Binding
site
Na+
K+
CLINICAL APPLICATION
Myasthenia gravis is an autoimmune disease (chapter 15)
caused by antibodies that block the nicotinic ACh recep-
tors, particularly in the motor end plates (postsynaptic
membranes) of skeletal muscle cells. This produces muscle
weakness, especially in the eyes, eyelids, and face. Neostig-
mine ( table 7.5 ) and related drugs, which block the enzyme
that degrades ACh (discussed shortly) in the synaptic cleft,
can help treat symptoms.
Saxitoxin, a poison produced by dinoflagellates in
the ocean that becomes concentrated in filter-feeding
clams and mussels, and tetrodoxin, a poison produced
by pufferfish, bind to voltage-gated Na^1 channels in
axons ( table 7.5 ), blocking action potentials. Saxitoxin can
thereby produce paralytic shellfish poisoning. Saxitoxin
and tetrodoxin can be fatal if paralysis of the diaphragm
causes suffocation.
Clinical Investigation CLUES
Denise had a seizure after eating fresh mussels at the
restaurant.
- If the mussels were contaminated with saxitoxin,
how would they have obtained it? - If the mussels contained saxitoxin, are the
symptoms it produces compatible with the
symptoms that Denise displayed?