Human Physiology, 14th edition (2016)

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).

1. Channel closed until
   neurotransmitter
   binds to it


2. 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?