Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

630 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes
major classes—ionotropic or metabotropic—on the basis of their signaling mechanisms:
1. Ionotropic receptors are ion channels and feature at fast chemical synapses.
Acetylcholine, glycine, glutamate, and GABA all act on transmitter-gated
ion channels, mediating excitatory or inhibitory signaling that is
generally immediate, simple, and brief.
2. Metabotropic receptors are G-protein-coupled receptors (discussed in
Chapter 15) that bind to all other neurotransmitters (and, confusingly, also
acetylcholine, glutamate, and GABA). Signaling mediated by ligand-binding
to metabotropic receptors tends to be far slower and more complex
than that at ionotropic receptors, and longer-lasting in its consequences.
The Acetylcholine Receptors at the Neuromuscular Junction Are
Excitatory Transmitter-Gated Cation Channels
A well-studied example of a transmitter-gated ion channel is the acetylcholine
receptor of skeletal muscle cells. This channel is opened transiently by acetylcholine
released from the nerve terminal at a neuromuscular junction—the specialized
chemical synapse between a motor neuron and a skeletal muscle cell (Figure
11–37). This synapse has been intensively investigated because it is readily accessible
to electrophysiological study, unlike most of the synapses in the central nervous
system, that is, the brain and spinal cord in vertebrates. Moreover, the acetylcholine
receptors are densely packed in the muscle cell plasma membrane at
a neuromuscular junction (about 20,000 such receptors per μm 2 ), with relatively
few receptors elsewhere in the same membrane.
The receptors are composed of five transmembrane polypeptides, two of one
kind and three others, encoded by four separate genes (Figure 11–38A). The four
genes are strikingly similar in sequence, implying that they evolved from a single
ancestral gene. The two identical polypeptides in the pentamer each contribute
one acetylcholine-binding site. When two acetylcholine molecules bind to the
pentameric complex, they induce a conformational change that opens the channel.
With ligand bound, the channel still flickers between open and closed states,
but now it has a 90% probability of being open. This state continues—with acetylcholine
binding and unbinding—until hydrolysis of the free acetylcholine by
the enzyme acetylcholinesterase lowers its concentration at the neuromuscular
junction sufficiently. Once freed of its bound neurotransmitter, the acetylcholine
receptor reverts to its initial resting state. If the presence of acetylcholine persists
for a prolonged time as a result of excessive nerve stimulation, the channel inactivates.
Normally, the acetylcholine is rapidly hydrolyzed and the channel closes
within about 1 millisecond, well before significant desensitization occurs. Desensitization
would occur after about 20 milliseconds in the continued presence of
acetylcholine.
The five subunits of the acetylcholine receptor are arranged in a ring, forming
a water-filled transmembrane channel that consists of a narrow pore through
the lipid bilayer, which widens into vestibules at both ends. Acetylcholine binding
opens the channel by causing the helices that line the pore to rotate outward, thus
disrupting a ring of hydrophobic amino acids that blocks ion flow in the closed
state. Clusters of negatively charged amino acids at either end of the pore help to
exclude negative ions and encourage any positive ion of diameter less than 0.65
nm to pass through (Figure 11–38B). The normal through-traffic consists chiefly
of Na + and K + , together with some Ca 2+ . Thus, unlike voltage-gated cation channels,
such as the K + channel discussed earlier, there is little selectivity among cations,
and the relative contributions of the different cations to the current through
the channel depend chiefly on their concentrations and on the electrochemical
driving forces. When the muscle cell membrane is at its resting potential, the net
driving force for K + is near zero, since the voltage gradient nearly balances the K +
concentration gradient across the membrane (see Panel 11–1, p. 616). For Na + ,
in contrast, the voltage gradient and the concentration gradient both act in the
same direction to drive the ion into the cell. (The same is true for Ca 2+ , but the
muscle cell myelinated axon nerve
body of
Schwann cell
axon terminals
10 µm
Figure 11–37 A low-magnification
scanning electron micrograph of a
neuromuscular junction in a frog. The
termination of a single axon on a skeletal
muscle cell is shown. (From J. Desaki and
Y. Uehara, J. Neurocytol. 10:101–110,
1981. With MBoC6 permission m11.36/11.38
from Kluwer
Academic Publishers.)