Essential Cell Biology 5th edition

414 CHAPTER 12 Transport Across Cell Membranes
Figure 12–35 An action potential is
triggered by a depolarization of a
neuron’s plasma membrane. The resting
membrane potential in this neuron is –60 mV,
and a stimulus that depolarizes the plasma
membrane to about –40 mV (the threshold
potential) is applied. This depolarizing
stimulus is sufficient to open voltagegated
Na + channels in the membrane and
thereby trigger an action potential. As the
membrane rapidly depolarizes further, the
membrane potential (red curve) swings past
zero, reaching +40 mV before it returns
to its resting negative value as the action
potential terminates. The green curve shows
how the membrane potential would simply
have relaxed back to the resting value after
the initial depolarizing stimulus if there had
been no amplification by voltage-gated ion
channels in the plasma membrane.
Figure 12–36 A voltage-gated Na +
channel can flip from one conformation
to another, depending on the membrane
potential. When the membrane is at rest
and highly polarized, positively charged
amino acids in the voltage sensors of the
channel (red bars) are oriented by the
membrane potential in a way that keeps
the channel in its closed conformation.
When the membrane is depolarized,
the voltage sensors shift, changing the
channel’s conformation so the channel has
a high probability of opening. But in the
depolarized membrane, the inactivated
conformation is even more stable than the
open conformation, and so, after a brief
period spent in the open conformation,
the channel becomes temporarily
inactivated and cannot open. The red
arrows indicate the sequence that follows
a sudden depolarization, and the black
arrow indicates the return to the original
conformation after the membrane has
repolarized.
plasma membrane potential (mV)
+40
0
–40
–60
0 1 2
time (msec)
STIMULUS
ACTION
POTENTIAL
threshold
potential
resting membrane
potential
membrane has shifted from its resting value of about –60 mV to about
+40 mV (Figure 12–35).
The voltage of +40 ECB5 mV is E12.31/12.35
close to the membrane potential at which the
electrochemical driving force for movement of Na + across the membrane
is zero—that is, the effects of the membrane potential and the concentration
gradient for Na + are equal and opposite; therefore Na + has no further
tendency to enter or leave the cell.
If these voltage-gated channels continued to respond to the depolarized
membrane potential, the cell would get stuck with most of its Na + channels
open. The cell is saved from this fate because voltage-gated Na +
channels have an automatic inactivating mechanism—a kind of “timer”
that causes them to rapidly adopt (within a millisecond or so) a special
inactivated conformation in which the channel is closed, even though
the membrane is still depolarized. The Na + channels remain in this inactivated
state until the membrane potential has returned to its resting,
negative value. A schematic illustration of these three distinct states of
the voltage-gated Na + channel—closed, open, and inactivated—is shown
in Figure 12–36. How they contribute to the rise and fall of an action
potential is shown in Figure 12–37.
During an action potential, voltage-gated Na + channels do not act alone.
The depolarized axonal membrane is helped to return to its resting
potential by the opening of voltage-gated K + channels. These also open
in response to depolarization, but not as promptly as the Na + channels,
and they stay open as long as the membrane remains depolarized. As the
local depolarization reaches its peak, K + ions (carrying positive charge)
therefore start to flow out of the cell, down their electrochemical gradient,
plasma
membrane
EXTRACELLULAR
SPACE
CYTOSOL
– – –
– – –
+
+
+ + +
+ + +
INACTIVATED
+
+
+
+
CLOSED
+ + +
+ + +
–––
RECOVERY AND
MEMBRANE
REPOLARIZATION
REFRACTORY
PERIOD
voltage sensors
+
+
–––
voltage-gated
Na + channel
ARRIVAL OF
ACTION POTENTIAL
– – –
– – –
+
+
+ + +
+ + +
OPEN
+
+
membrane
at rest
membrane
depolarized