Essential Cell Biology 5th edition

Ion Channels and Nerve Cell Signaling
413
cannula for perfusion
plasma membrane
giant
axon
rubber
roller
axoplasm
perfusion fluid
stream of perfusion fluid
(A)
rubber mat
cannula
(B)
plasma membrane of giant axon
Figure 12–33 The cytoplasm in a squid axon can be removed and replaced with an artificial solution of pure ions. (A) The axon
cytoplasm (axoplasm) is extruded using a rubber roller. (B) A perfusion fluid containing the desired concentration of ions is pumped
gently through the emptied-out axon.
ECB5 e12.33/12.33
equal importance when it comes to the action potential?
Because the squid axon is so large and strong, investigators
could extrude the cytoplasm from the axon like
toothpaste from a tube (Figure 12–33A). The emptiedout
axon could then be reinflated by filling it with a pure
solution of Na + , K + , or Cl – (Figure 12–33B). Thus, the
ions inside the axon and in the bath solution could be
varied independently (see Figure 12–32A). Using this
set-up, the researchers discovered that the axon would
generate a normal action potential if, and only if, the
concentrations of Na + and K + approximated the natural
concentrations found inside and outside the cell.
Thus, they concluded that the cell components crucial
to the action potential are the plasma membrane, Na +
and K + ions, and the energy provided by the concentration
gradients of these ions across the membrane; all
other components, including other sources of metabolic
energy, were presumably removed when the axon was
emptied and refilled.
Channel traffic
Once Na + and K + had been singled out as critical for an
action potential, the questions then became: What does
each of these ions contribute to the action potential?
How permeable is the membrane to each, and how does
the membrane permeability change as an action potential
sweeps by? Again, the squid giant axon provided
some answers. The concentrations of Na + and K + inside
and outside the axon could be altered, and the effects
of these changes on the membrane potential could be
measured directly. From such studies, it was determined
that, at rest, the membrane potential of an axon is close
to the equilibrium potential for K + : when the external
concentration of K + was varied, the resting potential of
the axon changed roughly in accordance with the Nernst
equation (see Figure 12–24). The results suggested that
at rest, the membrane is chiefly permeable to K + ; we
now know that K + leak channels provide the main pathway
that these ions can take through the resting plasma
membrane.
The situation for Na + is very different. When the external
concentration of Na + was varied, there was no effect on
the resting potential of the axon. However, the height of
the peak of the action potential varied with the concentration
of Na + outside the axon (Figure 12–34). During
the action potential, therefore, the membrane appeared
to be chiefly permeable to Na + , presumably as the result
of the opening of Na + channels. In the aftermath of the
action potential, the Na + permeability decreased and the
membrane potential reverted to a negative value, which
depended on the external concentration of K + . As the
membrane lost its permeability to Na + , it became even
more permeable to K + than before, presumably because
additional K + channels opened, accelerating the resetting
of the membrane potential to the resting state, and
readying the membrane for the next action potential.
These studies on the squid giant axon made an enormous
contribution to our understanding of nerve cell
excitability, and the researchers who made these discoveries
in the 1940s and 1950s—Alan Hodgkin and Andrew
Huxley—received a Nobel Prize in 1963. However, it was
years before the various ion channel proteins that they
had hypothesized to exist would be biochemically identified.
We now know the three-dimensional structures of
many of these channel proteins, allowing us to marvel
at the fundamental beauty of these molecular machines.
mV
40
0
–40
100%
50%
33%
0 1 2
time (msec)
Figure 12–34 The shape of the action potential depends
on the concentration of Na + outside the squid axon.
Shown here are action potentials recorded when the external
medium contains 100%, 50%, or 33% of the normal extracellular
concentration of Na + .