Essential Cell Biology 5th edition

412
HOW WE KNOW
SQUID REVEAL SECRETS OF MEMBRANE EXCITABILITY
Each spring, Loligo pealei migrate to the shallow
waters off Cape Cod on the eastern coast of the United
States. There they spawn, launching the next generation
of squid. But more than just meeting and breeding,
these animals provide neuroscientists summering
at the Marine Biological Laboratory in Woods Hole,
Massachusetts, with a golden opportunity to study the
mechanism of electrical signaling along nerve axons.
Like most animals, squid survive by catching prey and
escaping predators. Fast reflexes and an ability to accelerate
rapidly and make sudden changes in swimming
direction help them avoid danger while chasing down a
decent meal. Squid derive their speed and agility from a
specialized biological jet propulsion system: they draw
water into their mantle cavity and then contract their
muscular body wall to expel the collected water rapidly
through a tubular siphon, thus propelling themselves
through the water.
Controlling such quick and coordinated muscle contraction
requires a nervous system that can convey signals
with great speed down the length of the animal’s body.
Indeed, Loligo pealei possesses some of the largest nerve
cell axons found in nature. Squid giant axons can reach
10 cm in length and are over 100 times the diameter of
a mammalian axon—about the width of a pencil lead.
Generally speaking, the larger the diameter of an axon,
the more rapidly signals can travel along its length.
In the 1930s, scientists first started to take advantage
of the squid giant axon for studying the electrophysiology
of the nerve cell. Because of its relatively large
size, an investigator can isolate an individual axon and
insert an electrode into it to measure the axon’s membrane
potential and monitor its electrical activity. This
experimental system allowed researchers to address a
variety of questions, including which ions are important
for establishing the resting membrane potential
and for initiating and propagating an action potential,
and how changes in the membrane potential control ion
permeability.
Set-up for action
Because the squid axon is so long and wide, an electrode
made from a glass capillary tube containing a conducting
solution can be thrust down the axis of the isolated
axon so that its tip lies deep in the cytoplasm. This set-up
allowed investigators to measure the voltage difference
between the inside and the outside of the axon—that is,
the membrane potential—as an action potential sweeps
past the tip of the electrode (Figure 12–32). The action
potential itself would be triggered by applying a brief
electrical stimulus to one end of the axon. It didn’t matter
which end was stimulated, as the action potential
could travel in either direction; it also didn’t matter how
big the stimulus was, as long as it exceeded a certain
threshold (see Figure 12–35), indicating that an action
potential is an “all or nothing” response.
Once researchers could reliably generate and measure
an action potential, they could use the preparation
to answer other questions about membrane excitability.
For example, which ions are critical for an action
potential? The three most plentiful ions, both inside
and outside an axon, are Na + , K + , and Cl – . Do they have
axon plasma
membrane
bath
solution
intracellular
electrode
40
action potential
mV
0
resting membrane
potential
–40
(A)
axoplasm
1 mm
(B)
0 2 4 6
msec
Figure 12–32 Scientists can study nerve cell excitability using an isolated axon from
squid. (A) An electrode can be inserted into the cytoplasm (axoplasm) of a squid giant axon
to (B) measure the resting membrane potential and monitor action potentials induced when
the axon is electrically stimulated.
ECB5 e12.32/12.32