Action Potentials
What are they?
• Rapid, brief, depolarizations of the
membrane potentials of excitable cells
(neurons, muscle cells, some gland cells),
initiated by an appropriate stimulus to a
sensory receptor (chemical, physical or
electromagnetic), or chemical signals
released by neurons and received by other
neurons, muscle cells or gland cells.
Characteristics of Action Potentials
• Show threshold for initiation
• Are “all-or-nothing” - their magnitude is
not graded to stimulus intensity.
• Spread throughout the plasma membrane by
a non-decremental process- the magnitude
of the AP does not diminish with distance.
• Followed by a refractory period during
which it is difficult or impossible to initiate
another action potential.
Physiological Function
• Action potentials are the means of rapid
(milliseconds), long-distance (up to meters)
communication in the body
• As opposed to
• chemical messages - which can be longdistance, but slow (seconds to minutes)
• decremental electric currents - which are
rapid, but can only operate over short
distances (a few tens of microns)
We will examine the change in the voltage of a small
piece of excitable membrane as we drive a current
across it.
Volts
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+_ +_ +_ +_
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Notice that the
same current
strength causes a
smaller voltage
change for
hyperpolarizing
pulses than
depolarizing pulses
- it’s easier to
depolarize the
membrane than to
hyperpolarize it.
As the strength of depolarizing current is
increased, there is a sudden transition to an
action potential - a threshold has been
crossed.
• A threshold stimulus - defined in terms of
current intensity and duration - is one that is
able to initiate an action potential 50% of
the time.
The next slide shows a complete action
potential as it would be recorded in squid giant
axon. (Not all cell types show a
hyperpolarizing afterpotential as obvious as is
seen here.)
In this set of
experiments the axonal
membrane is voltagestepped to -40mV, 0 mV
or +40 mV, and the
resulting conductance
changes for Na+ and K+
are plotted over time.
Note that the Na+
conductance is selfterminating; the K+
conductance is not (at
least within the
timeframe shown here).
The next slide shows that the conductance change
of the stimulated (and unclamped) cell explains
the voltage change of an action potential.
The conductance change is the sum of the two
separate conductance (g) changes for Na+ and K+
that were recorded under clamp conditions.
Conductance = permeability x driving force,
so the conductance changes are essentially
permeability changes.
The Refractory Period
Following an action potential, there is a brief
period during which an excitable cell cannot
initiate a second action potential. This is the
absolute refractory period.
Following the absolute refractory period is a
longer period during which it is more difficult
to bring the membrane to threshold for a
second action potential - this is the relative
refractory period.
In the experiment shown on the next slide, a cell
is given a pair of stimulus pulses with a variable
time interval . As the interval is made shorter, the
threshold rises, because the second stimulus
starts to fall in the cell’s relative refractory
period.
The threshold becomes essentially infinite in the
millisecond or so just after the first AP (I.e. the
second stimulus comes during the absolute
refractory period.
Questions
• How does depolarization cause an AP and
what is responsible for threshold?
• What factors determine the duration of an
AP?
• Why is the membrane refractory to a second
stimulus for a time after an AP?
• Why is it easier to depolarize the membrane
than it is to hyperpolarize it?
These questions can now be answered
in terms of the behavior of voltagegated channels for Na+ and K+
Na+ channels can exist in three distinct
states, which form a cycle
• Most channels are closed, but available, at
rest potential - depolarization increases
open probability. At threshold the channels
enter a positive feedback cycle in which .
depolarization
activation
This positive feed back explains the rapid rise or
upsweep of the spike when threshold is reached.
The open state or activated state of Na+ channels is
followed by inactivation - a closed state in which the
channel cannot be reopened by depolarization. This
explains the downturn of the spike before it has time to
reach Ena.
Inactivation is removed by some combination of
repolarization and time, returning the channel to the
available state.
The Na+ channel has
two gates - an activation
gate in the interior of
the channel and an
inactivation gate
suspended from the
intracellular domain.
Na+ channel behavior is revealed by patch
clamping
The experiment on the next slide shows the singlechannel currents recorded from 7 individual Na+
channels in response to a depolarizing voltage step.
Notice how random the behavior is - the different
channels open at different times, stay open for different
times, and may flicker closed a time or two before each
finally inactivates. The summed current from the
channels is shown in the bottom trace. The Na+current
that flows across a patch of membrane during an action
potential is the sum of its many single-channel
currents.
Recording from voltage-sensitive
channels using a patch-clamp: A.
the membrane surface is pulled
gently to allow the glass pipette to
form a tight seal with the
membrane. This isolates the patch
of membrane and allows the
activity of one or a few channels
to be detected.
B. The opening of individual
channels in response to
depolarization is not synchronous
and does not last a standard
amount of time, but the cell’s total
response is the sum of the
individual responses (shown in 3).
In this slide
inward currents
are shown as
downward
deflections
K+ channels differ from Na+ channels in
that they
• Open more slowly in response to
depolarization
• Close (slowly) in response to repolarization
• Do not show time-dependent inactivation.
Note that the
trace of the
summed
current rises
more slowly
than for the
Na+ channel,
and that the
channels
continue to be
active as long
as the
depolarization
is maintained
Threshold can now be redefined
• as the stimulus intensity/duration that has a
50% probability of opening enough Na+
channels to cause the inward Na+ current to
exceed the outward K+ current.
• When the inward Na+ current exceeds the
outward K+ current, the system enters the
positive feedback cycle that leads to an AP
Three factors make threshold hard to reach
• Depolarization increases the driving force
for K+ and decreases it for Na+
• Depolarization opens K+ channels as well as
Na+ channels.
• Open Na+ channels do not stay open - they
inactivate.
The refractory period is determined by two
separate factors: Na+ channel recovery
from inactivation and K+ channel closing.
• Early in the refractory period, most Na+
channels are still in the inactivated state,
and so not available.
• In the middle of the refractory period, some
Na+ channels have become available, but
the number of open K+ channels is still
greater than at rest.
• The refractory period wanes as the last
voltage-sensitive K+ channels close.