Action Potential Flashcards
Action potential
a self-propagating wave of depolarization that travels down the axon; ALL-OR-NONE; uses the
existing Na+ and K+ gradients
and is generated by opening
and closing selective Na+ and
K+ channels
DEPOLARIZATION PHASE
voltage-gated Na+ channels fast to open up to bring membrane potential way above threshold towards ENa (+60mV)
Overshoot
d/t voltage-gated Na+ channels slow to close
REPOLARIZATION PHASE
voltage-gated K+ channels slow to open, gradually bringing membrane potential towards EK (-90mV)
Undershoot
d/t K+ channels slow to close
Return to RMP after undershoot is due to:
K+ leak channels
Properties of an action potential: Regenerative
once started goes through entire cycle
Properties of an action potential: All-or-none response
fixed amplitude, independent of stimulus strength if above threshold
Properties of an action potential: Time limited
same duration & does not vary with length of stimulus
Is there an effect on [ion] with the movement of Na+/K+ charges?
NO. Na+ in and K+ out not
enough to change intracellular concentrations because an AP is driven by potential difference of the membrane
Effect of the Na+/K+ ATPase pump on generating an action potential
NONE. Na+/K+ ATPase only pumps 3 molecules of Na+ out and 2K+ in, which contributes very little of a potential difference
Influx of Na+ during the rising
phase of the action potential:
1. Results in Vm reaching
ENa.
2. Changes intracellular
concentration of Na+ by
several mmoles.
3. Results in no change of
ENa.
4. Takes place after
opening of the voltage-gated K channels.
- Results in no change of
ENa.
- this is a constant
Tetradotoxin (TTX)
voltage-gated Na+ channel inhibitor
Tetraethyl ammonium (TEA)
voltage-gated K+ channel inhibitor
Relative Refractory Period
A second action potential can fire during the undershoot of the first action potential but the threshold for firing is higher
Absolute Refractory Period
A second action potential can
NOT fire during the depolarization of the first action potential
Advantages of a refractory period:
- limits the rate of firing
- results in a stimulus dependent
firing rate - ensures propagation in one
direction (from cell body to axon terminal) since threshold is higher
Encoding stimulus strength of an action potential: Frequency
more frequent APs–> stronger stimulus
- due to Refractory period, soonest another AP can be fired (a STRONG stimulus would cause another AP to be fired as soon as absolute refractory period is over)
Encoding stimulus strength of an action potential: Timing
weak stimulus–> brief AP
The Frequency of Action Potentials:
1. Is unlimited (they can
fire at any frequency)
2. Depends on the
strength of the stimulus
(i.e. the amount of
membrane
depolarization).
3. Depends on the
amplitude of the action
potential.
4. Depends on the rate
activation of the Na/K
ATPase
- Depends on the
strength of the stimulus
(i.e. the amount of
membrane
depolarization).
Conduction Velocity
how quickly the AP propagates down the axon
Determinants of conduction velocity: Length constant
Faster Conductance:
- Low internal resistance and High membrane resistance= more current
moves along the axon= longer length constant
* A Larger diameter axon= Longer length constant = faster conduction
velocity
* High membrane resistance= longer length constant = faster conduction
velocity
Myelination
- properties of the Schwann cells INCREASE MEMBRANE resistance
- insulation (no conductance through the membrane)
- INCREASES conduction velocity
Node of Ranvier
site of an action potential
Saltatory conduction
propagation of action potentials along myelinated axons
Multiple Sclerosis
auto-immune disease that causes breakdown myelin –> slower conduction velocity
- no insulation leads to dissipation of current and signals are slower and weaker
The speed of an Action
Potential:
1. Is slower with
increased
membrane
resistance.
2. Is not important to
the function of the
neuron in the body.
3. Is unaffected by
Myelination.
4. Is slower in
narrower axons.
- Is slower in
narrower axons.
Which of the following statements
is correct?
1. The undershoot of the action potential
is generated by the electrogenic Na-K
pump.
2. During the overshoot of the action
potential there is a slight excess of
positive charge on the inside of the
membrane.
3. At the peak of the action potential the
driving force on Na+ ions is 0.
4. If the membrane was voltage clamped
at the threshold voltage for an action
potential all of the voltage-gated Na+
channels would open.
5. During the relative refractory period
all voltage-gated Na+ channels are
inactivated.
- During the overshoot of the action
potential there is a slight excess of
positive charge on the inside of the
membrane
