Action Potential

Question 1

Action potential

Answer 1

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

Question 2

DEPOLARIZATION PHASE

Answer 2

voltage-gated Na+ channels fast to open up to bring membrane potential way above threshold towards ENa (+60mV)

Question 3

Overshoot

Answer 3

d/t voltage-gated Na+ channels slow to close

Question 4

REPOLARIZATION PHASE

Answer 4

voltage-gated K+ channels slow to open, gradually bringing membrane potential towards EK (-90mV)

Question 5

Undershoot

Answer 5

d/t K+ channels slow to close

Question 6

Return to RMP after undershoot is due to:

Answer 6

K+ leak channels

Question 7

Properties of an action potential: Regenerative

Answer 7

once started goes through entire cycle

Question 8

Properties of an action potential: All-or-none response

Answer 8

fixed amplitude, independent of stimulus strength if above threshold

Question 9

Properties of an action potential: Time limited

Answer 9

same duration & does not vary with length of stimulus

Question 10

Is there an effect on [ion] with the movement of Na+/K+ charges?

Answer 10

NO. Na+ in and K+ out not
enough to change intracellular concentrations because an AP is driven by potential difference of the membrane

Question 11

Effect of the Na+/K+ ATPase pump on generating an action potential

Answer 11

NONE. Na+/K+ ATPase only pumps 3 molecules of Na+ out and 2K+ in, which contributes very little of a potential difference

Question 12

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.

Answer 12

3. Results in no change of
   ENa.
   - this is a constant

Question 13

Tetradotoxin (TTX)

Answer 13

voltage-gated Na+ channel inhibitor

Question 14

Tetraethyl ammonium (TEA)

Answer 14

voltage-gated K+ channel inhibitor

Question 15

Relative Refractory Period

Answer 15

A second action potential can fire during the undershoot of the first action potential but the threshold for firing is higher

Question 16

Absolute Refractory Period

Answer 16

A second action potential can
NOT fire during the depolarization of the first action potential

Question 17

Advantages of a refractory period:

Answer 17

• 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

Question 18

Encoding stimulus strength of an action potential: Frequency

Answer 18

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)

Question 19

Encoding stimulus strength of an action potential: Timing

Answer 19

weak stimulus–> brief AP

Question 20

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

Answer 20

2. Depends on the
   strength of the stimulus
   (i.e. the amount of
   membrane
   depolarization).

Question 21

Conduction Velocity

Answer 21

how quickly the AP propagates down the axon

Question 22

Determinants of conduction velocity: Length constant

Answer 22

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

Question 23

Myelination

Answer 23

• properties of the Schwann cells INCREASE MEMBRANE resistance
• insulation (no conductance through the membrane)
• INCREASES conduction velocity

Question 24

Node of Ranvier

Answer 24

site of an action potential

Question 25

Saltatory conduction

Answer 25

propagation of action potentials along myelinated axons

Question 26

Multiple Sclerosis

Answer 26

auto-immune disease that causes breakdown myelin –> slower conduction velocity
- no insulation leads to dissipation of current and signals are slower and weaker

Question 27

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.

Answer 27

4. Is slower in
   narrower axons.

Question 28

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.

Answer 28

2. During the overshoot of the action
   potential there is a slight excess of
   positive charge on the inside of the
   membrane