7 - Hodgkin Huxley AP Flashcards
Even when the AP is not propagating, the membrane ____ and ____ are changing with both time and space
Even when the AP is not propagating, the membrane voltage and current are changing with both time and space
Importance of using two axial intracellular electrodes and ground shield during giant squid axon experiments?
Made the whole intracellular compartment of the axon isopotential
Complications of ri and wavelength were eliminated
Internal wire has negligible resistance and current is passed evenly throughout the length of the axon
How did H&H command the membrane potential across the axon
Step in voltage
- amplifiers injected sufficient current to charge up the membrane almost instantly such that the phenomenon associated with slow charging of the membrane capacitance was minimized
Isolation of early inward and delayed outward currents:
- Early inward current known to be carried by ____
- How was this eliminated?
- Net early inward current was calculated by subtracting ____ with ______
Isolation of early inward and delayed outward currents:
- Early inward current known to be carried by Na+
- eliminated when ionic gradient for Na+ was adjusted to have a reversal potential that was identical to depolarization step (-9mV)
- Na+ conductance was activated but no net na+ current will flow
- late outward current can be observed in isolation (bottom trace in image)
- eliminated when ionic gradient for Na+ was adjusted to have a reversal potential that was identical to depolarization step (-9mV)
- Net early inward current was calculated by subtracting the trace with both currents with the trace with only the late outward current
- Na+ current eliminated
In the image:
- How was (C) obtained?
- How was (D) recorded?
- (C) was obtained by subtraction (D from A)
- (D) was recorded when ENa was adjusted to the the same as the depolarization step
- (B) is the _______ current and very small steady ______
- (C) is the _______ ; isolated by subtracting the total current with _____ (in D)
- (D) the _______ current
- recorded by adjusting ______ such that ENa was -9mv
- (B) is the capacitative current and very small steady leak current
- (C) is the net inward I<u>Na</u> ; isolated by subtracting the total current with I<u>k</u> (in D)
- (D) the net outward I<u>k</u> current
- recorded by adjusting the extracellular [Na+] such that ENa was -9mv (potential of the command step)
- No net Na+ current even if Na+ conductance was activated
- recorded by adjusting the extracellular [Na+] such that ENa was -9mv (potential of the command step)
why does Tetraethylammonium (TEA) only work well for squid axon when applied intracellularly
Squid K+ channel does not have an extracellular TEA binding site
What is the threshold in H&H’s model?
threshold is a membrane potential that the activated INa and Ik exactly reach an unstable balance
Value determined by both the resting membrane potential as well as the amplitude and shape of the stimulus
What is the absolute refractory period in H&H model?
Duration after an action potential while the combined effect of INa inactivation and/or IK activation is sufficient to prevent the generation of another AP by an external stimulus
What is the relative refractory period in H&H model?
Duration after an ap while the combined effects of INa inactivation and Ik activation are sufficient to raise the threshold for AP activation
H and H postulated that the activation of gNa or gK was controlled by the _______ of a certain number of independent “m” or “n” “gating particles”
H and H postulated that the activation of gNa or gK was controlled by the voltage-dependent translocation of a certain number of independent “m” or “n” “gating particles”
If the gating particles were charged (most likely way they sense the membrane potential) their translocation might be detected as a _______
If the gating particles were charged (most likely way they sense the membrane potential) their translocation might be detected as a gating current
How was a “gating current” detected:
- Gating current expected to occur only when channels were _________ (not with _____)
- Other than direction, positive and negative voltage commands were _______
- The summation of the current traces from (a) and (b) was expected to cancel all “_______” and “____” currents - thus isolating the _______
How was a “gating current” detected:
- Gating current expected to occur only when channels were activated to open with depolarization (figA) (not with hyperpolarization (fig B))
- Other than direction, positive and negative voltage commands were identical
- The summation of the current traces from (a) and (b) was expected to cancel all “capacitative” and “leak” currents - thus isolating the gating current
To prevent the voltage-activated ionic currents from masking the gating current, the ionic currents were suppressed by one of two ways:
- Replacing Na+ and K+ with impermeant cations (internally and externally)
- Use of TEA and TTX
- Caution: Some drugs may interfere with gating
Do individual na+ or K+ channels open and close with
a) the same “smooth” kinetics of the macroscopic currents
or
b) stochastically (like the ligand-gated channels) but with a mean population kinetics?
B) Individual Na+ channels open stochastically
