Action Potentials: Module 1.7 - 1.8 Flashcards
Who won the Nobel Prize in Medicine and physiology in 1963?
Hodgkin and Huxley
For their work in ionic mechanisms involved in action potentials.
When was the first published intracellular recording of an Action Potential?
1939 By Hodgkin and Huxley
Used Giant squid Axon and electrodes
The giant squid axon contracts the ____ causing the expulsion of ____ for propulsion(siphon) against danger.
The giant squid axon contracts the mantel causing the expulsion of water for porpulsion(siphon) against danger.
1mm diameter in Squid axon
What are 4 distinct properties of an action potential?
- Threshold
- All or None Event
- AP conducted without decrement. Has a self-regenerative feature that keeps the amplitude constant, even when it is condected over great distances.
- Refractory Period
What are the different parts of an action potential?
- Rising Phase - depolarizing phase; rapid increase in Vm from resting potential to maximum value of 10mV - 40mV
- Overshoot - part of the AP above 0mV
- Repolarizing Phase - Negative going from Peak to resting potential.
- Afterhyperpolarization: repolarization undershoots to a voltage more negative than resting.
What do the threshold, amplitude, time course, and duration of an AP depend on?
- The gating and permeability properties of ion channels
- the intracellular and extracellular concentrations of the ions
- Membrane Properties such as Geometry of the Cell
What ion is the depol/rising phase largely due to?
Due to the Na Conductance
What ion is the peak of the AP largely due to?
Na
What ion is the repolarization phase of the AP largely due to?
Due to the decrease of Na conductance and an increase in K conductance.
What ion is the afterhyperpolarization phase of the AP largely due to?
Due to the conductance of K
Total Absence of Na
Does the membrane potential of a AP ever reach E(Na)?
No
Due to inactivation of Na Channels and activation of K Conductance
Explain the how the permeability of Na and K change during the AP.
At higher membrane potentials closer to the peak of AP the cells **permeability to Na is higher than K **due to activation of voltage gated Na Channels.
At lower membrane potentials closer to resting potentials the cells permeability to K is greater than Na due to inactivation of Na channels and more K leak channels.
The greater the ____ for a given ion, the greater its contribution to the ____.
The greater the permeability for a given ion, the greater its contribution to the membrane potential.
What are the 2 types of refractory periods?
Absolute: Impossible to fire another AP.
Relative: the minimal stimulus necessary for activation is stonger or longer than predicted by the first AP
Why is there an absolute refractory period and until when does it last?
Absolute Refractory period exists because Na channels remain inactivated.
Why is there a relative refractory period and until when does it last?
Due to increased K+ conductance leading to the AHP (more negative Vm closer to Ek). Need more depolarization to reach threshold. Also Na channels are starting to come out of Inactivation.
What potential do Voltage-gated sodium channels activate at?
Approx -55mV
Approx What percentage of sodium channels needs to recover from inactivation to ilicit another AP.
20%
Refractory period is associated with what?
Recovery of Voltage gated Na Channels from inactivation.
What is needed for the recovery of Na channels from inactivation?
Time and Negative Potential
What are 2 consequences of the Refractory period?
- Minimum inter-spike interval
- Direction of Propogation
Why is there a peak in the AP?
Results in the rate difference between Na channel activation and K. Both activate at the same time however Na cycle is faster.
If both were same rate - no peak and both currents would cancel.
What direction is Na and K currents?
Na+ = Inward = -
K+ = Outward = +
What molecule blocks K+ current only revealing Na current?
TEA - Tetraethlyammonium
What molecules black Na+ current only revealing K+ current?
TTX - Tetrodotoxin - Pufferfish
STX - Saxitoxin - Dinoflagellates
What does it mean that Na current is Biphasic?
Has activation phase before the peak and inactivation phase after the peak.
Na+ inactivation can only be seen over ____.
Na+ inactivation can only be seen over time.
Given that Na channels and current are biphasic
Which phase is fast acting and which is slow?
Activation - Fast
Inactivation - slow
What are the three possible states of a Na ion channel?
- Resting
- Activated
- Inactivated
Which ion has greater dependance on depolarization Na or K for current release?
Na
What is the negative resistance region?
Region where a more depolarized Vm leads to increase in negative inward current instead of positive outwards.
Do you need more Depolarization to achieve an Open probability of 50% (P0 = 0.5) in Na or K?
K
What are difference in graphs between voltage gated channels and non voltage gate graphs?
What is the difference between an ohmic channel and a rectifying channel?
Ohmic - Current varies linearly with driving force - channels behave as simple resistors
Rectifying - Current is a non linear function of driving force. Conducts more ions in one direction than the other.
What are 3 things you can see from current-voltage relationships?
- Activation Voltages - Current begins at these voltages
- Direction of Ion flow
- Reversal Potential
