Action potential and conduction Flashcards
What is an action potential?
An all-or-nothing event resulting from an increase in ionic conductance of the membrane. Requires an influx of Na+ that depolarizes the cell above the threshold.
What happens in the current clamp experiment?
The experimenter injects current across the membrane and different times. The more current injected, the more spikes meaning more action potentials. We also see a shorter lag time when more current is injected.
Why do all voltage-gated Na+ channels inactivate while only some voltage-gated K+ channels inactivate?
Na+ channel inactivation is necessary for repolarization (downstroke). When Na+ channels inactivate, K+ channels are opened (not a result of), K+ efflux is driving the repolarization allowing for the non inactivated K+ to deactivate.
What roles do voltage-gated K+ channels play in action potential conduction along unmyelinated axons?
K+ is responsible for repolarization, which allows the Na+ VGC to recover from inactivation. K+ is also still open at the segment behind the site of the action potential, where there is a shunt that leaks of K+. This prevents retrograde axial current from the action potential. No reverse spike.
How does an inactive gate open?
They must recover during repolarization. It can’t reopen while the membrane is repolarizing. The K+ leaving the cell allows for repolarization to occur, therefore closing the channel again.
Describe the structure of cation-gated channels.
Cation-gated channels have alpha subunits with six membrane segments (S1 - S6). P-loop connects S5 and S6 and dips in and out of the extracellular side to form the selectivity filter.
S4 segment has a net positive charge and is responsible for voltage gating, acting as a voltage sensor. During depolarization, more + charges cause the S4 to change the confirmation of the channel and open. To close repolarization is when the cell becomes - and causes the arginine residues to close. Driven by an electrostatic environment.
How do you inhibit K+, Na+ and Ca++ channels?
Tea/4 - AP, TTX, and Cadmium.
How would you interpret IV plots for Na+ and K+ voltage-gated channels?
K+ IV plot will be more positive and shoot upward because it is a positive current with a positive charge leaving the cell.
Na+ plot will shoot downward because it is a negative current with positive charges flowing out of the cell.
Ik=(Vcom - Ek)*gk
What happens when negative charges enter the cell?
This displaces positive charges outside of the cell, causing a negative capacitance current (downward).
What happens when positive charges enter the cell?
This displaces a negative charge outside of the cell, creating a positive capacitance current (upward).
What is the difference between voltage clamp and current clamp?
Voltage clamp you are fixing the voltage command to look at how Vm affect current. Looking at current flow.
Current clamp you or fixing the current command to see how the Vm is affected
What is the reference direction?
The positive charge leaving the cell is the positive current and the positive charge entering the cell is the negative current.
Why doesn’t the AP travel back up the axon?
As Na+ channels begin to close and K+ channels begin to open where the stimulus region is, the Na+ in the adjacent region will begin to open. It can’t move back because Na+ is inactivated during absolute refractory stage in the region behind.
What is myelination?
The process involves glial cells that wrap myelin, and an insulator sheath, around axons. Does not conduct AP, instead, the AP travels between the gaps called nodes of Ranvier. Lots of na+ here.
