Lecture 6: Voltage Clamp and Membrane Currents Flashcards
What can APs be generated by? (2)
- artificially depolarizing the cell through current injections
- summation of graded receptor potentials or PSPs that spread electrotonically into vicinity of axon hillock
What happens to AP if you increase the strength of a depolarizing stimulus?
does NOT change shape or size of APs, but does increase their frequency (APs/sec) up to a maximum firing rate (where frequency no longer increases with stronger current)
Why do maximum firing rates occur?
because action potentials have a refractory period
What is an absolute refractory period?
after AP has been triggered, there is a certain period of time in which a second AP cannot be generated, no matter how strong the stimulus
How can the duration of an absolute refractory period be measured?
can be easily measured from the time between AP peaks when cell is at its maximum firing rate
What is the relative refractory period?
period shortly after the firing of a nerve fiber when partial repolarization has occurred and a greater than normal stimulus can stimulate a second response
for ongoing stimuli that are suprathreshold but not as strong, frequency of APs being generated will be determined by how rapidly the ongoing depolarization can bring the membrane back to threshold voltage, despite repolarization and after-hyperpolarization phases
What is the likely reason why crossing VTh triggers an AP?
(probably) due to a very rapid loss of Rm (“membrane breakdown”)
↓ in Rm indicates ↑ in G
Why was it so hard to study action potentials with electrophysiology?
cannot tell difference between voltage-dependent conductance and voltage change based on conductance – how do you study a conductance that is controlled by changes in voltage, and also produces more changes in voltage as soon as you trigger it?
What type of novel technique was required to study APs?
technique that could keep an axon’s Vm from changing
What was the solution to keep an axon’s Vm from changing?
incorporate negative feedback – constantly monitoring Vm for any deviations and injecting an equal and opposite current to keep neuron at its set point
What occurs when command voltage is altered?
Vm changes after a brief delay (< 0.1ms) due to capacitance in cell membrane
capacitive current (difference current) briefly flows through electrodes while Vm is less negative than Vcom is telling it to be (because Vm has to deal with Cm) – but after that, nothing else happens in this trace (no other currents are flowing across the membrane at this voltage)
Why do membranes take some time to charge?
because they are lipid bilayers
What does the voltage clamp reveal with a strong depolarization which would normally trigger AP?
- when cell membrane was depolarized past VTh, active current(s) are triggered, in addition to the passive capacitative current
- capacitative current now flows the other way, because the Vm is moving in the opposite direction from VCom
- early on (< 1ms), there is a ‘negative’ current which appears and then goes away (transient inward current)
- later, a ‘positive’ current (delayed outward current) gradually appears – this current persists as long as depolarization is maintained
The ion replacement experiment confirms that the early current is carried by Na+. H&H correctly guessed the late current is carried by K+, but didn’t try to do an equivalent test where they altered [K+]o to confirm that guess. Why not?
K+ is more permeable at RMP so changing its concentrations might disrupt RMP and axon function before the voltage step even occurs
What happens when you depolarize the membrane potential of the axon beyond a threshold voltage?
trigger two overlapping, but distinguishable currents that don’t flow at more negative membrane potentials
- these are two distinct currents that have different time courses, and are sensitive to different perturbations
- early current carried by Na+ ions
- late current carried by K+ ions
