elecrtophysiology 3

Question 1

Provide a conceptual description of the voltage clamp technique and its rationale

Answer 1

We start with having a stimulating electrode (injects current, also known as a current-passing electrode) and recording electrodes (which record Vm).
The recoding electrode is connected to the voltage clamp amplifier, which will compare the membrane potential to the desired membrane potential.

(clamp Vm to a value normally above threshold)
So when the Vm deviates from the desired potential, the voltage clamp amplifier injects current in equal and opposite directions, which is the feedback arrangement that allows the membrane potential to be the same as the desired potential.

rationale
The clamp method allows for the Vm to be stable at a certain current flow via clamping so it doesn’t deviate from this desired Vm.
This provides information about the flow of ions across the neuronal membrane.

Question 2

describe why net current flow is 0 despite ionic movement at resting membrane potential

Answer 2

Although the Na conductance (not conductivity!) is low, the driving force is high (Vm far from ENa), and while the K conductance is high, the driving force is low (Vm near EK). The result is that net inward and outward ion flows are equal (since I ion = g ion x driving force on ion) and there is no net current across the membrane

Question 3

describe the Na vs K conductance at resting membrane, depolarisation, hyperpolarisation, and back to RMP

Answer 3

t1: gK > gNa
t2: gNa&raquo_space; gK
t3: gK&raquo_space; gNa
t4: gK > gNa

Question 4

[Provide a qualitative explanation of the theoretical model used by Hodgkin and Huxley to reconstruct the action potential and describe their hypothesis to do with AP membrane permeability

Answer 4

Membrane permeability will change for both sodium and potassium during the time course of an AP, such that the sodium permeability increases rapidly but transiently when generating an AP, followed by a slight delay in K permeability. These permeability changes vary with membrane potential. These changes are reversible.

At first, the incorporation of conductance and eq potential for any given value for an ion provides static information for that time point and this doesn’t allow for changes in Vm to be seen. Therefore, they added a time dimension to this, which enables the acknowledgement of the membrane capacitor. With this, we know that when current flows through, it does not immediately lead to a change in Vm; instead, a portion of that current goes to charge the membrane capacitor (which stores and separates charge), which causes a slight delay. After this charging, current flows through the membrane, allowing Vm to change during the course of AP.

Question 5

what is a fixed property of the membrane?

Answer 5

membrane capacitor

Question 6

rationale for membrane clamp technique based on the new Hodgkin and Huxley hypothesis

Answer 6

that as everything changes at the same time, we cannot determine what Vm is; therefore, the rationale of the clamp technique is that we can fix certain values so that we can input them into the equation and thus work out Vm.

Reasoned that if Vm could be held (‘clamped’) at any desired test voltage, then I ion could be measured, and hence, g ion determined for
that voltage

Question 7

whats early and late current

Answer 7

Early or negative current is ions moving into the cell

late or positive current is ions moving out of the cell

Question 8

describe the inward and outward current based on eq potential and conductance of sodium from -26,0,26,50,65 Mv

Answer 8

At -26 mV, the inward current is quite small as the membrane potential is closer to RMP than it is to reversal potential and the conductance of sodium is not at its maximum.

at 0 mV, has a slightly smaller driving force between here at -26 mV, , the conductance is very high, such that there is more inward current.

at 26 mV, it has an even smaller driving force, so outward current becomes slightly greater than inward current as it starts to get closer to the sodium reversal potential

at 50 mV, the driving force is very small and conductance is at its maximum so there is little to no inward current

at 65 its reached its reversal potential, such that there’s no more inward current and no more conductance as its at its maximum, so greater outward current.

Question 9

what does inward current depend on that also determines Vm

Answer 9

the conductance of that ion as well as its driving force (comparing its reversal potential to the Vm)

Question 10

what pharmacological compound do you add to block potassium? current to view sodium movement

Answer 10

tetraethyl ammonium

Question 11

what pharmacological compound do you add to block sodium? current to view potassium movement

Answer 11

tetrodotoxin