voltage vs current clamp bme202

What does “clamp” mean in electrophysiology?

It refers to the variable the experimenter controls—either voltage (voltage clamp) or current (current clamp).

What is controlled in voltage clamp mode?

The membrane potential (Vm) is held at a chosen command value by the amplifier.

How does the voltage clamp amplifier work?

It measures Vm and injects current rapidly to keep the membrane at the command voltage.

What happens if Vm gets too high during voltage clamp?

The amplifier decreases the injected current.

What happens if Vm gets too low during voltage clamp?

The amplifier increases the injected current.

What variable is recorded in voltage clamp mode?

The clamp current (the amount of current injected to maintain the command voltage).

What does a changing clamp current indicate?

That underlying membrane ionic currents are changing over time.

Why do voltage clamp experiments use sudden jumps between steady voltages?

To eliminate capacitive currents and isolate purely ionic currents during constant voltage periods.

What does Ohm’s law tell us during periods of constant voltage?

Ionic current through a channel is proportional to its conductance.

Why is conductance always positive while current can be positive or negative?

Current direction depends on ion flow, but conductance is a magnitude (ability to pass current).

In a spreadsheet model of voltage clamp, what determines V values?

V is set directly by the experimenter (typed in manually), not calculated by a differential equation.

What happens to Hodgkin-Huxley state variables in voltage clamp mode?

They respond to the imposed V values, updating toward voltage-dependent asymptotes with specific time constants.

What do the α and β functions determine?

They define the voltage-dependent asymptotes (“inf”) and time constants (τ) for gating variables.

How can you test a voltage clamp spreadsheet model?

Plot the asymptote and τ functions, then check if n, m, and h variables approach their asymptotes with correct time constants.

How are K⁺ and Na⁺ channel conductances computed?

Instantaneously as functions of the gating variables (e.g. gK = gKmax·n⁴, gNa = gNamax·m³h).

How are ionic currents computed in voltage clamp simulations?

Using Ohm’s law: Iion = gion(V − Eion).

In current clamp mode, what is controlled?

The injected current (I) is the command variable, and Vm becomes a dependent variable.

What governs Vm in current clamp mode?

A differential equation describing how Vm evolves with its own V∞ and τV.

How do you update V in a spreadsheet current clamp model?

Using a recursive update rule based on previous V and current I, instead of manually setting V.

What happens with no current injection in current clamp mode?

Vm settles to its equilibrium potential (around −70 mV).

Why is the resting potential not exactly −70 mV?

Because ionic conductances and battery values (Eion) combine to produce a slightly different equilibrium.

What happens when a positive current is injected?

V∞ immediately increases, and Vm exponentially approaches the new equilibrium.

What occurs as Vm nears −50 mV during current injection?

The activation variable m increases rapidly, boosting gNa and depolarizing the cell further.

Why does increased gNa accelerate depolarization?

It raises V∞ and lowers τV, pulling Vm quickly toward the peak of the action potential.

Overall, what does voltage clamp reveal?

The time course and magnitude of ionic currents at controlled voltages.

Overall, what does current clamp reveal?

How the membrane potential dynamically responds to injected currents—showing full action potential waveforms.