Questions: Voltage Clamp: Measuring Ionic Currents in Isolation

Question 1

A voltage clamp holds membrane potential at −20 mV. The feedback amplifier injects +2 nA to maintain this voltage. What can you conclude about the ionic currents flowing through the membrane?

• A. The ionic currents sum to +2 nA inward
• B. The ionic currents sum to −2 nA (2 nA net outward), because the amplifier injects equal and opposite current to maintain the clamp
• C. There is exactly 2 nA of sodium current, because sodium drives depolarization toward −20 mV
• D. There are no ionic currents — the voltage clamp prevents channel opening

Answer: (B)

The voltage clamp logic: if channels carry net outward ionic current (e.g., K⁺ leaving), the membrane potential would rise, so the amplifier injects inward current to counteract it. If +2 nA outward is being injected, the ionic current must be −2 nA (2 nA inward) — the amplifier is compensating for inward ionic current (e.g., Na⁺ entering). The measured amplifier current is equal in magnitude but opposite in sign to the ionic current. This is the key: measuring the injected current is equivalent to measuring the ionic current it is exactly canceling.

Question 2

Hodgkin and Huxley applied tetrodotoxin (which blocks Na⁺ channels) to a voltage-clamped axon and observed that the fast inward current disappeared while a slow outward current remained. What does this demonstrate?

• A. Tetrodotoxin causes potassium channels to open more slowly
• B. The voltage clamp alone cannot isolate individual ionic currents because sodium and potassium currents overlap in time, requiring pharmacological blockers for separation
• C. Tetrodotoxin blocks the feedback amplifier, distorting current measurements
• D. The remaining outward current must be sodium current because the initial inward current was blocked

Answer: (B)

Under a depolarizing step, Na⁺ (fast inward) and K⁺ (slower outward) currents flow simultaneously, and the voltage clamp measures their sum. Adding tetrodotoxin selectively eliminates the sodium current, isolating the potassium current. Adding tetraethylammonium (K⁺ channel blocker) instead isolates the sodium current. This combination — voltage clamp plus pharmacological dissection — allowed Hodgkin and Huxley to characterize each current's voltage dependence and kinetics independently and write their landmark conductance equations.

Question 3

The voltage clamp can reveal ionic currents that are invisible during a normal action potential because the membrane potential changes too rapidly during an action potential to isolate individual channel contributions.

• T. True
• F. False

Answer: True

During a natural action potential, the entire event lasts ~1 ms, with sodium and potassium currents overlapping in time as the voltage sweeps from rest to peak and back. There is no way to ask 'how much Na⁺ current flows at exactly −20 mV?' because the membrane passes through −20 mV in a fraction of a millisecond. The voltage clamp freezes the membrane at any desired potential indefinitely, allowing the experimenter to observe how currents evolve over time at a fixed voltage — revealing kinetics, inactivation, and voltage dependence that are hopelessly convolved during an unclamped action potential.

Question 4

In a voltage clamp experiment, the current measured by the feedback amplifier is equal to the ionic current in both magnitude and sign.

• T. True
• F. False

Answer: False

The amplifier current and ionic current are equal in magnitude but opposite in sign. If net ionic current is inward (Na⁺ entering, negative by convention), the membrane would depolarize, so the amplifier injects outward current (+) to counteract it. The measured amplifier current is therefore the mirror image of the ionic current. Confusing the signs leads to misinterpreting inward versus outward currents — mistaking a depolarizing Na⁺ inflow for a hyperpolarizing current, for example.

Question 5

Explain the core logic of how the voltage clamp allows direct measurement of ionic currents that would otherwise be impossible to isolate during an action potential.

Model answer: During a normal action potential, Na⁺ influx depolarizes the membrane, which opens more Na⁺ channels in a positive feedback loop — the voltage changes so fast that all currents overlap. The voltage clamp breaks this by using a feedback amplifier: it continuously compares the actual membrane potential to a command voltage and injects whatever current is needed to eliminate any difference. When Na⁺ channels open and begin depolarizing the membrane, the amplifier instantly injects equal and opposite outward current, keeping the voltage constant. The injected current must exactly equal the ionic current it is canceling, so by measuring the amplifier output, the experimenter directly measures the ionic current at a fixed, controlled voltage. This converts the problem from 'measure rapidly changing currents at a moving voltage' to 'measure steady current at a fixed voltage,' enabling precise characterization of channel conductance as a function of voltage and time.

The key insight: maintaining constant voltage means the amplifier's injected current is a real-time mirror of the ionic current — turning a coupled, explosive process into a controlled, measurable one.