At rest, neurons are negatively charged with respect to the extracellular fluid. The magnitude of this electrical difference is referred to as Vm, the cell's membrane potential. An action potential is a positive change in membrane voltage initiated, carried, and terminated by different electrical currents across the membrane. In the 1940s and 1950s, Andrew Huxley and Alan Hodgkin used an apparatus called a voltage clamp to study the changes in electrical currents throughout the course of an action potential. As an action potential progresses down an axon, the membrane depolarizes and then repolarizes. Vm moves from –60 mV to more than 0 mV and then returns to –60 mV within milliseconds, making it difficult to observe the current's pattern. An experimenter can set the voltage clamp to hold the axon at a certain voltage, called the command potential, freezing the action potential at a moment in time. A voltmeter measures Vm and that value is compared to the command potential. If the values match, the machine does nothing. If they do not, the apparatus will supply a current to the axon sufficient to change the measured Vm to the command potential.Whatever current the axon's membrane naturally allows in or out will be countered with equal and opposite current from the apparatus. If the experimenters know what current they have applied, they know what current the membrane has passed. By performing repeated experiments, they measure the membrane currents that control the action potential.Figure 1 shows Hodgkin and Huxley's voltage clamp. A voltmeter measures Vm; one of its electrodes is placed within a section of axon, and the other is grounded in the extracellular fluid. The measured value is compared to the command potential and the voltage clamp amplifier supplies the current necessary to equalize the two values. The amount of current delivered is calculated by measuring the voltage drop over a series of two resistors.Figure 1 Voltage clamp apparatus, connected to axonQuestion 15Suppose an axon is at its resting potential and the command potential is set to 0 mV. What would happen if the ground at point g was removed and the electrode originally at h was placed intracellularly, near point a?A.The measurement of Vm would be unaltered.B.Current would flow continuously from the axon to the clamp amplifier.C.Minimal current would flow between the clamp amplifier and the axon.D.Voltage-gated channels on the axon membrane would be activated.
Question
At rest, neurons are negatively charged with respect to the extracellular fluid. The magnitude of this electrical difference is referred to as Vm, the cell's membrane potential. An action potential is a positive change in membrane voltage initiated, carried, and terminated by different electrical currents across the membrane. In the 1940s and 1950s, Andrew Huxley and Alan Hodgkin used an apparatus called a voltage clamp to study the changes in electrical currents throughout the course of an action potential. As an action potential progresses down an axon, the membrane depolarizes and then repolarizes. Vm moves from –60 mV to more than 0 mV and then returns to –60 mV within milliseconds, making it difficult to observe the current's pattern. An experimenter can set the voltage clamp to hold the axon at a certain voltage, called the command potential, freezing the action potential at a moment in time. A voltmeter measures Vm and that value is compared to the command potential. If the values match, the machine does nothing. If they do not, the apparatus will supply a current to the axon sufficient to change the measured Vm to the command potential.Whatever current the axon's membrane naturally allows in or out will be countered with equal and opposite current from the apparatus. If the experimenters know what current they have applied, they know what current the membrane has passed. By performing repeated experiments, they measure the membrane currents that control the action potential.Figure 1 shows Hodgkin and Huxley's voltage clamp. A voltmeter measures Vm; one of its electrodes is placed within a section of axon, and the other is grounded in the extracellular fluid. The measured value is compared to the command potential and the voltage clamp amplifier supplies the current necessary to equalize the two values. The amount of current delivered is calculated by measuring the voltage drop over a series of two resistors.Figure 1 Voltage clamp apparatus, connected to axonQuestion 15Suppose an axon is at its resting potential and the command potential is set to 0 mV. What would happen if the ground at point g was removed and the electrode originally at h was placed intracellularly, near point a?A.The measurement of Vm would be unaltered.B.Current would flow continuously from the axon to the clamp amplifier.C.Minimal current would flow between the clamp amplifier and the axon.D.Voltage-gated channels on the axon membrane would be activated.
Solution
The answer is B. Current would flow continuously from the axon to the clamp amplifier.
Here's why:
The ground at point g is important because it provides a reference point for the voltage measurements. If it is removed, the system loses this reference point.
The electrode originally at h is used to measure the voltage difference across the membrane. If it is moved intracellularly near point a, it would no longer be measuring the voltage difference across the membrane. Instead, it would be measuring the voltage difference within the cell.
As a result, the voltage clamp would detect a difference between the command potential and the measured potential. It would then supply a current to try to equalize these two values.
However, because the electrode is now inside the cell and not measuring the membrane potential correctly, the voltage clamp would continuously supply current, trying to correct a voltage difference that it cannot correct. This would result in a continuous flow of current from the axon to the clamp amplifier.
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