Membrane Permeability
Questions & Short Answer
1. What is the voltage clamp method?
Answer: The voltage clamp method allows characterization of permeability changes as a function of
membrane potential and time.
Textbook Reference: Ion Currents across Nerve Cell Membranes
2. Explain how Hodgkin and Huxley used the voltage clamp method to show that changes in
permeability to Na+ and K+ underlie the action potential.
Answer: Using the giant axon of a squid, they showed that depolarization is followed by transient
inward current and lasting outward current. This inward current does not flow if the membrane
potential is clamped at +52 mV, the equilibrium potential for Na+, indicating that it is attributable to Na+
permeability. They also removed Na+ from the extracellular medium and found that this reverses the
polarity of the early Na+ current but has no effect on the delayed lasting current. Using radioactive K+,
they confirmed the involvement of K+ in the delayed current.
Textbook Reference: Ion Currents across Nerve Cell Membranes
3. Which way does current flow across the membrane during: a) the rising phase, and b) the falling
phase of the action potential?
Answer:
a) Inward
b) Outward
Textbook Reference: Two Types of Voltage-Dependent Ion Currents
4. Suppose you are recording action potentials from a neuron. How will the action potential be
affected if you remove: a) Na+, or b) K + from the external medium?
Answer:
a) It will be suppressed; the rising phase will not occur.
b) It will be more difficult to trigger because the resting potential of the neuron will become even more
negative.
Textbook Reference: Two Types of Voltage-Dependent Ion Currents
Questions & Short Answer
1. What is the voltage clamp method?
Answer: The voltage clamp method allows characterization of permeability changes as a function of
membrane potential and time.
Textbook Reference: Ion Currents across Nerve Cell Membranes
2. Explain how Hodgkin and Huxley used the voltage clamp method to show that changes in
permeability to Na+ and K+ underlie the action potential.
Answer: Using the giant axon of a squid, they showed that depolarization is followed by transient
inward current and lasting outward current. This inward current does not flow if the membrane
potential is clamped at +52 mV, the equilibrium potential for Na+, indicating that it is attributable to Na+
permeability. They also removed Na+ from the extracellular medium and found that this reverses the
polarity of the early Na+ current but has no effect on the delayed lasting current. Using radioactive K+,
they confirmed the involvement of K+ in the delayed current.
Textbook Reference: Ion Currents across Nerve Cell Membranes
3. Which way does current flow across the membrane during: a) the rising phase, and b) the falling
phase of the action potential?
Answer:
a) Inward
b) Outward
Textbook Reference: Two Types of Voltage-Dependent Ion Currents
4. Suppose you are recording action potentials from a neuron. How will the action potential be
affected if you remove: a) Na+, or b) K + from the external medium?
Answer:
a) It will be suppressed; the rising phase will not occur.
b) It will be more difficult to trigger because the resting potential of the neuron will become even more
negative.
Textbook Reference: Two Types of Voltage-Dependent Ion Currents
