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Neuronal Action Potential -
Exercises
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Questions requiring short answers
  1. Name a toxin that inhibits voltage-gated Na+ channels of neurons.
    Tetrodotoxin (TTX)
  2. Name a reagent that inhibits voltage-gated K+ channels of neurons.
    Tetraethylammonium ion (TEA)
  3. This proposed model accounts for the molecular mechanism governing the inactivation process of voltage-gated Na+ channels.
    Ball and chain model of Na+ channel inactivation
  4. During this phase of the action potential, if a second stimulus is applied to the neuron (no matter how strong the stimulus), a second action potential will not be generated.
    Absolute refractory period
  5. During this phase of the action potential, only a stronger than normal stimulus will lead to the generation of a new potential.
    Relative refractory period
  6. Name at least two agents that inhibit voltage-gated Na+ channels of neurons.
    (1) Tetrodotoxin (TTX)
    (2) Lidocaine
  7. In a typical neuron, what are the values for the duration of the absolute refractory period and relative refractory period?
    (1) Absolute refractory period: 1-3 ms
    (2) Relative refractory period: 3-5 ms
  8. This positive feedback cycle is responsible for the spike phase of the action potential.
    Hodgkin cycle
Multiple-choice questions
  1. A typical neuron has a resting membrane potential of about
    (A) +70 mV
    (B) +70 V
    (C) −70 mV  ✔  
    (D) −70 V
    (E) All of the above are observed at rest.
  2. Which of the following ions are involved in neuronal action potentials?
    (A) Na+
    (B) K+
    (C) Cl
    (D) A and B only  ✔  
    (E) A, B, and C
  3. At the peak of the action potential, the membrane potential is:
    (A) exactly at the Na+ equilibrium potential
    (B) close to but more positive than the Na+ equilibrium potential
    (C) close to but less positive than the Na+ equilibrium potential  ✔  
    (D) exactly at 0 mV
    (E) the same as the resting membrane potential
  4. In the nervous system, the strength of the stimulus is coded into:
    (A) the frequency of action potentials generated  ✔  
    (B) the amplitude of action potentials generated
    (C) both the frequency and amplitude of action potentials generated
  5. At what membrane voltage do neuronal voltage-gated Na+ channels become activated?
    (A) −70 mV
    (B) −50 mV  ✔  
    (C) 0 mV
    (D) +50 mV
    (E) None of the above
  6. At what membrane voltage do neuronal voltage-gated K+ channels become activated?
    (A) −90 mV
    (B) −70 mV
    (C) −50 mV  ✔  
    (D) 0 mV
    (E) +50 mV
  7. The spike phase of the action potential is due to:
    (A) the opening of voltage-gated Na+ channels  ✔  
    (B) the opening of voltage-gated K+ channels
    (C) the closure of resting K+ channels
    (D) the opening of voltage-gated Cl channels
    (E) None of the above
  8. The hyperpolarization phase of the action potential:
    (A) Is due to the opening of voltage-gated Cl channels
    (B) Is due to the prolonged opening of voltage-gated K+ channels  ✔  
    (C) Is due to the closure of resting Na+ channels
    (D) Is due to the closure of Cl channels
    (E) None of the above
  9. Which of the following is NOT true about the refractory period?
    (A) It is thought that the refractory period is caused by the hyperpolarization phase of the action potential.
    (B) The refractory period is important in preventing the overlap of succeeding action potentials.
    (C) The absolute refractory period refers to that time during which a stronger stimulus will lead to the generation of a new action potential.  ✔  
    (D) The relative refractory period refers to that time during which a stronger stimulus will lead to the generation of a new action potential.
    (E) The relative refractory period coincides with the hyperpolarization phase of the action potential.
  10. Which of the following is NOT true of the absolute refractory period?
    (A) Na+ channel inactivation is responsible for the absolute refractory period.
    (B) The absolute refractory period is the time during which another stimulus given to the cell (no matter how strong) will not lead to a second action potential.
    (C) In typical neurons, the absolute refractory period takes about 5 ms.  ✔  
    (D) The molecular basis of the absolute refractory period is described by the ball-and-chain model.
    (E) All of the above are true about the absolute refractory period.
  11. Na+ and K+ permeation through their respective ion channels represents an example of:
    (A) Passive transport  ✔  
    (B) Primary active transport
    (C) Secondary active transport
    (D) A and C only
    (E) A, B, and C
  12. Which of the following is NOT consistent with the function of neuronal voltage-gated Na+ channels?
    (A) After becoming activated (opening) at the threshold voltage, these channels very rapidly enter an inactive state.
    (B) Na+ channels may go from the inactive state to the open state.  ✔  
    (C) Na+ channels may go from the inactive state to the closed state.
    (D) Na+ channels may go from the open state to the inactive state.
    (E) Na+ channels cannot go from the open state to the closed state.
  13. Which of the following is NOT true about the neuronal action potential?
    (A) Action potentials are all-or-nothing.
    (B) Action potentials travel along axons in a non-decremental fashion.
    (C) The spike phase of the action potential is due to the opening of voltage-gated Na+ channels.
    (D) Repolarization and hyperpolarization are due to the activity of K+ channels.
    (E) All of the above are true about action potentials.  ✔  
  14. The threshold potential refers to the voltage at which:
    (A) The axon blows up
    (B) The membrane breaks down
    (C) Voltage-gated Na+ and K+ channels open  ✔  
True / False questions
  1. Neuronal voltage-gated Na+ channels inactivate.
    True
  2. Neuronal voltage-gated K+ channels inactivate.
    False
  3. At the peak of the action potential, plasma membrane permeability to K+ (pK) is higher than the permeability to Na+ (pNa).
    False
  4. Graded potentials are all-or-nothing.
    False
  5. Threshold voltage is approximately the same for voltage-gated Na+ and K+ channels.
    True
  6. Influx of Na+ and K+ through their respective ion channels represents an electrogenic process.
    True
  7. The Hodgkin cycle represents an example of a positive feedback loop.
    True
  8. During a typical action potential, the intracellular and extracellular concentrations of K+ and Na+ change significantly.
    False
Essay questions
  1. Explain how pharmacological inhibition of Na+ and/or K+ channels can help physiologists gain a better understanding of the role of these ion channels in the action potential.
    Please see the section on Pharmacological Inhibition of Na+ and K+ Channels.
  2. Describe the inactivation mechanism for neuronal voltage-gated Na+ channels.
    Please study the events involved in Na+ channel inactivation in the section on the Important Features of the Neuronal Action Potential.
  3. Describe how excitable and non-excitable cells respond differently to hyperpolarizing and depolarizing electrical stimulation.
    Please see the introductory section of this lecture (Introduction).
  4. List and explain a few differences between graded potentials and action potentials.
    Please study the section on Graded Potentials versus Action Potentials.
  5. Explain the molecular basis of the neuronal refractory period. Be sure to discuss both the absolute refractory period and the relative refractory period.
    Please study the section on Refractory Periods.
  6. Explain in detail the molecular and ionic basis of action potentials in neurons.
    Please study the entire lecture on the Neuronal Action Potential.
Calculation problems
  1. At the peak of the action potential, Vm is approximately +50 mV. Assuming normal intracellular and extracellular K+ concentrations, (1) calculate the driving force (in mV) that acts on K+ ions, and (2) use the information obtained in part 1 to determine the direction in which K+ ions will flow (i.e., into the cell or out of cell).
    1. First use the Nernst equation to determine the equilibrium potential for K+ (VK). Then use the electrochemical driving force calculator to calculate the driving force (in mV) that acts on K+ ions.
    2. Use the sign (i.e., positive or negative) of the driving force calculated above, and the information given in the lecture on the resting membrane potential, to determine whether K+ ions will move into or out of the cell under these conditions.
  2. Assume the following concentrations of Na+ and K+ ions: [Na+]o = 120 mM, [Na+]i = 6 mM, [K+]o = 2 mM, and [K+]i = 150 mM. Assume further that at the peak of the action potential, pK : pNa is 1 : 12. Calculate Vm at the peak of the action potential.
    Use the Goldman-Hodgkin-Katz equation to determine the Vm under the conditions given.
  3. In a typical vertebrate axon, the absolute refractory period is 1.0 ms and the relative refractory period is 4.0 ms. Thus, the axon is refractory for a total of 5.0 ms. If the axon is continuously stimulated with stimuli only large enough in amplitude to ensure excitation when the neuron is at rest, what is the highest frequency of action potentials that can be generated?
    A similar calculation has been done in the section on Frequency Coding in the Nervous System.
  4. In a typical vertebrate axon, the absolute refractory period is 1.0 ms and the relative refractory period is 4.0 ms. If the axon is continuously stimulated with stimuli large enough in amplitude to ensure excitation, what is the highest frequency of action potentials that can be generated?
    A similar calculation has been done in the section on Frequency Coding in the Nervous System.
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Posted: Thursday, July 5, 2012
Last updated: Friday, January 17, 2014