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Problem Set 2 - Action Potential, MEPP

Page history last edited by PBworks 14 years, 5 months ago

Physiology Facilitator Problem Set 2

Action potential, motor endplate potential, transport



The first eight questions ask information regarding the figure below. To keep things simple, assume that currents produced by only two ions, K+ and Na+, contribute to this squid action potential.



1. The straight dashed line at about –80 mV represents the equilibrium potential for what ion?.


2. The straight dashed at about + 65 mV represents the equilibrium potential for what ion?


3. What does curve T represent?


4. What does curve U represent?


5. What prevents the “peak” of the action potential from reaching ENa?


6. What causes EM to be more negative at point W than it was prior to the initiation of the action potential?


7. What is the reason that a normally suprathreshold stimulus if given at a time corresponding to the upward arrow on the X axis will not elicit another action potential?


8. In terms of the abbreviated Chord equation given below, describe the events that change EM during the entire action potential.


Chord equation: EM = ENa (gENa/gEtotal) + EK (gEK/gEtotal)


9. An action potential will propagate from its site of initiation in the cell body of a spinal cord motorneuron to the presynaptic terminal at the neuromuscular endplate (i.e. anterograde propagation), but upon depolarizing the presynaptic terminal, the action potential does not repropagate back to the cell body (i.e., retrograde propagation) Why is this?

  • A. Not enough time is available to restore the loss of intracellular Na+ that occurs during the anterograde action potential to allow for immediate retrograde propagation.
  • B. Nerve axons are incapable of propagating an action potential in a retrograde direction.
  • C. Distal regions of nerve axons are incapable of initiating action potentials.
  • D. Na+ channel inactivation prevents the depolarized nerve action from immediately propagating in a retrograde direction.
  • E. Distal regions of nerve axons lack K+ channels, and therefore can not repolarize to a potential sufficient to be re-excited again.



For the next several questions you may wish to refer to the figure below.



10. As a motoneuron-action potential reaches the end of the axon:

  • A. Na+ conductance in the presynaptic membrane decreases.
  • B. Ca+2 conductance in the presynaptic membrane increases.
  • C. Intracellular Ca+2 concentration in the cytoplasm of the presynaptic cell decreases.


11. In view of the fact that acetylcholine opens non-selective cation-channels in the postsynaptic membranes, what causes the postsynaptic membrane to depolarize, i.e., shouldn't inward Na+ current match outward K+ current?


12. Define miniature end-plate potential (MEPP). Define end-plate potential (EPP). What can modify the frequency of MEPPs? What can modify the amplitude of EPPs?


13. Where in the synapse is acetylcholine-esterase located? How would administering an effective acetylcholinesterase inhibitor alter the amplitude and frequency of the MEPPs? How would the magnitude of the EPP be altered?







Select the SINGLE BEST answer.


In answering the following next three questions, assume that if a presynaptic vesicle is released and its acetylcholine reaches and opens some channels in the motor end plate, a MEPP will result, and no matter how small the resulting MEPP, it is still considered to be a MEPP.


14. Curare binds to and blocks the acetylcholine receptor at the neuromuscular junction. An isolated nerve muscle preparation treated with a less-than-saturating dose of curare would be associated with a decrease in the:

  • A. frequency of MEPPs.
  • B. amplitude of MEPPS.
  • C. release of transmitter from the presynaptic terminal of the motor axon.
  • D. amplitude of the presynaptic action potential.
  • E. amplitude of the action potential recorded from the innervated muscle - assuming that an action potential is produced.


15. An isolated nerve-muscle preparation exposed to a low concentration of extracellular Ca2+ would be associated with a decrease in the:

  • A. amplitude of the EPP.
  • B. release of transmitter from the presynaptic terminal of the motor axon.
  • C. influx of Ca2+ into the presynaptic terminal.
  • D. frequency of the MEPPs.
  • E. all of the above are correct.


16. The endplate potential (EPP) that normally occurs at a vertebrate neuromuscular junction:

  • A. can be reduced in amplitude by reducing extracellular [Ca2+] below normal.
  • B. can be increased in amplitude by the use of drugs that block the enzyme acetylcholinesterase.
  • C. is normally suprathreshold for the generation of a post-synaptic action potential.
  • D. results from the release of the contents of a large number of presynaptic vesicles.
  • E. all of the above are correct.


17. The unequal distribution of Na+ and K+ across a resting, excitable membrane:

  • A. provides a source of potential energy.
  • B. is maintained by the expenditure of metabolic energy.
  • C. is maintained even in the face of several action potentials.
  • D. is abolished by any poison which interrupts the Na+ pump.
  • E. all of the above.


18. A counter transport mechanism that exchanges one extracellular Na+ for one intracellular H+ in the membrane of a cell whose resting potential is -60 mV and whose intracellular [Na+] is 12 mM and extracellular [Na+] is 120 mM can achieve what maximum theoretical pH gradient across the cell membrane? (Remember that pH = -log [H+], for example, pH = 7 means [H+] = 10-7M.)

  • A. 2 pH units, inside more acid.
  • B. 2 pH units, inside more alkaline.
  • C. 1 pH unit, inside more acid.
  • D. 1 pH unit, inside more alkaline.
  • E. 1.5 pH units, inside more alkaline.


19. A sodium-coupled amino acid cotransport mechanism exists that transports one Na+ ion inward for each glycine molecule that is transported inward. Since one positive charge is transported inward for each cycle of the cotransport mechanism, what theoretical maximum internal concentration of glycine could be achieved when there is an external glycine concentration of 1 mM and the cell has a resting potential of -60 mV, an internal [Na+] of 12 mM and external [Na+] of 120 mM?

  • A. 0.1 mM
  • B. 1 mM
  • C. 10 mM
  • D. 100 mM
  • E. 1000 mM

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