Prob Set 3 Q&A - Problem Set 3 Cellular...

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Problem Set 3 Cellular Neurobiology Fall 2010 1. You perform an anode-break excitation experiment to generate an action potential. A. You close the circuit; what occurs at the cathode and anode? Cathode: negative charge is supplied to the outside of the cell, which depolarizes the cell locally and may elicit an action potential. At steady state the depolarized voltage causes more voltage-gated sodium channels to open and become inactivated. Additionally, some voltage-gated potassium channels open. Now less INa can be generated and more IKout is generated so that the threshold voltage (where INa = IKout) is not attained. This is the phenomenon of accommodation. Anode: positive charge is supplied to the outside of the cell, hyperpolarizing the cell locally. This causes removal of inactivation of the small proportion of voltage-gated sodium channels that were inactivated at rest, lowering the threshold voltage because no more GNa is available. The small proportion of voltage-gated potassium channels that were open will also close, lowering resistance to depolarization. B. You leave the switch closed for a long time and then reopen it; what occurs at the cathode and anode? Cathode: the extracellular environment near the cathode regains positive charge and causes the membrane voltage to hyperpolarize back to the resting potential. Anode: the membrane potential is rapidly repolarized toward the resting potential. Many voltage-gated sodium channels that are no longer inactivated and were closed will now open, which may be enough to generate INa that brings the membrane potential to threshold and elicit an action potential. 2. A. What causes the absolute refractory period? Closure of the inactivation gates prevents the voltage-gated sodium channels from opening immediately after the firing of an action potential. B. What might happen to an action potential if τ h were larger or smaller? τ h refers to the time the inactivation gate takes to open or close Larger: would lead to an prolongation of the Na + currents that would counteract the K + currents, increasing the duration of the action potential and decreasing the maximal firing rate for the cell 1
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Smaller: could lead to early closure of Na + channels. This could make action potentials briefer and increase the maximal firing rate or it could prevent Na + flux, blocking action potentials. C. How does the absolute refractory period differ from the relative refractory period? The absolute refractory period is due the inactivation of Na + channels. The relative refractory period is due to the presence of open K + channels that lead to hyperpolarization of the membrane potential, making it more difficult for a depolarizing stimulus to bring the membrane potential to threshold. This occurs because K + channels take longer to both open and close in comparison to Na + channels.
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  • Fall '07
  • SPITZER
  • Voltage-gated sodium channels, Potassium channel, voltage-gated potassium channels

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