MIT Department of Biology 7.013: Introductory Biology - Spring 2005 Instructors: Professor Hazel Sive, Professor Tyler Jacks, Dr. Claudette GardelSolutions to: 7.013 Neurobiology Section Problem Shown below is a plot of an action potential at a single point on an axon. +50 mV Membrane 0 mV potential -70 mV time A B C Fill in the table below, for each of the three timepoints (A, B and C) indicated by the arrows in the diagram above. Time Point A B C Na+ voltage-sensitive channel status (Inactivated, Closed, or Open) Closed Open Inactivated K+ voltage-sensitive channel status (Inactivated, Closed, or Open) Closed Closed Open Na+ flow through Na+ voltage-sensitive channels (In, None, or Out ) None In None K+ flow through K+ voltage-sensitive channels (In, None, or Out ) None None OutSolutions: Figure by MIT OCW. 0+80Not enough depolarization to open Na+ channelsPump "cleans up"Pump "cleans up"Depolarization to open Na+ channels Na+ flow depolarizes membrane = action potential Na+ channels spontaneously close and inactivateK+ flow repolarizes membraneAfter membrane repolarizes, Na+ channels reactivateNa+ Injection into NeuronNa+ FlowNa+ Channel StateK+ Channel StateK+ FlowVmem(mV)+60+40+20-20-40-60-80000outoutOOCCIininTime Pump "cleans up"b) Figure by MIT OCW. 0+80Without the voltage-gated Na channels, the membrane depolarizes but cannot fire an actionpotential. The pump repolarizes the membrane (with perhaps some help from the K channels).Na+ Injection into NeuronNa+ channel blockedNa+ FlowNa+ Channel StateK+ Channel StateK+ FlowVmem(mV)+60+40+20-20-40-60-80000outoutOOCCIininTimec) Figure by MIT OCW. 0+80Na+ Injection into NeuronK+ channel blockedAction potential fires as usual, but cell can only re-polarize via the pump, which takes a lot longer than to repolarize via the K channels.Na channel remains inactive, because membrane has not yet repolarized.Na+ FlowNa+ Channel StateK+ Channel StateK+
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