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Lecture 5 Synaptic Transmission web

Lecture 5 Synaptic Transmission web - Action...

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A ti P t ti l ( ti d) Action Potential (continued) & Synaptic transmission Chapter 5 What happens after an action potential reaches the end of the axon?
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Significance of Voltage Clamp working hypothesis, mid 1900s: at threshold, voltage-dependent increase in g Na is involved in i i h & i d i f lli h f AP rising phase & increased g K in falling phase of AP Difficult to test this hypothesis since Vm & conductance during an AP conductance ______ _______ during an AP Voltage clamp amplifier holds Vm constant despite in g & g while measuring I ________________in g Na K ions This allowed measurement of g Na & g K at a ______ voltage & how g N & g K changed across time: Na 1. g Na rapidly increases with steep relationship on Vm > threshold 2. g Na rapidly decreases after a slight delay even when Vm >> threshold 3. g K increases after a slight delay with steep relationship to Vm 4. g K does not decrease if Vm is held at a level above threshold
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Patch Clamp Technique Patch clamp technique measures current through a ______ ion channel. Sensitive amplifier connected to glass pipette with tiny opening. Tight contact between glass pipette and membrane hi d ith ti M b achieved with suction. Membrane potential controlled to study voltage dependence of current through _____ ion channel ion channel. Current is ~1 to 2 pA (10 -12 ampere). Several orders of magnitude smaller h d b l than current measured by voltage clamping entire _____
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Patch-clamp recording of voltage-gated ____channel Properties of voltage-gated h l l i ti ____channels explain action potential properties Vm depolarized to threshold, -40 mV → ↑ chance of opening Channels open soon after depolarization – thus rapid rising phase Channels inactivate quickly – thus brief action potential duration _____________needed to relieve inactivation – explains __________ period
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Averaged current recorded from many single channels has same time ___ course as ‘macroscopic’ ____current (current recorded from whole axon by traditional voltage clamp method)
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Voltage-gated potassium channels contribute to falling phase • Help bring ___ back to resting membrane potential after peak of action potential: also known as after peak of action potential: also known as ‘_________ rectifier channels’ • Do not open immediately upon depolarization: delay allows open channels to take to delay allows open ____channels to take ___ to _____ of action potential • Averaged current from many single openings has same time course as macroscopic K + current
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Summary: How voltage-gated Na + and K + channels produce an action potential ____channels open soon after depolarization Inward current through ____ channels Rapid ___influx brings V m close t to ____ ___channels inactivate quickly ___channels open with a _____ __ efflux brings V m back to resting membrane potential, close to __ The undershoot is explained by The undershoot is explained by increased g K compared to rest, ___ briefly dips below resting membrane potential
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Action potential down axon Action potential ___________ down axon Positive charges entering during AP entering during AP spread and cause membrane just ahead
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