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Session 16 - Life Sci 2C03 L16 Session 16 Action Potentials...

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Life Sci 2C03 – L16 Session 16 – Action Potentials, Nerve Conduction and Synapse Action Potentials 1. Differential permeability of ion-channels in rest and impulse generation 1. Ion channels are 100 times more permeable to K+ ions than Na+ ions in resting state ( resting membrane potential) 2. However, during an action potential they are 10-20 times more permeable to Na- ions than K+ ions 1. The change in voltage is equivalent of 100,000 volts 2. Na+ and K+ currents are in the opposite directions 1. Steps: 1. Na+ channels open --? Cell becomes more and more positive (40 mV) 2. Na+ starts closing before peak of 40 mV) 3. K+ channels now open K+ exit cell, cell becomes repolarized 4. Hyperpolarization occurs because too much K+ leaves cell (Na+/K+ pumps involved) 5. Membrane comes back to restoration b/c the ions that exit diffuse in extracellular fluid and leave; only a few remain around membrane --? Membrane Potential reached 1. Ionic Basis 1. Na+ entry … followed closely in time (milliseconds) by…
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Life Sci 2C03 – L16 2. K+ exit (Notes. Events 1 and 2 partially overlap) 1. AP is summed up activity of Na+/K- activity 2. K+ regulated by channels and pumps 1. Components of an AP 1. Threshold : minimum strength of current required 2. All or none phenomenon a) Either a complete response or no response at all b) Once generated, conducted without drop or gain in amplitude (no way of stopping it) 3. Always followed by Refractoriness 1. Refractory Period – a period during which a second stimulus does not evoke another AP from occurring 1. Rate Law of Action Potential 1. Strength of a stimulus is represented by rate of firing of AP. The size of each AP is constant 2. Amplitude of spikes remain same, but firing rate changes (whether you increase or decrease strength of stimulus) 1. Cesium blocks K + channels 1. Macroscopic recording using Voltage Clamp 2. Na+ always opens w/ change in voltage 2. Tetrodotoxin blocks Na + channels 1. Diff in Na+ compared to K+ 1.
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