MCB130lectures11to14 - MCB130 R Schekman Action Potential...

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MCB130 Lectures 11-14 R. Schekman Action Potential, Synaptic Transmission and Membrane Fusion A. Action potential 1. Because of the Na + , K + ATPase, nerve cells have a high external [Na + ] 440 mM relative to cytoplasmic concentration (50 mM) and conversely a low external [K + ] 20 mM relative to the cytoplasmic concentration (400 mM). a. Nerve cells have a K + leak channel which allows potassium to return to the cell exterior to an extent governed by the K + equilibrium potential. b. Na + can enter weakly through the leak channel and reduces the resting potential to - 60 mV. 2. Voltage gated Na + channels allow membrane depolarization when the membrane potential rises above a threshold of about -60 to -40 mV. a. The Na + channel opens and closes randomly in a resting state. b. When threshold potential reached, Na + influx opens additional channels until the Na + equilibrium potential (30 mV) is reached at which time the Na + channels close and different K + channels open. This hastens a return to the K + equilibrium potential. c. Signals are propagated in a wave of depolarization which in mammalian neurons is conducted between nodes separated by myelinated areas. The nodes are rich in Na + channels, the internodal region is depleted of channels. 3. Channels selectively inhibited a. Na + channel blocked by tetrodotoxin. Active site of channel probably contains carboxylate side chain. Selective permeability to Na + assured by small radius of channel. Tetrodotoxin binding used as assay to purify channel.
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