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Lecture 4 The Action Potential

Aps relative permeabilities and currents rising phase

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APs, relative permeabilities, and currents rising phase characterized by an influx of sodium into cell, we have a reversal relative permeability. A cell now is much more permeable to sodium than to potassium. Given concentration gradient, causes positive ions attracted to inside. Opening extra Na+ channels falling phase (going down) characterized by eflux of potassium from inside of cell to outside. Returns inside of cell to a more negative state relative to outside. So at resting phase, cell more permeable to potassium. This is reserved during rising phase, cell much more permeable to sodium than potassium. Following phase returns to normal. Ion channels and gating Ion channels mediating Aps two types of voltage-gated ion channels mediate changes in membrane permeability
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during action potential. Inactivating voltage-gated Na+ channel delayed (rectifying) voltage-gated K+ channel gated channels are those that undergo change in the probability of being open or closed resulting in changes in relative permeability of the membrane to specific ions. Channels are a “family” of structurally and functionally similar proteins form a pore allowing ions to move across membrane comprised of membrane-spanning domains (either one polypeptide or separate subunits) sodium channels composed of one polypeptide chain potassium channels are four separate subunits Voltage-gated Na+ channels inactivating voltage-gated Na+ channel single polypeptide four domains each domain contains 6 transmembrane spans/domains – contains hydrophobic and hydrophilic parts voltage-sensitive selective pore channel opens and closes bases on electrical charge of inside of cell relative to outside inactivating voltage-gated Na+ channel – voltage-sensitive activation of selective pore is due to modest depolarization selective pores Na+ vs K+ ions equal charge different hydrated sizes allows Na+ selective pore (Na+ plus water b/c hydrated ions) removal of inactivation occurs only when membrane repolarizes (ie at -65 mV)
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APs relative permeabilities and currents rising phase...

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