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5-Elec Signals Neurons Figs BW

low sodium inside high outside due to sodium

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Unformatted text preview: ue to sodium-potassium pump. sodium diffuse in, down its concentration gradient making inside positive, chloride can't follow. this create electrical gradient that drive sodium out. this create equilibrium and get nerst equation for sodium. this equation of postive value (because outer concentration is greater, log greater than 1 is positive). for Ek for potassium negative (b/c outer concentration is less than inside, log less than 1 is negative) AT REST, most of potassium channel will open and sodium channel will close. resting membrane potential is negative (assume only k+ is permeable). when nerve cell is active (peak of action potential), extra sodium channel open (sodium permeability increase, potassium permeability may go down b/c some channel close) and membrane reach positive. the gates can switch open/close very rapidly. equilibrium shift very rapidly. membrane potential can shift very rapidly as gates open/close. Vm When Several Ions are Permeable +70 ENa Vm 0 Actual membrane potential ? (mV) ECl -90 but cells never permeable to just one ion. jn particular, most cells have measurable permeability to potassium (most permeable at rest), some for sodium and chloride. usually potassium permeability is largest of 3 although chloride can be large too. the ratio of potassium to sodium permeability can be 20 to 1, but there are open sodium channel at rest. -what happens when several ions (more than one) channels are open?. the potassium equilibrium potential (previous slide, negative value -80). sodium equilibrium potential (positive value, +50). chloride is negative (more positive than potassium, but still negative). these Ek,na,cl are equilibrium potential for potassium, sodium, and chloride. -in typical cell permeable to 3, where's the actual membrane potential will be? well, we assume it will be in between the 3 (factors affect this: ions most permeable make most contribution to membrane potential, depend on permeability of ion and relative concentration of ion across membrane). if cell is most permeable to potassium, th...
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