BIOLOGY 152 1st Edition Lecture 16Outline of Last LectureA The Nervous System a. Neuron Morphology b. Resting Membrane Potential Outline of Current Lectureb. Resting Membrane PotentialCurrent LectureResting Membrane Potential (Actually lecture 17)March 6th**Clicker Question**Why does potassium determine the resting membrane potential in most cells?- Answer = the plasma membrane has high K permeability - related to high concentration, but is determined by K instead of anyother ion is because the K permeability is very high • Voltage that will hold a particular concentrations gradient starts ofpositive, then becomes negative (as K moved towards higher) • Membrane potential is a measure of the diference in electrical potentialsbetween two spots: in this case, the extracellular medium and inside thecell • can be measured by voltage • negative in the cell, because when you measure voltage you must set something to zero (so if the outside of the cell is positive the inside of the cell is negative with respect to the outside) = the resting membranepotential These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.**Clicker Question**If I altered the concentrations of Na+ both inside and out so that they were the same as K would both the ions now have the same Nernst potential?YES — univalent cation in both cases, if you plug the numbers into the Nernst equation you’ll get the same answer• when you’re determining for a single ion, the membrane must bepermeable that ion but permeability doesn’t matter for any other ion(unless you’re looking at a bunch of other ions) • if Chloride was changed to 140 and 5 then it would NOT have the same Nernst potential, because for an anion the equation looks slightly diferent (would be the exact opposite, or positive instead of negative) **Clicker Question**Inside mammalian cell is 100mmK, and outside is 10 mM K, what would happen if I lowered the outside twofold (so 10 changed to 5)?• The membrane potential would become more negative because thegradient is larger — closer to the Nernst potential for K • -make the gradient bigger, take more voltage (looking at the Nernstpotential equation, if you make ratio diferent, the voltage willchange) Approach the Na Nernst potential — increase the permeability of Na, sodium will have the biggest efect on the membrane potential (like K usually does)• in problem, sodium started as the same as the potassium Nernstpotential = would be 0 because Na and K are balancing each other • The total number of positive and negative charges are balanced on eitherside of the membrane• the negative charge inside the cell is a result of only a tiny numberof charges directly along with membrane • -very little K actually needs to leak out in a normal cell • Membrane potential determined by ions with high permeability • membrane potential will approach that ions Nernst potential • if something is permeable it does matter if you change the concentration,if it’s NOT permeable then it DOESN’T matter **Clicker Question**How does Na get so high outside and K so high inside?• sodium-potassium pump that establishes a gradient by pumping Na out andK inside (in cells a majority of your ATP is going to maintain this membranepotential) • A neuron has the Na/K pump working all the time and K leak channelsare working all the time • they’re allowing K to slowly leak outside the cell — this slow leak isgiving you the membrane potential. It doesn’t change the concentration **Clicker Question**Cyanide inhibits mitochondrial aerobic respiration, Ouabain does not, yet the efect on the membrane potential is similar (it becomes more positive)Why might this be true?• Ouabain inhibits the Na/K ATP-ase and stops it from working • still have potassium leak and you don’t have the pump working so youget rid of your gradient over time In a mammalian neuron the resting membrane potential is usually about -70 mV Threshold potential = membrane potential change that must happen in order for the firing to happen- has to depolarize (polarized when at -70) or become more positive alittle bit and shoots all the way up to the sodium Nernst potential (seegraph in lecture slides)- shoots down again which goes beyond the resting membrane potential (hyper polarization) and it becomes more negative than the resting membrane potential **Clicker Question**What event could cause the dramatic change seen in the rising phase? I. Increasing permeability of the membrane for NaII. Increasing the permeability of the membranefor K III. Changing Nernst potential for NaIV. Changing Nernst potential for KIf I was true (which it is) it would move the sodium closer to the sodium Nernst potential, increasing the permeability of K would make it more negative if anything- answer = C (I) - Nernst potential does NOT have a permeability component - when an action potential happens, the change in concentration is TINY Voltage gated channels = potassium one and a sodium one• open only in presence of depolarizing membrane potential (so if it goesPOSITIVE) and Na or K will move through depending on theirconcentration gradient • these channels all have 3 states closed state (resting membrane potential) open state (ions can move through them) inactive state (a ball of protein blocks the hole and stays there for a certain amount of time, depending on the channel. This causes the refractory period, or a period in which the neuron cannot fire) How does this happen?• hit the threshold potential, the membrane is depolarized — causes voltage gated sodium channels to open, sodium difuses down electrochemical gradient (cell doesn’t have much Na so it wants to difuse in) • very quickly after this (tiny percentage of Na coming in) the potentialrises and it reaches the Na Nernst potential • Na channels inactivate, and K voltage gated channels open (now membrane potential has gone from negative to positive) — K rushes inside and it wants to return the Nernst potential to K Nernst potential (so potassium is back in control) — membrane potential goes way down• hyper polarizes briefly • K voltage channels inactivate and the leak channels take over, returningit to the resting membrane • Threshold