Summary of Nernst Potentials Salt concentrations are the primary controllers of
ID: 199302 • Letter: S
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Summary of Nernst Potentials Salt concentrations are the primary controllers of the electrical potentials across plasma membranes, Over the short term the ionic concentrations will collapse as more and more r. ns cross the plasma membrane. What is the main ion pump required to maintain equilib- rium concentrations of Na and K'? Goldman@ 37°C Start the simulator and select the Goldman @ 37'C tab on the upper right side of the display window Ex Eci The Goldman equation combines the individual equilibrium potentials for all the ions involved in controlling the membrane potential with the permeability of the plasma membrane for those ions. (See discussion of permeability listed on page 65). In this simulator only K', Na' and Cl are included as they are the 3 most important ions for most cells. As we will be emphasizing only K and Na* in this simulator you may turn off the CI permeability. a. What value should you set the chloride permeability (PCI) to prevent Cl from passing through the plasma membrane in this simulator? (Hint: you can set PCI to this value and then change the outside or inside concentrations of CI and it will not affect the Em (mem- brane potential). Set the K' concentrations to 4.5 and 140 mM outside and inside, respectively, with the mem- brane permeability for K' set to 25. Set the Na' concentrations to 140 and 15 mM outside and inside, respectively, with the membrane permeability for Na' set to I b. What is the current resting membrane potential (Em)? c. Why is it so negative? d. Why is the permeability of K' relatively higher than Na" under resting e. During electrical depolarization of a neuron, what channel type is activated:? conditions? (Hint: What channel type is open during resting conditions?)Explanation / Answer
d) The membrane potential in resting state is referred as resting membrane potential. In resting nerve fibre, the plasma membrane is negatively charge inside the plasma membrane and positively charged from outside the membrane. Minus sign indicates that inside of the membrane is always negatively charged.
Repolarization – The change in membrane potential flows passively along the axon resulting in the depolarization of the adjacent region of the axon. With increase in positive charge inside the membrane, Na+ ions channels get closed leading to the repolarization of the membrane. It is achieved by the increased permeability of K+ ions due to K+ ion channel opening, causing the K+ ion flow outward so the membrane potential returns to negative value i.e., the resting potential of -60mv is restored.
Hyperpolarization – Before K+ ion channel closes, excess K+ ions have moved out of the cell generating the polarized potential (-80mv). Thus the membrane becomes hyperpolarized.
e) When a nerve fibre receives a stimulus it result in the change of permeability in the membrane. Opening of voltage-gated Na+ channels increases the permeability of membrane to Na+ ions and voltage gated K+ ion channel increases movement of K+ ion outside the membrane. It results in the depolarization or the change in membrane potential from -60mv to +30mv. An action potential is generated when the membrane is depolarized.
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