1. Imagine a hypothetical neuron that is permeable to only K+ ions. If the EXTRA

Question

1. Imagine a hypothetical neuron that is permeable to only K+ ions. If the EXTRAcellular [K+] is 10 X greater than the INTRAcellular [K+], then the resting membrane potential for this hypothetical cell at standard temperature (20°C) would be…? I know the answer is +58 mV, but how?

B. Changing the temperature of a giant squid axon from 20°C to 40°C would change the K+ Nernst potential from about -75 mV to about…? Answer: -80 mV , how?

C. In molluscan neurons, the carrier of inward current during the rising phase of the action potential is usually Ca2+. This ion has an usually steep concentration gradient. Given the values below, what is the Nernst potential for Ca2+ at 20°C? [Ca2+]O = 10-2 M; [Ca2+]i = 1.7 x 10-7 M. Answer: +137 mV

Explanation / Answer

1. The resting potential of a cell permeable to only potassium ions, the resting membrane potential is determined by the equilibrium potential of K+ ions. The equilibrium potential of K+ ion is determined by Nernst Equation ------

RT/ zF ln [Xo] / [Xi].

R=Gas constant, T= Temperature, Z=valance of the ion, F= Farady constant, [Xi],[Xo] = Concentration of ions inside and outside respectively.

RT/F = 25 mV at 20 0 centigrade. The constant for converting natural logerithm to common logerithm is 2.3. For K+ value of z= +1. so the equation can be written as

Equilibrium potential (resting potential) = 58mV/1 log [10X] / [X] =58mV. ( 25 x 2.3 =57.5)

2. RT/F at 40 degree celcius is about 62 mV. Standard K+ ion concentration at this temperature is [20]out and [400]in

So membrane potential will be 62/z log [ion]out / [ion]in = -80mV

3. at 20 degree centigrade in molluscan neuron [Ca++o] / [Ca++i] = 4.77

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