Question

When an electrode is stuck inside an intact squid giant axon, the membrane potential registers

-70mV. When the axon, suspended in a bath of seawater, is stimulated to conduct an impulse, the membrane potential changes transiently to +40mV. For univalent ions at 20^oC, the Nernst equation reduces to:

V_{(membrane potential)} = 58mV x log₁₀(C_o/C_i)

Where C_o and C_i are the extra and intracellular concentrations of the ion respectively (see table below).

Ion	[cytoplasm]	[seawater]
Na+	65mM	430mM
K+	344mM	9mM

A. Using this equation, calculate the potential across the resting membrane;

1. Assuming that it is solely due to K⁺. (2 points)

2. Assuming that it is solely due to Na⁺. (2 points)

B. Which calculation is closer to the measured resting potential and what conclusion do you draw from this? (4 points)

C. Which calculation is closer to the measured action potential? What does this mean? (4 points)

Answer 1

A.Resting Membrane Potential due to Potassium = 58 X log 10 (9/344) = 58 X -1.6 = -93 mV

Resting Membrane Potential due to Sodium = 58 X log 10 (430/65) = 58 X 0.82 = 46 mV

B. The Measured Resting potential -70mV is closer to the potential of Potassium as the potassium is responsible for maintaining the Resting Membrane Potential.The Resting Membrane Potential is determined by the movement of potassium ions across the membrane. So,the concentration of potassium inside the cell increases, the negativity of the membrane potential increases,Usually at rest the membrane potential is near or slightly lower  than the potential of potassium ions.The Potassium freely moves in and out of the cell, even at rest to maintain resting membrane potential as this causes a postive charge potassium ion moves out of cell causes an increase in negative charge inside the cell but the potassium is repelled by the postitive charge outside which is opposses the movement of potassium concentration gradient. The Conclusion is Potassium ions move outside the cell as the more negativity inside the cell leads to efflux of potassium to maintain ar rest.

C.The Measured Action Potential is closer to the potential of Sodium as the sodium increases the positivity inside the cell the cell do not allow the sodium to enter the cell .As the Concentration of sodium is higher outside the cell it potential is usually positive.The movement of Sodium is carried through Sodium-Potassium ATPase  Pump which are opened as the negativity inside the cell increases this is because of more concentration of potassium inside the cell as this causes increase in negativity of resting membrane potential to decrease it negativity the sodium ions enter inside the cell and potassium ion exit the cell. The action potential changes to +40mV because of the influx of the more sodium ions into the cell to maintain its concentration gradient.