The membrane potential of the squid giant axon is modeled by the Goldman equatio

Question

The membrane potential of the squid giant axon is modeled by the Goldman equation.

1.

Calculate the membrane potential of this axon when the nerve cell is quiescent (not firing) .use reasonable parameters.

2.

The membrane potential during firing changes from -70 mV to +45 mV. This change is because of an increase in membrane permeability to Na. By what factor should PNa increase so that the membrane potential goes from -70 to +45?

3.

Assume that instead of changing the permeability of the membrane for Na, the membrane potential is changed instead by changing the internal concentration of Na. By what factor must the internal concentration be changed to move the membrane potential from -70 mV to +45 mV?

4. Consider a typical squid giant axon. This axon has a diameter of 0.5 mm. The space constant of this giant axon is given by . Use standard values for rm and ri to calculate a value for the space constant. During an action potential, 3.7 picomoles/cm2 of Na enters the cell through the membrane of a length of the axon corresponding to . How many action potentials have to fire before the internal concentration of Na changes to the value calculated in part c above?

Explanation / Answer

1). Membrane potential can be calculated from Goldman’s equation. The equation is given below.

Em = -61.5mV log ([Na]out/ [K]in)

= -61.5mV log (150/5) = -61.5mV (1.47) = -90mV (approximately).

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