Becker's World of the Cell 9th Edition by Lewis Kleinsmith, Jeff Hardin, Gregory Paul Bertoni

Exercise 5

Heart Throbs. An understanding of muscle cell stimulation involves some of the same principles as nerve cell stimulation, except that calcium ions play an important role in the former. The following ion concentrations are typical of those in human heart muscle and in the serum that bathes the muscles:
[K + ]: 150 m M in cell, 4.6 m M in serum
[Na + ]: 10 m M in cell, 145 m M in serum
[Ca 2+ ]: 0.001 m M in cell, 6 m M in serum
Figure depicts the change in membrane potential with time upon stimulation of a cardiac muscle cell.
(a)Calculate the equilibrium potential for each of the three ions, given the concentrations listed.
(b)Why is the resting membrane potential significantly more negative than that of the squid axon (-75 mV versus -60 mV)
(c)The more positive membrane potential in region
of the graph could in theory be due to the movement across the membrane of one or both of two cations. Which cations are they, and in what direction would you expect each of them to move across the membrane
(d)How might you distinguish between the possibilities suggested in part c
(e)The rapid decrease in membrane potential that is occurring in region
is caused by the outward movement of potassium ions. What are the driving forces that cause potassium to leave the cell at this point Why aren't the same forces operative in region
of the curve
FIGURE The Action Potential of a Muscle Cell of the Human Heart.