Chapter 22, Problem 22.5PS

QUANTITATIVE 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:

$\begin{array}{l}[{\text{K}}^{+}]:\text{150 m}M\text{ in cell},\text{ 4}\text{.6 m}M\text{ in serum}\\ [{\text{Na}}^{\text{+}}]:\text{10 m}M\text{ in cell},\text{ 145 m}M\text{ in serum}\\ [{\text{Ca}}^{2+}]:\text{0}\text{.001 m}M\text{ in cell, 6 m}M\text{ in serum}\end{array}$

Figure 22-19 depicts the change in membrane potential with time upon stimulation of a cardiac muscle cell.

Figure 22-19 The Action Potential of a Muscle Cell of the Human Heart.

See Problem 22-5. (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?