378

PART 3 Integration and Control Systems

that is established when the tendency for K + to diffuse out of the cell,because of the K + concentration gradient, is equal to the tendency forK + to move into the cell because of the attraction of the positivelycharged K + to negatively charged proteins and other molecules.

TABLE 11.3

Characteristics Responsible forthe Resting Membrane Potential

+ 1. The concentration of K is higher inside the cell than outside, and the + concentration of Na is higher outside the cell than inside.

+ 2. Due to the K leak channels, the plasma membrane is 50–100times more permeable to K + than to other positively charged ions,such as Na + .

Predict 4

Given that tissue A has significantly more K leak ion channels thantissue B, which tissue has the larger resting membrane potential?

+

Other ions, such as Na + , Cl − , and Ca 2 , have a minor influ-ence on the resting membrane potential, but the major influenceis from  K + . The resting plasma membrane is not very perme-able to Na + . In fact, because the resting plasma membrane is50–100 times less permeable to Na + than to K + , very little Na + candiffuse into the resting cell. The resting plasma membrane is not + very permeable to Ca 2 , either. The plasma membrane is relatively − permeable to Cl , but these negatively charged ions are repelled bythe negative charge inside the cell.The resting membrane potential is proportional to the tendencyfor K + to diffuse out of the cell, not to the actual rate of flow for K + .At equilibrium, very few of these ions pass through the plasma mem-brane because their movement out of the cell is opposed by the nega-tive charge inside the cell. Still, some Na + and K + diffuse continuouslyacross the plasma membrane, although at a low rate. The large con-centration gradients for Na + and K + would eventually disappearwithout the continuous activity of the sodium-potassium pump.As already noted, the sodium-potassium pump maintains theuneven distribution of Na + and K + across the plasma membrane. Thepump is also responsible for a small portion of the resting membranepotential, usually less than 15 mV, because it transports approxi-mately three Na + out of the cell and two K + into the cell for each ATPmolecule used (see figure 3.21). The outside of the plasma membranebecomes more positively charged than the inside, because more posi-tively charged ions are pumped out of the cell than are pumped into it.The characteristics responsible for the resting membrane poten-tial are summarized in table 11.3.

+

3. The plasma membrane is impermeable to large, intracellular,negatively charged molecules, such as proteins. In other words,these anions are “trapped” inside the cell.

4. Potassium ions tend to diffuse across the plasma membrane fromthe inside to the outside of the cell.

5. Because negatively charged molecules cannot follow the positivelycharged K + , a small negative charge develops inside the plasmamembrane.

+ 6. The negative charge inside the cell attracts positively charged K .When the negative charge inside the cell is great enough to preventadditional K + from diffusing out of the cell through the plasmamembrane, an equilibrium is established.

7. The charge difference across the plasma membrane at equilibriumis reflected as a difference in potential, which is measured in millivolts(mV).

+ 8. The resting membrane potential is proportional to the potential for K + to diffuse out of the cell but not to the actual rate of flow for K .

+ 9. At equilibrium, very little movement of K or other ions takes placeacross the plasma membrane.

Changing the Resting Membrane Potential

The resting membrane potential can become more positive ormore negative. The charge difference across a particular area ofmembrane is changed when ions move across the plasma mem-brane due to a change in the ion concentration gradients or ionpermeability of the plasma membrane. Depolarization (d ē -p ō ′l ă r-i-z ā ′sh ŭ n) occurs when the membrane potential becomes morepositive and is the movement of the membrane potential closer tozero (figure 11.8 a ). On the other hand, hyperpolarization (h ī ′per-p ō ′l ă r-i-z ā ′sh ŭ n) occurs when the membrane potential becomesmore negative and is the movement of the membrane potentialfurther away from zero (figure 11.8 b ). We will further discussspecific situations in which depolarization or hyperpolarizationoccurs in the “Graded Potentials” section.

Potassium Ions

We will now consider some scenarios where the resting mem-brane potential is influenced by K + . Under normal action potential

conditions, K + diffuses out of the cell; however, changes in the K + concentration gradient and permeability could change the restingmembrane potential. An increase in the extracellular concentrationof K + decreases its concentration gradient because the concentra-tion of K + is normally lower outside than inside a cell. Thus, K + tends to stay inside the cell. In addition, because there is less“pull” on K + to leave the cell, the negative charge inside the celldoes not need to be as large to attract K + back into the cell. At thisnew equilibrium, the smaller charge difference across the plasmamembrane changes the resting membrane potential to a more posi-tive level than normal because more positive charge is inside thecell. Thus, we say the cell is depolarized.In contrast, a decrease in the extracellular concentration of K + increases the K + concentration gradient. As a result, the tendency forK + to diffuse out of the cell increases, and a larger negative chargeinside the cell is required to resist that diffusion. At this new equi-librium, the larger charge difference across the plasma membranechanges the resting membrane potential to a more negative levelthan normal because less positive charge is inside the cell. Thus,we say the cell is hyperpolarized.Although K + leak ion channels allow some K + to diffuse acrossthe plasma membrane, the resting membrane is not completely per-meable to K + . However, there are gated K + channels in the plasmamembrane; if they open, membrane permeability to K + increases, andmore K + diffuses out of the cell. The increased tendency for K + todiffuse out of the cell is opposed by the greater negative charge thatdevelops inside the plasma membrane. Because the negative chargeis greater inside the cell, the cell is now hyperpolarized.