The Resting Membrane Potential

All cells under resting conditions have a potential difference across their plasma membranes oriented with the inside of the cell negatively charged with respect to the outside (Figure 8-7a). This potential is the resting membrane potential.

Resting Membrane Potential
(b)

FIGURE 8-7

(a) Apparatus for measuring membrane potentials. (b) The potential difference across a plasma membrane as measured by an intracellular microelectrode. The asterisk indicates the time the electrode entered the cell.

Time

FIGURE 8-7

(a) Apparatus for measuring membrane potentials. (b) The potential difference across a plasma membrane as measured by an intracellular microelectrode. The asterisk indicates the time the electrode entered the cell.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

By convention, extracellular fluid is assigned a voltage of zero, and the polarity (positive or negative) of the membrane potential is stated in terms of the sign of the excess charge on the inside of the cell. For example, if the intracellular fluid has an excess of negative charge and the potential difference across the membrane has a magnitude of 70 mV, we say that the membrane potential is —70 mV.

The magnitude of the resting membrane potential varies from about —5 to —100 mV, depending upon the type of cell; in neurons, it is generally in the range of —40 to —75 mV (Figure 8-7b). The membrane potential of some cells can change rapidly in response to stimulation, an ability of key importance in their functioning.

The resting membrane potential exists because there is a tiny excess of negative ions inside the cell and an excess of positive ions outside. The excess negative charges inside are electrically attracted to the excess positive charges outside the cell, and vice versa. Thus, the excess charges (ions) collect in a thin shell tight against the inner and outer surfaces of the plasma membrane (Figure 8-8), whereas the bulk of the in-tracellular and extracellular fluids are neutral. Unlike the diagrammatic representation in Figure 8-8, the number of positive and negative charges that have to be separated across a membrane to account for the potential is an infinitesimal fraction of the total number of charges in the two compartments.

The magnitude of the resting membrane potential is determined mainly by two factors (a third factor will

TABLE 8-2 Distribution of Major Ions Across the Plasma Membrane of a Typical Nerve Cell

Extracellular

fluid

+ 1 + + + - Cell

- + +\- — +

The excess of positive charges outside the cell and the excess of negative charges inside collect tight against the plasma membrane. In reality, these excess charges are only an extremely small fraction of the total number of ions inside and outside the cell.

TABLE 8-2 Distribution of Major Ions Across the Plasma Membrane of a Typical Nerve Cell

Concentration, mmol/L

Ion

Extracellular

Intracellular

Na+

150

15

Cr

110

10

K+

5

150

be given later): (1) differences in specific ion concentrations in the intracellular and extracellular fluids, and (2) differences in membrane permeabilities to the different ions, which reflect the number of open channels for the different ions in the plasma membrane. The rest of this section analyzes how these two factors operate.

The concentrations of sodium, potassium, and chloride ions in the extracellular fluid and in the intra-cellular fluid of a typical nerve cell are listed in Table 8-2. Although this table appears to contradict our earlier assertion that the bulk of the intra- and extracellular fluids are electrically neutral, there are many other ions, including Mg2+, Ca2+, H+, HCO3", HPO42~, SO42~, amino acids, and proteins, in both fluid compartments. Of the mobile ions, sodium, potassium, and chloride ions are present in the highest concentrations, and the membrane permeabilities to these ions are restricted, although, as we shall see, to different degrees. Sodium and potassium generally play the most important roles in generating the resting membrane potential. Note that the sodium and chloride concentrations are lower inside the cell than outside, and that the potassium concentration is greater inside the cell. As we described in Chapter 6, the concentration differences for sodium and potassium are due to the action of a plasma-membrane active-transport system that pumps sodium out of the cell and potassium into it. We will see later the reason for the chloride distribution.

To understand how such concentration differences for sodium and potassium create membrane potentials, let us consider the situation in Figure 8-9. The assumption in this model is that the membrane contains potassium channels but no sodium channels. Initially, compartment 1 contains 0.15 M NaCl, and compartment 2 contains 0.15 M KCl. There is no potential difference across the membrane because the two compartments contain equal numbers of positive and negative ions; that is, they are electrically neutral. The positive ions are different—sodium versus potassium, but the total numbers of positive ions in the two compartments are the same, and each positive ion is balanced by a chloride ion.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

(a)

Compartment 1

Compartment 2

0.15 M

0.15 M

NaCl

KCl

(b)

K+

Na+

(c)

K+ Na+

ÎZ)

K+

(d)

+

-

K+

+

-

K+

Na+

(e)

K+

+ +

-

K+

Na+

+ +)

-

Generation of a diffusion potential across a membrane that contains only potassium channels. Arrows represent ion movements.

This initial state will not, however, last. Because of the potassium channels, potassium will diffuse down its concentration gradient from compartment 2 into compartment 1. After a few potassium ions have moved into compartment 1, that compartment will have an excess of positive charge, leaving behind an excess of negative charge in compartment 2. Thus, a potential difference has been created across the membrane.

Now we introduce a second factor that can cause net movement of ions across a membrane: an electrical potential. As compartment 1 becomes increasingly positive and compartment 2 increasingly negative, the membrane potential difference begins to influence the movement of the potassium ions. They are attracted by the negative charge of compartment 2 and repulsed by the positive charge of compartment 1.

As long as the flux due to the potassium concentration gradient is greater than the flux due to the membrane potential, there will be net movement of potassium from compartment 2 to compartment 1, and the membrane potential will progressively increase. However, eventually the membrane potential will become negative enough to produce a flux equal but opposite the flux due to the concentration gradient. The membrane potential at which these two fluxes become equal in magnitude but opposite in direction is called the equilibrium potential for that type of ion—in this case, potassium. At the equilibrium potential for an ion, there is no net movement of the ion because the opposing fluxes are equal, and the potential will undergo no further change.

The value of the equilibrium potential for any type of ion depends on the concentration gradient for that ion across the membrane. If the concentrations on the two sides were equal, the flux due to the concentration gradient would be zero, and the equilibrium potential would also be zero. The larger the concentration gradient, the larger the equilibrium potential because a larger electrically driven movement of ions will be required to balance the larger movement due to the concentration difference.

If the membrane separating the two compartments is replaced with one that contains only sodium channels, a parallel situation will occur (Figure 8-10). A sodium equilibrium potential will eventually be established, but compartment 2 will be positive with respect to compartment 1, at which point net movement of sodium will cease. Again, at the equilibrium potential the movement of ions due to the concentration gradient is equal but opposite to the movement due to the electrical gradient.

Thus, the equilibrium potential for one ion species can be different in magnitude and direction from those for other ion species, depending on the concentration gradients for each ion. (Given the concentration gradient for any ion, the equilibrium potential for that ion can be calculated by means of the Nernst equation, Appendix D.)

Compartment 1 Compartment 2

0.15 M NaCl

0.15 M KCl

FIGURE 8-10

Generation of a diffusion potential across a membrane that contains only sodium channels. Arrows represent ion movements.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Our examples were based on the membrane being permeable to only one ion at a time. When more than one ion species can diffuse across the membrane, the permeabilities and concentration gradients for all the ions must be considered when accounting for the membrane potential. For a given concentration gradient, the greater the membrane permeability to an ion species, the greater the contribution that ion species will make to the membrane potential. (Given the concentration gradients and membrane permeabilities for several ion species, the potential of a membrane permeable to these species can be calculated by the Goldman equation, Appendix D.)

It is not difficult to move from these hypothetical examples to a nerve cell at rest where (1) the potassium concentration is much greater inside than outside (Figure 8-11a) and the sodium concentration profile is just the opposite (Figure 8-12a); and (2) the plasma membrane contains 50 to 75 times as many open potassium channels as open sodium channels.

Given the actual potassium and sodium concentration differences, one can calculate that the potassium equilibrium potential will be approximately —90 mV (Figure 8-11b) and the sodium equilibrium potential about +60 mV (Figure 8-12b). However, since the membrane is permeable, to some extent, to both

Extracellular fluid

(a)

Low Na+

i-High Na+

(b)

Na+ movement due to

concentration gradient

Key

Na+ movement due to

electrical gradient

FIGURE 8-12

Forces acting on sodium when the membrane of a neuron is at (a) the resting potential (-70 mV, inside negative), and (b) the sodium equilibrium potential (+60 mV, inside positive).

Extracellular fluid

(a)

-70 mV

(b)

K+ movement due to

concentration gradient

Key

K+ movement due to

electrical gradient

FIGURE 8-11

Forces acting on potassium when the membrane of a neuron is at (a) the resting potential (-70 mV, inside negative), and (b) the potassium equilibrium potential (-90 mV, inside negative).

potassium and sodium, the resting membrane potential cannot be equal to either of these two equilibrium potentials. The resting potential will be much closer to the potassium equilibrium potential because the membrane is so much more permeable to potassium than to sodium.

In other words, a potential is generated across the plasma membrane largely because of the movement of potassium out of the cell down its concentration gradient through open potassium channels, so that the inside of the cell becomes negative with respect to the outside. To repeat, the experimentally measured resting membrane potential is not equal to the potassium equilibrium potential, because a small number of sodium channels are open in the resting state, and some sodium ions continually move into the cell, canceling the effect of an equivalent number of potassium ions simultaneously moving out.

An actual resting membrane potential when recorded is about -70 mV, a typical value for neurons, and neither sodium nor potassium is at its equilibrium potential. Thus, there is net movement through ion channels of sodium into the cell and potassium out. The concentration of intracellular sodium and potassium ions does not change, however, because active-transport mechanisms in the plasma membrane utilize

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

energy derived from cellular metabolism to pump the sodium back out of the cell and the potassium back in. Actually, the pumping of these ions is linked because they are both transported by the Na,K-ATPase pumps in the membrane (Chapter 6).

In a resting cell, the number of ions moved by the pump equals the number of ions that move in the opposite direction through membrane channels down their concentration and/or electrical gradients. Therefore the concentrations of sodium and potassium in the cell do not change. As long as the concentration gradients remain stable and the ion permeabilities of the plasma membrane do not change, the electric potential across the resting membrane will also remain constant.

Thus far, we have described the membrane potential as due purely and directly to the passive movement of ions down their electrical and concentration gradients, the concentration gradients having been established by membrane pumps. There is, however, as mentioned in Chapter 6, another component to the membrane potential that reflects the direct separation of charge across the membrane by the transport of ions by the membrane Na,K-ATPase pumps. These pumps actually move three sodium ions out of the cell for every two potassium ions that they bring in. This unequal transport of positive ions makes the inside of the cell more negative than it would be from ion diffusion alone. A pump that moves net charge across the membrane contributes directly to the membrane potential and is known as an electrogenic pump.

In most cells (but by no means all), the electrogenic contribution to the membrane potential is quite small. It must be reemphasized, however, that even when the electrogenic contribution of the Na,K-ATPase pump is small, the pump always makes an essential indirect contribution to the membrane potential because it maintains the concentration gradients down which the ions diffuse to produce most of the charge separation that makes up the potential.

Figure 8-13 summarizes the information we have been presenting. This figure may mistakenly be seen to present a conflict: The development of the resting membrane potential depends predominantly on the diffusion of potassium out of the cell, yet in the steady state, sodium diffusion into the cell, indicated by the black Na+ arrow in Figure 8-13, is greater than potassium diffusion out of the cell. The reason is that although there are relatively few open sodium channels, sodium has a much larger electrochemical force acting upon it—that is, it is far from its equilibrium potential. The greater diffusion of sodium into the cell than potassium out compensates for the fact that the membrane pump moves three sodium ions out of the cell

Plasma

Intracellular fluid membrane Extracellular fluid

Extracellular Fluid Ions

FIGURE 8-13

Movements of sodium and potassium ions across the plasma membrane of a resting neuron in the steady state. The passive movements (black arrows) are exactly balanced by the active transport (red arrows) of the ions in the opposite direction.

FIGURE 8-13

Movements of sodium and potassium ions across the plasma membrane of a resting neuron in the steady state. The passive movements (black arrows) are exactly balanced by the active transport (red arrows) of the ions in the opposite direction.

for every two potassium ions that are moved in. Figure 8-13 shows ion movements once steady state has been achieved, not during its achievement.

We have not yet dealt with chloride ions. The plasma membranes of many cells have chloride channels but do not contain chloride-ion pumps. Therefore, in these cells chloride concentrations simply shift until the equilibrium potential for chloride is equal to the resting membrane potential. In other words, the negative membrane potential moves chloride out of the cell, and the chloride concentration outside the cell becomes higher than that inside. This concentration gradient produces a diffusion of chloride back into the cell that exactly opposes the movement out because of the electric potential.

In contrast, some cells have a non-electrogenic active transport system that moves chloride out of the cell. In these cells, the membrane potential is not at the chloride equilibrium potential, and net chloride diffusion into the cell contributes to the excess negative charge inside the cell; that is, net chloride diffusion makes the membrane potential more negative than it would otherwise be.

We noted earlier that most of the negative charge in neurons is accounted for not by chloride ions but by negatively charged organic molecules, such as proteins and phosphate compounds. Unlike chloride, however, these molecules do not readily cross the plasma membrane but remain inside the cell, where their charge contributes to the total negative charge within the cell.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

PART TWO Biological Control Systems

8. Neural Control Mechanisms

© The McGraw-Hill Companies, 2001

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Responses

  • esther
    How are these forces acting on ions in both intra and extracellular fluids during resting states?
    7 years ago
  • julia
    What is the magnitude of a resting potential?
    7 years ago
  • Julia
    What are functions of the resting membrane potential?
    7 years ago
  • bucca
    Why is magnitude of membrane potential determined by concentration gradient?
    7 years ago
  • michelino
    WHAT STEPS ARE THERE FOR RESTING MEMBRANE POTENTIAL?
    6 years ago
  • Tolman
    When does the potential across the plasma membrane become positive?
    6 years ago
  • LEARCO MANFRIN
    Does The resting membrane potential exists only on the cell body and dendrites.?
    6 years ago
  • William
    How a neuron maintains a resting state active and passive processes?
    6 years ago
  • nicola
    Why would the resting membrane potential have the same value in the cell body and the axon?
    6 years ago
  • asphodel took-brandybuck
    Why do chloride has little movement in resting potential?
    5 years ago
  • arabella
    What is the resting potential of a neuron?
    5 years ago
  • raymond
    Why is the inside of the plasma membrane of a neuron negative?
    5 years ago
  • camelia
    When a muscle or nerve is at rest which ions are in excess in the extracellular fluid?
    5 years ago
  • Kimi
    What is the typical resting potential of a neuron 48?
    5 years ago
  • folcard
    WHAT IS RESTING MEMBRANE POTENSIAL FOR HUMAN BODY?
    2 years ago
  • REBECCA
    Why steady state membrane potential is not equal to zero?
    2 years ago
  • Lorenzo McKenzie
    What ismagnitude of resting membrane potential depends on?
    2 years ago
  • Gorbulas
    Does a positive membrane potential attract negative ions?
    2 years ago
  • vala
    Why is the intracellular fluid and extracellular fluid assigned a voltage of zero?
    11 months ago
  • benjamin
    Can we say that the membrane potential is a range?
    7 months ago

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