How the resting membrane potential is established in a neuron.

Key points:

  • A resting (non-signaling) neuron has a voltage across its membrane called the resting membrane potential, or simply the resting potential.
  • The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion.
  • In a resting neuron, there are concentration gradients across the membrane for Na+\text {Na}^+ and K+\text K^+. Ions move down their gradients via channels, leading to a separation of charge that creates the resting potential.
  • The membrane is much more permeable to K+\text K^+ than to Na+\text {Na}^+, so the resting potential is close to the equilibrium potential of K+\text K^+ (the potential that would be generated by K+\text K^+ if it were the only ion in the system).

Introduction

Suppose you have a dead frog. (Yes, that's kind of gross, but let's just imagine it for a second.) What would happen if you applied an electrical stimulus to the nerve that feeds the frog's leg? Creepily enough, the dead leg would kick!
The Italian scientist Luigi Galvani discovered this fun fact back in the 1700s, somewhat by accident during a frog dissection. Today, we know that the frog's leg kicks because neurons (nerve cells) carry information via electrical signals.
How do neurons in a living organism produce electrical signals? At a basic level, neurons generate electrical signals through brief, controlled changes in the permeability of their cell membrane to particular ions (such as Na+\text{Na}^+ and K+\text K^+). Before we look in detail at how these signals are generated, we first need to understand how membrane permeability works in a resting neuron (one that is not sending or receiving electrical signals).
In this context, permeability refers to the ability of a particular molecule to cross the plasma membrane of a cell by diffusion.
  • If a molecule can cross the membrane, the membrane is said to be permeable to that molecule.
  • If a molecule cannot cross the membrane, the membrane is not permeable (is impermeable) to that molecule.
Permeability also comes in degrees. That is, a membrane may be more permeable to one type of molecule than another (even though it is "permeable" to both). If a membrane is more permeable to a molecule, it’s easier for that molecule to diffuse across the membrane. If a membrane is less permeable to a molecule, it’s harder for that molecule to diffuse across the membrane.
In this article, we'll see how a neuron establishes and maintains a stable voltage across its membrane – that is, a resting membrane potential.

The resting membrane potential

Imagine taking two electrodes and placing one on the outside and the other on the inside of the plasma membrane of a living cell. If you did this, you would measure an electrical potential difference, or voltage, between the electrodes. This electrical potential difference is called the membrane potential.
At a conceptual level, voltage means the same thing in a neuron as it does in an electrical circuit. However, current in wires is carried by electrons. In contrast, in neurons and other cells, current is carried through the movement of ions. These may include both positively charged ions (cations) and negatively charged ions (anions).
Diagram of a voltmeter measuring the membrane potential. One electrode is outside the cell. The other electrode is in the interior of the cell. The voltmeter shows a -70 mV voltage across the membrane.
_Image modified from "How neurons communicate: Figure 2," by OpenStax College, Biology (CC BY 4.0)._
Like distance, potential difference is measured relative to a reference point. In the case of distance, the reference point might be a city. For instance, we can say that Boston is 190190 miles\text{miles} northeast, but only if we know that our reference point is New York City.
For a cell’s membrane potential, the reference point is the outside of the cell. In most resting neurons, the potential difference across the membrane is about 3030 to 9090 mV\text{mV} (a mV\text{mV} is 1/10001/1000 of a volt), with the inside of the cell more negative than the outside. That is, neurons have a resting membrane potential (or simply, resting potential) of about 30-30 mV\text{mV} to 90- 90 mV\text{mV}.
Because there is a potential difference across the cell membrane, the membrane is said to be polarized.
  • If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized.
  • If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized.
Diagrams of voltmeters with one electrode inside the cell and one in the fluid outside of the cell. The first voltmeter shows hyperpolarization: it reads -80 mV. The second voltmeter shows the resting potential: it reads -70 mV. The third voltmeter shows depolarization: it reads +40 mV.
_Image modified from "How neurons communicate: Figure 2," by OpenStax College, Biology (CC BY 4.0)._
All of the electrical signals that neurons use to communicate are either depolarizations or hyperpolarizations from the resting membrane potential.

Where does the resting membrane potential come from?

The resting membrane potential is determined by the uneven distribution of ions (charged particles) between the inside and the outside of the cell, and by the different permeability of the membrane to different types of ions.

Types of ions found in neurons

In neurons and their surrounding fluid, the most abundant ions are:
  • Positively charged (cations): Sodium (Na+\text{Na}^+) and potassium (K+\text{K}^+)
  • Negatively charged (anions): Chloride (Cl\text{Cl}^-) and organic anions
In most neurons, K+\text{K}^+ and organic anions (such as those found in proteins and amino acids) are present at higher concentrations inside the cell than outside. In contrast, Na+\text{Na}^+ and Cl\text{Cl}^- are usually present at higher concentrations outside the cell. This means there are stable concentration gradients across the membrane for all of the most abundant ion types.
This diagram represents the relative concentrations of various ion types inside and outside of a neuron.
  • K+ is more concentrated inside than outside the cell.
  • Organic anions are more concentrated inside than outside the cell.
  • Cl- is more concentrated outside than inside the cell.
  • Na+ is more concentrated outside than inside the cell.

How ions cross the membrane

Because they are charged, ions can't pass directly through the hydrophobic ("water-fearing") lipid regions of the membrane. Instead, they have to use specialized channel proteins that provide a hydrophilic ("water-loving") tunnel across the membrane. Some channels, known as leak channels, are open in resting neurons. Others are closed in resting neurons and only open in response to a signal.
Ion channels. The channels extend from one side of the plasma membrane to the other and have a tunnel through the middle. The tunnel allows ions to cross. One of the channels shown allows Na+ ions to cross and is a sodium channel. The other channel allows K+ ions to cross and is a potassium channel. The channels simply give a path for the ions across the membrane, allowing them to move down any electrochemical gradients that may exist. The channels do not actively move ions from one side to the other of the membrane.
Some ion channels are highly selective for one type of ion, but others let various kinds of ions pass through. Ion channels that mainly allow K+\text{K}^+ to pass are called potassium channels, and ion channels that mainly allow Na+\text{Na}^+ to pass are called sodium channels.
There are chloride channels that allow Cl\text {Cl}^- ions to cross the plasma membrane, though we won't focus much on them in this article. The chloride channels are conceptually similar to the sodium and potassium channels described in the section above.
The situation is different for organic anions present in the interior of the cell. Often, these anions are negatively charged amino acid side chains in proteins. The proteins are typically large and bulky and remain trapped inside the cell. Thus, organic anions generally cannot cross the membrane like Na+\text {Na}^+ and K+\text {K}^+.
In neurons, the resting membrane potential depends mainly on movement of K+\text {K}^+ through potassium leak channels. Let's see how this works.

What happens if only K+\text K^+ can cross the membrane?

The membrane potential of a resting neuron is primarily determined by the movement of K+\text K^+ ions across the membrane. So, let's get a feeling for how the membrane potential works by seeing what would happen in a case where only K+\text K^+ can cross the membrane.
We'll start out with K+\text{K}^+ at a higher concentration inside the cell than in the surrounding fluid, just as for a regular neuron. (Other ions are also present, including anions that counterbalance the positive charge on K+\text K^+, but they will not be able to cross the membrane in our example.)
Starting state:
Zero voltage across the membrane, as measured by a voltmeter with one electrode inside and one electrode outside the cell. The inside of the cell and the outside of the cell are separated by a membrane with potassium channels, which are initially closed. There is a higher concentration of potassium ions on the inside of the cell than on the outside. Each potassium ion (on either side of the membrane) is balanced by an anion, so the system as a whole is electrically neutral.
For clarity, the diagrams in this section show only K+\text K^+ and the anions (negatively charged ions) that counterbalance the positive charge on K+\text K^+. We don't say exactly what types of anions these are, so some of them may be Cl\text {Cl}^- ions.
Many other ions that are not shown in the diagram may be present in the system, including Cl\text {Cl}^- and Na+\text {Na}^+. However, since these ions cannot cross the membrane, they will not influence the membrane potential. This is why we can ignore them as we focus on the special case where the membrane is permeable only to K+\text K^+.
If potassium channels in the membrane open, K+\text K^+ will begin to move down its concentration gradient and out of the cell. Every time a K+\text{K}^+ ion leaves the cell, the cell's interior loses a positive charge. Because of this, a slight excess of positive charge builds up on the outside of the cell membrane, and a slight excess of negative charge builds up on the inside. That is, the inside of the cell becomes negative relative to the outside, setting up a difference in electrical potential across the membrane.
System moving towards equilibrium:
If K+ can cross via channels, it will begin to move down its concentration gradient and out of the cell. (Channels are shown opening, potassium is shown moving from the interior to the exterior of the cell through channels.)
The movement of K+ ions down their concentration gradient creates a charge imbalance across the membrane. (The potassium ions that have crossed from the interior to the exterior of the cell are not partnered with anions on the outside of the cell. They line up along the membrane on the outside, and the unpartnered anions they left behind on the inside line up along the membrane on its inside face. The voltmeter now registers a slight negative voltage.)
The charge imbalance opposes the flow of K+ down the concentration gradient.
For ions (as for magnets), like charges repel each other and unlike charges attract. So, the establishment of the electrical potential difference across the membrane makes it harder for the remaining K+\text K^+ ions to leave the cell. Positively charged K+\text K^+ ions will be attracted to the free negative charges on the inside of the cell membrane and repelled by the positive charges on the outside, opposing their movement down the concentration gradient. The electrical and diffusional forces that influence movement of K+\text K^+ across the membrane jointly form its electrochemical gradient (the gradient of potential energy that determines in which direction K+\text K^+ will flow spontaneously).
Eventually, the electrical potential difference across the cell membrane builds up to a high enough level that the electrical force driving K+\text{K}^+ back into the cell is equal to the chemical force driving K+\text{K}^+ out of the cell. When the potential difference across the cell membrane reaches this point, there is no net movement of K+\text{K}^+ in either direction, and the system is considered to be in equilibrium. Every time one K+\text{K}^+ leaves the cell, another K+\text{K}^+ will enter it.
At equilibrium:
At equilibrium, the concentration gradient of K+ is exactly balanced by the electrical potential difference across the membrane. Although K+ ions still cross the membrane via channels, there is no net movement of K+ from one side to the other. The voltmeter registers a negative membrane potential that is equal to the K+ equilibrium potential (for the K+ concentrations present in the cell and in the surrounding fluid).

The equilibrium potential

The electrical potential difference across the cell membrane that exactly balances the concentration gradient for an ion is known as the equilibrium potential. Because the system is in equilibrium, the membrane potential will tend to stay at the equilibrium potential. For a cell where there is only one permeant ionic species (only one type of ion that can cross the membrane), the resting membrane potential will equal the equilibrium potential for that ion.
The steeper the concentration gradient is, the larger the electrical potential that balances it has to be. You can get an intuitive feeling for this by imagining the ion concentrations on either side of the membrane as hills of different sizes and thinking of the equilibrium potential as the force you'd need to exert to keep a boulder from rolling down the slopes between them.
Left panel: Two compartments separated by a semi-permeable membrane, labeled A and B. There is a voltmeter between A and B. The ion of interest is much more concentrated in A than in B, and the voltmeter with electrodes in A and B registers a large negative voltage. The voltage is analogous to the force we would have to exert to keep a boulder from rolling from a very high place down a hill to a very low place.
Right panel: Same setup, but with A and B having a much slighter difference in concentration of the ion of interest (B slightly less concentrated than A). In this case, the voltage is only slightly negative. This is analogous to the case where we have a very high place and a slightly lower place and are exerting a force to keep a boulder from rolling down this not-very-steep hill.
If you know the K+\text{K}^+ concentration on both sides of the cell membrane, then you can predict the size of the potassium equilibrium potential.

Does membrane potential equal K+\text K^+ equilibrium potential?

In glial cells, which are the support cells of the nervous system, the resting membrane potential is equal to the K+\text K^+ equilibrium potential.
In neurons, however, the resting membrane potential is close but not identical to the K+\text K^+ equilibrium potential. Instead, under physiological conditions (conditions like those in the body), neuron resting membrane potentials are slightly less negative than the K+\text K^+ equilibrium potential.
What does that mean? In a neuron, other types of ions besides K+\text K^+ must contribute significantly to the resting membrane potential.
Suppose that you did an experiment in which you measure the resting membrane potential of a glial cell while systematically changing the K+\text K^+ concentration of the extracellular fluid. If you did this and graphed the results, you would find that the resting membrane potential of the glial cell was consistently equal to the potassium equilibrium potential. That suggests that, in glial cells, the resting membrane potential is determined solely by the resting permeability to K+\text {K}^+.
If you repeated the experiment with a neuron, you would find that the measured resting membrane potential was close to the potassium equilibrium potential, but deviated from it a bit (especially when the concentration of K+\text K^+ outside the cell was low). The resting membrane potential under physiological conditions is less negative than the potassium equilibrium potential. This tells us that, for neurons, the K+\text K^+ permeability and K+\text K^+ concentration gradient are not the only factors that determine the resting membrane potential.
Graph depicting membrane potential (mV) on the Y-axis and extracellular [K+] (mM) on the X-axis.
The potassium equilibrium potential forms a straight diagonal line with a positive slope on this graph.
The actual membrane potential of an neuron follows the potassium equilibrium potential in most of the graph. However, it deviates at low K+ concentrations in that it is higher (less negative) than the potassium equilibrium potential.
This discrepancy tells us that ions other than K+ are also involved.

Both K+\text K^+ and Na+\text {Na}^+ contribute to resting potential in neurons

As it turns out, most resting neurons are permeable to Na+\text {Na}^+ and Cl\text{Cl}^- as well as K+\text K^+. Permeability to Na+\text{Na}^+, in particular, is the main reason why the resting membrane potential is different from the potassium equilibrium potential.
Let’s go back to our model of a cell permeable to just one type of ion and imagine that Na+\text{Na}^+ (rather than K+\text K^+) is the only ion that can cross the membrane. Na+\text{Na}^+ is usually present at a much higher concentration outside of a cell than inside, so it will move down its concentration gradient into the cell, making the interior of the cell positive relative to the outside.
Because of this, the sodium equilibrium potential—the electrical potential difference across the cell membrane that exactly balances the Na+\text{Na}^+ concentration gradient—will be positive. So, in a system where Na+\text{Na}^+ is the only permeant ion, the membrane potential will be positive.
Starting state:
Zero voltage across the membrane, as measured by a voltmeter with one electrode inside and one electrode outside the cell. The inside of the cell has a low concentration of sodium ions, and the outside of the cell has a higher concentration of sodium ions. Each sodium ion is counterbalanced by an anion that is found on the same side of the membrane as the sodium ion. There are sodium channels in the membrane, but they are initially closed.
The channels open and Na+ can move through them.
At equilibrium:
The voltmeter now registers a positive voltage equal to the sodium equilibrium potential for this particular pair of sodium concentrations.. The Na+ ions have moved down their concentration gradient until their further movement is opposed by a countervailing electrical potential difference across the membrane. There are extra positive charges on the inside of the cell in the form of Na+ ions, and these Na+ ions line up along the membrane. On the opposite side of the membrane, there are extra anions (the former partners of the Na+ ions, which are unable to cross), which also line up at the membrane.
In a resting neuron, both Na+\text{Na}^+ and K+\text{K}^+ are permeant, or able to cross the membrane.
  • Na+\text{Na}^+ will try to drag the membrane potential toward its (positive) equilibrium potential.
  • K+\text{K}^+ will try to drag the membrane potential toward its (negative) equilibrium potential.
You can think of this as being like a tug-of-war. The real membrane potential will be in between the Na+\text{Na}^+ equilibrium potential and the K+\text{K}^+ equilibrium potential. However, it will be closer to the equilibrium potential of the ion type with higher permeability (the one that can more readily cross the membrane).
The membrane potential is a weighted mean of the equilibrium potentials of the different permeant ions.
If only one permeant ionic species is present, the membrane potential will be determined by that ion's equilibrium potential. It doesn't matter how permeable it is (how readily it can cross the membrane), because there is nothing to counter it.
If multiple permeant ionic species are present (and not at equilibrium), the resting potential will be between the equilibrium potentials for the different permeant ions. The greater the permeability to a given ionic species, the more it will dominate the final membrane potential.
For the membrane potential to be constant, the Na+\text{Na}^+ current has to equal the K+\text{K}^+ current (assuming Cl\text {Cl}^- is at equilibrium, which is reasonable in many cells). Current equals permeability times driving force (driving force is the difference between the membrane potential and the ion's equilibrium potential), which is essentially Ohm's law. If the permeability to one ion is much greater than that of the other, the driving force has to be less (i.e., the membrane potential must be closer to that ion's equilibrium potential) in order for the currents to be equal.

Opening and closing ion channels alters the membrane potential

In a neuron, the resting membrane potential is closer to the potassium equilibrium potential than it is to the sodium equilibrium potential. That's because the resting membrane is much more permeable to K+\text K^+ than to Na+\text {Na}^+.
  • If more potassium channels were to open up—making it even easier for K+\text{K}^+ to cross the cell membrane—the membrane would hyperpolarize, getting even closer to the potassium equilibrium potential.
  • If, on the other hand, additional sodium channels were to open up—making it easier for Na+\text{Na}^+ to cross the membrane—the cell membrane would depolarize toward the sodium equilibrium potential.
Changing the number of open ion channels provides a way to control the cell’s membrane potential and a great way to produce electrical signals. (We will see the opening and closing of channels again when we discuss action potentials.)

The Na+\text{Na}^+-K+\text K^+pump maintains Na+\text{Na}^+ and K+\text{K}^+ gradients

The Na+\text{Na}^+ and K+\text{K}^+ concentration gradients across the membrane of the cell (and thus, the resting membrane potential) are maintained by the activity of a protein called the Na+\text{Na}^+-K+\text K^+ ATPase, often referred to as the sodium-potassium pump. If the Na+\text{Na}^+-K+\text K^+pump is shut down, the Na+\text{Na}^+ and K+\text K^+ concentration gradients will dissipate, and so will the membrane potential.
At the resting membrane potential of a neuron, neither Na+\text {Na}^+ nor K+\text K^+ is at its equilibrium potential. The membrane potential is less negative than the K+\text K^+ equilibrium potential, but less positive than the Na+\text {Na}^+ equilibrium potential. Thus, there will be a steady leak of K+\text K^+ out of the cell and of Na+\text {Na}^+ into the cell. The activity of the pump opposes these leaks and maintains the ions' concentration gradients.
In addition, the pump also has to deal with the Na+\text{Na}^+ and K+\text K^+ that cross the membrane when a neuron fires an action potential.
Like the ion channels that allow Na+\text{Na}^+ and K+\text K^+ to cross the cell membrane, the Na+\text{Na}^+-K+\text K^+ pump is a membrane-spanning protein. Unlike potassium channels and sodium channels, however, the Na+\text{Na}^+-K+\text K^+ pump doesn’t just give Na+\text {Na}^+ and K+\text K^+ a way to move down their electrochemical gradients. Instead, it actively transports Na+\text{Na}^+ and K+\text{K}^+ against their electrochemical gradients.
The energy for this "uphill" movement comes from ATP hydrolysis (the splitting of ATP into ADP and inorganic phosphate). For every molecule of ATP that's broke down, 33 Na+\text{Na}^+ ions are moved from the inside to the outside of the cell, and 22 K+\text K^+ ions are moved from the outside to the inside.
  1. Three sodium ions bind to the sodium-potassium pump, which is open to the interior of the cell.
  2. The pump hydrolyzes ATP, phosphorylating itself (attaching a phosphate group to itself) and releasing ATP. This phosphorylation event causes a shape change in the pump, in which it closes off on the inside of the cell and opens up to the exterior of the cell. The three sodium ions are released, and two potassium ions bind to the interior of the pump.
  3. The binding of the potassium ions triggers another shape change in the pump, which loses its phosphate group and returns to its inward-facing shape. The potassium ions are released into the interior of the cell, and the pump cycle can begin again.
_Image modified from "The sodium-potassium exchange pump," by Blausen staff (CC BY 3.0)._

The sodium-potassium pump cycle

  1. To begin, the pump is open to the inside of the cell. In this form, the pump binds readily to Na+\text {Na}^+ ions (has a high affinity for them) and will take up three of them.
  2. When the Na+\text {Na}^+ ions bind, they trigger the pump to hydrolyze (break down) ATP. One phosphate group from ATP is attached to the pump, which is then said to be phosphorylated. ADP is released as a by-product.
  3. Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space. In this conformation, the pump no longer binds readily to Na+\text {Na}^+ ions (has a low affinity for them), so the three Na+\text {Na}^+ ions are released outside the cell.
  4. In its outward-facing form, the pump switches allegiances and now readily binds to (has a high affinity for) K+\text K^+ ions. It will bind two of them, and this triggers removal of the phosphate group attached to the pump in step 2.
  5. With the phosphate group gone, the pump will change back to its original form, opening towards the interior of the cell.
  6. In its inward-facing shape, the pump no longer readily binds to K+\text {K}^+ ions, so the two K+\text {K}^+ ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again.
Image credit: OpenStax Biology. Image modified from original work by Mariana Ruiz Villareal.
The sodium-potassium pump transports sodium out of and potassium into the cell in a repeating cycle of conformational (shape) changes. In each cycle, three sodium ions exit the cell, while two potassium ions enter. This process takes place in the following steps:
  1. To begin, the pump is open to the inside of the cell. In this form, the pump binds readily to Na+\text {Na}^+ ions (has a high affinity for them) and will take up three of them.
  2. When the Na+\text {Na}^+ ions bind, they trigger the pump to hydrolyze (break down) ATP. One phosphate group from ATP is attached to the pump, which is then said to be phosphorylated. ADP is released as a by-product.
  3. Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space. In this conformation, the pump no longer binds readily to Na+\text {Na}^+ ions (has a low affinity for them), so the three Na+\text {Na}^+ ions are released outside the cell.
  4. In its outward-facing form, the pump switches allegiances and now readily binds to (has a high affinity for) K+\text K^+ ions. It will bind two of them, and this triggers removal of the phosphate group attached to the pump in step 2.
  5. With the phosphate group gone, the pump will change back to its original form, opening towards the interior of the cell.
  6. In its inward-facing shape, the pump no longer readily binds to K+\text {K}^+ ions, so the two K+\text {K}^+ ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again.
This may seem complicated, but it actually just involves the protein going back and forth between two forms: an inward-facing form with high affinity for sodium (and low affinity for potassium) and an outward-facing form with high affinity for potassium (and low affinity for sodium). The protein can be toggled back and forth between these forms by the addition or removal of a phosphate group, which is in turn controlled by the binding of the ions to be transported.
The contents of this pop-up is a modified derivative of “Active transport,” by OpenStax College, Biology (CC BY 3.0). Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:25/Biology.
Because 33 Na+\text{Na}^+ are exported for every 22 K+\text K^+ brought into the cell, the pump makes a small direct contribution to the resting membrane potential (making it slightly more negative than it would otherwise be). The pump's big contribution to the membrane potential, however, is indirect: It maintains steady Na+\text {Na}^+ and K+\text K^+ gradients, which give rise to the membrane potential as Na+\text {Na}^+ and K+\text K^+ move down their respective concentration gradients through leak channels.
The Na+\text{Na}^+-K+\text K^+ pump also plays a crucial role in maintaining the osmotic balance of the cell. Without the pump, intracellular osmolarity would exceed extracellular osmolarity at electrochemical equilibrium: Water would rush into the cell and the cell would burst!
From a pump-centric perspective, the resting membrane potential and the whole neuronal signaling apparatus can be viewed as a side effect of the cell’s attempt to avoid osmotic lysis.
This article is licensed under a CC BY-NC-SA 4.0 license.

Works cited:

  1. Purves, D., Augustine, G.J., Fitzpatrick, D., Katz, L.C., LaMantia, A.-S., McNamara, J.O., and Williams, S. M. (1997). Figure 2.3. Electrochemical equilibrium. In Neuroscience (2nd ed.). Sunderland, MA: Sinauer Associates. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK11054/figure/A134/?report=objectonly.

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