- Anatomia neuronu
- Przegląd budowy i funkcji neuronu
- Potencjał czynnościowy
- Potencjał czynnościowy
- Przewodzenie skokowe w neuronach
- Neuroprzekaźniki i receptory
- Pytania i odpowiedzi: Depolaryzacja neuronów, hiperpolaryzacja i zmiana potencjału.
- Przegląd ogólny funkcji kory mózgowej
How the resting membrane potential is established in a neuron.
- 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 and . 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 than to , so the resting potential is close to the equilibrium potential of (the potential that would be generated by if it were the only ion in the system).
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 and ). 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 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.
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.
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 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 to (a is 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 to .
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.
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 () and potassium ()
- Negatively charged (anions): Chloride () and organic anions
In most neurons, and organic anions (such as those found in proteins and amino acids) are present at higher concentrations inside the cell than outside. In contrast, and 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 to pass are called potassium channels, and ion channels that mainly allow to pass are called sodium channels.
In neurons, the resting membrane potential depends mainly on movement of through potassium leak channels. Let's see how this works.
What happens if only can cross the membrane?
The membrane potential of a resting neuron is primarily determined by the movement of 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 can cross the membrane.
We'll start out with 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 , but they will not be able to cross the membrane in our example.)
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.
If potassium channels in the membrane open, will begin to move down its concentration gradient and out of the cell. Every time a 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 ions to leave the cell. Positively charged 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 across the membrane jointly form its electrochemical gradient (the gradient of potential energy that determines in which direction will flow spontaneously).
Eventually, the electrical potential difference across the cell membrane builds up to a high enough level that the electrical force driving back into the cell is equal to the chemical force driving out of the cell. When the potential difference across the cell membrane reaches this point, there is no net movement of in either direction, and the system is considered to be in equilibrium. Every time one leaves the cell, another will enter it.
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 concentration on both sides of the cell membrane, then you can predict the size of the potassium equilibrium potential.
Does membrane potential equal equilibrium potential?
In glial cells, which are the support cells of the nervous system, the resting membrane potential is equal to the equilibrium potential.
In neurons, however, the resting membrane potential is close but not identical to the equilibrium potential. Instead, under physiological conditions (conditions like those in the body), neuron resting membrane potentials are slightly less negative than the equilibrium potential.
What does that mean? In a neuron, other types of ions besides must contribute significantly to the resting membrane potential.
Both and contribute to resting potential in neurons
As it turns out, most resting neurons are permeable to and as well as . Permeability to , 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 (rather than ) is the only ion that can cross the membrane. 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 concentration gradient—will be positive. So, in a system where is the only permeant ion, the membrane potential will be positive.
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.
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 and are permeant, or able to cross the membrane.
- will try to drag the membrane potential toward its (positive) equilibrium potential.
- 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 equilibrium potential and the 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).
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 than to .
- If more potassium channels were to open up—making it even easier for 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 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 -pump maintains and gradients
The and concentration gradients across the membrane of the cell (and thus, the resting membrane potential) are maintained by the activity of a protein called the - ATPase, often referred to as the sodium-potassium pump. If the -pump is shut down, the and concentration gradients will dissipate, and so will the membrane potential.
Like the ion channels that allow and to cross the cell membrane, the - pump is a membrane-spanning protein. Unlike potassium channels and sodium channels, however, the - pump doesn’t just give and a way to move down their electrochemical gradients. Instead, it actively transports and 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, ions are moved from the inside to the outside of the cell, and ions are moved from the outside to the inside.
- Trzy jony sodu wiążą się z pompą sodowo-potasową, która jest otwarta do wnętrza komórki.
- Pompa hydrolizuje ATP, sama się fosforylując (przyłączając do siebie grupę fosforanową) i uwalniając ADP. Fosforylacja powoduje zmianę kształtu pompy, która zamyka się do wnętrza komórki i otwiera się na zewnątrz komórki. Trzy jony sodu są uwalniane, a dwa jony potasu wiążą się z wnętrzem pompy
- Wiązanie jonów potasu powoduje kolejną zmianę kształtu pompy, która traci swoją grupę fosforanową i powraca do kształtu otwartego do wnętrza komórki. Jony potasu są uwalniane do wnętrza komórki, a cykl pompy może się rozpocząć ponownie.
Because are exported for every 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 and gradients, which give rise to the membrane potential as and move down their respective concentration gradients through leak channels.
Chcesz dołączyć do dyskusji?
Rozumiesz angielski? Kliknij tutaj, aby zobaczyć więcej dyskusji na angielskiej wersji strony Khan Academy.
Na razie brak głosów w dyskusji