This page is divided into the following sections:
All cells are separated from their environments by their cell membrane. This membrane, a plasma membrane, is good as a barrier between the intracellular and extracellular environments because of its basis in a lipid bilayer. Up until now, we have been thinking of this separation as a separation of two liquid environments. But now, you have to add a new way of thinking about this separation: the cell membrane also serves to separate two electrical environments.
The intracellular and extracellular compartments have slightly different charges. This difference is based on the electrically-charged components, ions, within each compartment. To begin to understand this, let's think about the fluids within the intracellular and extracellular compartments. There are many dissolved ions in these fluids. For example, the extracellular solution contains more sodium and chloride ions (as if our cells were all being bathed in a more salty (NaCl), like the sea, environment). They exist as ions there, rather than as the salt molecule NaCl, because all salts dissolve in a watery environment. The intracellular solution contains more potassium ions. All of these ions are small, and could possibly find a way to cross the membrane (using facilitated diffusion... remember?).
The intracellular compartment also contains some negatively charged molecules, ones that are larger and cannot cross the membrane. It is because of these intracellularly-trapped, negatively charged molecules that the intracellular compartment ends up being a bit more negative than the extracellular compartment.
Try to work through the above paragraphs to understand them. I know they are heavy with chemistry ideas and diffusion ideas, but you should be able to follow the logic I tried to create. The end result of all that information is the understanding that the intracellular compartment is more negative in its charge than the extracellular compartment.
You are familiar with 9-volt batteries. You know that if you cause the 9-volt charge to surge through your body, it would not be a comfortable experience (I never did this, but some people like to feel the zap... some people are crazy).
Well, one volt (abbreviated 1 V) is equal to one thousand millivolts (abbreviated 1000 mV). That is just like 1 m = 1000 mm in the length measurements. Electrical charges in cells are measured in mV, and the typical, normal difference between the intracellular and extracellular compartments is about 70 to 80 mV.
Because the inside is more negative than the outside, we can say that the intracellular compartment is at -70 to -80 mV, while the extracellular compartment is at 0 mV. The extracellular compartment is so huge-- it includes all the extracellular fluids of the body; its charge cannot really every change because it is so big. That's the other reason we call it 0 mV. Then, the only charge you have to consider is the intracellular one.
The negative charge that exists intracellularly when a neuron is just hanging out, and not electrically active, is the resting potential. In discussions of electrical charges, the word "potential" is always used to refer to a charge. So, the neuron has a resting potential, not a resting charge.
For the purposes of simplifying our discussion, I'll refer to the resting potential as -70 mV. Just keep in mind that it is typically -70 to -80 mV.
Also, the resting potential exists when the cell is not electrically active, but that doesn't mean that the cell isn't doing anything. It might be making proteins, undergoing cellular respiration, etc.
All cells, not just neurons, have a resting potential. But in neurons (and in muscle fibers) it is very important to understand the resting potential, since all electrical activity will be changes from this resting potential.
The negative charge inside and zero charge outside has to be maintained as part of homeostasis. So, how is that done?
I mentioned in the introduction to this page that there are certain ions that are found inside, and others that are found outside the neuron. Let's take a closer look now. Here's a drawing using that same neuron I drew for the neuron web page. Here we are looking in more detail at the cell body of this neuron. You can see that potassium ions (K+) tend to be in higher concentration inside the neuron than outside. Also, sodium ions (Na+) tend to be in higher concentration outside the neuron than inside.
The concentration difference is approximately ten-fold for both ions, so there is about 10 times the potassium ion concentration inside the neuron than outside. And there is about 10 times the sodium ion concentration outside the neuron as inside. Don't forget that there are large, negatively charged molecules inside the neuron as well (they are not drawn in here).
It is the maintenance of these concentration differences that, for the most part, maintains the difference in charge between the two compartments. Therefore, we will have to maintain the concentration differences to maintain the resting potential.
The big differences in concentration of these two ions cause very strong concentration gradients to exist for both of them. To illustrate that, I have made some more drawings, zooming in on one portion of the neuronal membrane.
In this drawing, the membrane is still illustrated as a blue line. The sodium and potassium ions are drawn in the appropriate compartments.
In which direction is the concentration gradient for sodium? In which direction is the concentration gradient for potassium? Can you tell? I figured that you could, but to make sure that I drove home this point, I took this drawing and animated it to show the directions of the concentration gradients.
At this point, stop and consider the following steps in logic:
These steps bring us to the next section.
With such HUGE gradients on these ions, why don't they just diffuse down their gradients? There is a huge driving force on them to do that.
Keep in mind that in order to diffuse, they have to cross the membrane by facilitated diffusion (because charged ions or molecules cannot cross the lipid bilayer unassisted). So, there have to be transport channels in the membrane for them. If there were tons of these channels, then the ions would diffuse rapidly down their concentration gradients, and the normal distribution of ions would disappear.
Neurons do not have many of these transport channels. They have very few. Here's a drawing that shows more detail of the plasma membrane:
In this drawing, five transport proteins are shown within the lipid bilayer. The two at the far left, with the question marks in them, are proteins that we will investigate next week. The other green, light blue, and purple channels are the ones we are going to work on now. The phospholipids of the lipid bilayer are drawn in dark blue, just like the membranes on the previous images.
Imagine that the green channel (without the "?") in the middle is a pore that is always open to allow potassium ions to cross. When a potassium ion encounters this pore, it will cross the membrane, by facilitated diffusion, in the direction of its concentration gradient (out of the cell). Imagine that the purple channel (without the "?") at the far right is a similar pore, but one that is specific for only letting sodium ions cross. When sodium ions encounter this pore, they will cross the membrane in the direction of their concentration gradient (into the cell).
When ions diffuse through these pores, they begin to ruin the normal distribution of ions. This type of movement of the ions is called leaking. And the pores that allow leaking to occur are called leak pores. Because this leaking is not a good thing for the cell, there are very, very few of these leak pores. And, to counteract them, there has to be a way to return the ions back to their appropriate compartments.
Returning a sodium ion to the extracellular fluid is moving it AGAINST its concentration gradient. Movement against the concentration gradient has to be accomplished by active transport, which requires protein pumps and ATP. There is one pump that simultaneously moves sodium ions back out and potassium ions back in. It is called the sodium-potassium pump. This pump is the one shown in light blue in my diagram.
Therefore, as sodium and potassium ions leak (just a bit) through the few leak pores that exist for them, the sodium-potassium pump shoves them back into their appropriate compartments. This constant leaking and shoving back is what keeps the concentrations of these ions steady. This is what maintains the concentration gradients on both ions. I have made an animation of this entire process... it's as good as I could do it (another 2 hour animation!). I hope it helps:
You should now be able to understand how the concentration gradients on these ions are maintained. It is time to rap this up with a few details...
You may have noticed in the animation above that every time the sodium-potassium pump operates, it pumps 2 K+ in and 3 Na+ out. That is not an even exchange, right?
By pumping more positive charges out than in, this pump actually helps to maintain a more negative (less positive) environment inside the neuron. This characteristic of the sodium-potassium pump causes it to be assigned the label of an electrogenic pump. The prefex "electro-" just stands for an electric charge, while the suffix "-genic" means generating. So, the term electrogenic means "electric charge generating." That is appropriate for the sodium-potassium pump since it helps to generate the normal, resting charge (or, resting potential) in neurons.
Now you should know what a resting potential is and how it is maintained. Please spend time working on this to understand it, since it is the basis for all that we will do in our next unit.
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