Before we can talk about the function of our neurons, we need to talk about ion channels, because those are critical for neuron function. So remember that in order for things to cross the cell membrane, particularly if they're polar or charged, they have to have some sort of transport protein. Very few things can go across the lipid bilayer on their own.
Well, some of the transport proteins are ion channels. meaning that they are transmembrane proteins or integral membrane proteins that have a pore in the middle for ions to move through. Now, some of these ion channels can be gated, which means they have like a little gate on one side or the other, and those gates can be opened or closed with a signal coming in. Now one type of those channels is what we call leakage channels.
Leakage channels alternate between open and closed, and these are for random movement of ions with their concentration gradient. So this one you're looking at has all the ions on the inside, so when it opens up, the ions want to diffuse and move outside. So that's called a leakage channel. You have sodium and potassium leakage channels, but it's important to note that...
potassium channels are much more numerous than sodium channels. So the potassium channels, because you have a lot more of them, means that potassium can leak through the membrane much more quickly than sodium can. Now you can have gated channels.
Gated channels don't open and close randomly. They open or close based on a signal. So ligand gated channels, a ligand is a small molecule. So for our intents and purposes, that's like a neurotransmitter or a hormone.
These neurotransmitters or hormones will actually bind to the protein. When they bind to the protein, that opens up the pore and allows the ions to move in or out of the cell. Now, these ion channels are highly specific, meaning you're going to have one for sodium, you have one for potassium, and so on and so forth.
So just because they open doesn't mean all ions move. Only the ions that are specific for that channel will move. Mechanically gated channels open usually by pressure or touch or some type of physical movement in the surrounding tissue is what causes them to open.
Thermoreceptors work very similarly, but they open based on local temperature change. Voltage gated channels will open when the membrane voltage gets to be a certain level. So this is the potential across the membrane. We're going to talk about how the membrane is going to have a net negative charge on the inside and a net positive charge on the outside. And that's called polarization because you have two different poles, a positive and a negative.
So when that polarization reaches a certain millivolt, that will cause the channel to open. All right, now you can measure this voltage difference. You put an electrode on the inside and one on the outside. And what you're measuring really is their difference in relation to each other. What is their relational difference?
Not necessarily the actual charge, but relatively the inside will be negative compared to the outside. Now this membrane potential or this difference is called resting membrane potential. Generally this is around minus 70 millivolts for most neurons and muscle cells, although it can vary, but that's a pretty good estimate is minus 70. Now the reason you have this positive outside, negative inside distribution is because you have unequal distribution of ions.
particularly mediated by the sodium potassium pump. Remember, a pump is something, it's an ATPase. It's something that's going to consume energy as it pumps ions. So the sodium potassium pump will pump three sodium out and two potassium in. So every time an ATP is cleaved, you're going to end up with one more positive charge.
Outside than inside, so this leads to this unequal distribution. Also, most of your anions, your negatively charged ions, stay inside the cell. They can't leave. Proteins, for example, there's lots of proteins in the cell, and they tend to be anionic.