Originally posted by hypnagogue
How is the Na+/K+ equilibrium maintained? From that graph it looks like Na+ always comes in and K+ always goes out. How/when do the reverse processes happen?
There is an ATP-dependent (i.e., requires the cell to expend energy) pump that moves sodium and potassium in the opposite direction of the diffusion.
Zoobyshoe, as you're reading through all that material, keep in mind the system you're studying is very dynamic. In other words, ions are moving all the time. It is the relative ratio of ions on the inside of the cell compared to outside the cell that is determining the charge. You can achieve a "negative" membrane potential by having less positive ions inside the cell than outside. It doesn't necessarily require having more negative ions. That's a concept that many college students have had trouble with, and if you understand that, then the rest gets a lot easier.
When a neuron is resting, there is a lot of sodium on the outside and a lot of potassium on the inside. There is also a little sodium on the inside and a little potassium on the outside. There are also negatively charged anions inside the cell (lots of them) and lots of chloride outside the cell (also negatively charged). It is easier for potassium to flow out of the cell than for sodium to flow into the cell, so overall, you get more positive ions flowing out than in. That is what gives you a negative resting potential...inside the cell winds up more negative than the outside because the positively charged potassium has flowed out.
When an appropriate stimulus comes along, such as a neurotransmitter binding to its receptor, this can open a "gate" on a sodium channel (a structure in the membrane that sodium can move through). This allows sodium to move very quickly into the cell and make the inside more positive. If enough sodium moves into reach a threshold, then this is what provides the stimulation to open lots more sodium channels and change the inside of the cell to a more positive charge than the outside. This rapid change to a relative positive charge inside compared to outside the cell is what is called the action potential. The cell then "recovers" by closing the sodium channels and opening potassium channels to let potassium out and make the cell more negative on the inside again (by letting out the positive ions). Because potassium channels are somewhat slow to close, you will get a hyperpolarization, in other words, more potassium will go out than is necessary to return to just the resting potential, so you wind up with an even lower charge inside the cell than you started with. Once the potassium channels close, the cell returns to its resting potential.
The propagation of this down the axon means that this process doesn't happen in the whole cell all at one time, but a depolarization and repolarization in one place triggers the next segment to depolarize. This is why the hyperpolarization at the end of an action potential is important. This helps prevent the action potentials from moving backward instead of forward down the axon.
Calcium comes into play at a synapse, which is the way two neurons communicate with each other. Most commonly, these are chemical synapses. Think of the neuron terminal as containing lots of tiny balloons (synaptic vesicles) filled with chemicals (neurotransmitters). Calcium helps these balloons attach to the membrane and open up a place to dump out their contents into the very tiny space between two cells (synaptic cleft). These chemicals then can attach to a receptor on the next cell, and this event opens up sodium channels and starts the whole process over again in the next cell.
Yes, this is a lot to absorb all at once. Now I hope you see why I didn't jump straight into this explanation until you had some pictures to look at. Hopefully now that you have some pictures in front of you, and have been reading the text, this will help clarify some places that tend to be tricky to understand. I'm glad you're finding it so fascinating! It's really fun stuff!