Saturday, January 29, 2011

Action Potentials!

Hooray for a post with actual substance!

So here is a great picture of a typical neuron cell. This is taken from, a fantastic website with very clear diagrams.

So let's talk a bit about the morphology of this cell a bit. What we have is a normal cell that has a bit of an unusual shape and some specific changes that allow it to have a bit of a different function.

The cell body in this picture is the main portion of the cell, with the nucleus and all of the typical cellular machinery: mitochondria, rough endoplasmic reticulum, ribosomes, smooth endoplasmic reticulum, golgi apparati, and so forth according to the function of the cell.

It also has little processes that come off of the cell body called dendrites. These processes allow for signals that come in from other cells. Dendrites are how a cell receives information. Depending on the type of cell, these processes can either be concurrent with other cells, i.e. their cytoplasm is continuous from cell to cell, or there is a gap between cells called a synapse. Both are means by which signals are sent from cell to cell. We will come back to this concept.

See that long big process coming off of the cell body? That's an axon. Signals that are received from the dendrites travel down these axons and hit the axon terminals, which connect to dendrites or cell bodies of other cells depending on whether they are an electrical synapse (or continuous) or a chemical synapse (gap between cells).

Imagine you have a balloon filled with and submerged in water. The water inside the balloon has a different mix of stuff than the water outside the balloon. The stuff that we are talking about here are ions, or atoms with charge. If you aren't sure what an ion is, here is a clear article on the topic by Wikipedia.

Since the mixture of ions is different inside than outside, it is called a gradient. There are two different types of gradients: electrical, based on the overall charge of the mixture, and chemical, based on the overall concentration of the mixture.

Imagine that the balloon has little holes that allow the ions to move. The little holes can just stay open to let the ions move freely, or they can be regulated to only allow the ions to move at certain times. These holes are called channels. The ones that stay open all the time are called leak channels. They allow for small amounts of movement in and out of the cell. There are many types of channels that are regulated, but the ones we will concern ourselves with are channels that mainly allow only one type of ion through at different times, and pumps. Pumps allow for exchange of ions between the inside and the outside and depend on actual energy to operate. The best known is the sodium/potassium/ATPase pump, which burns the currency of metabolic energy, ATP, in order to exchange sodium (hereafter referred to by its atomic ion symbol, Na+) and potassium (K+) between the inside and the outside.

Our balloon analogy can only go so far here, so let's replace in our minds the balloon with a cell. The walls of the balloon are the walls of the cell, called a membrane. The channels are proteins that are embedded in the membrane and allow for passage of ions. The pump is a channel but it has a very specific purpose. The fluid inside of the cell is called the intracellular fluid (ICF) and the fluid outside is the extracellular fluid (ECF). This will become important later.

So bear in mind that leak channels allow ions through freely, but that there are many of these pumps that maintain concentration gradients. The question, of course, is what effect these pumps have on maintaining the electrical gradient.

Stay tuned for the next installment, where I will talk about setting up electrical gradients and how the membrane has a resting potential. I anticipate there being a part three that discusses what an action potential is and why they are important.


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