A few random questions :-) (A2)

1.) how is an action potential generated? I know about voltage gated sodium channels opening and the membrane depolarizing etc, but how does a receptor start the AP?
2.) whats the difference between an epithelium and an endothelium, how can I tell which is which?
3.) can anyone briefly explain the role of the loop of henle please? (ascending and descending limb) I bit confused about what its purpose is
4.) how does myleination increase the speed of the action potential? since Na+ has to diffuse from node to node to continue the AP rather from sodium channel to sodium channel it seems like this would of made it slower?

thanks

3.) can anyone briefly explain the role of the loop of henle please? (ascending and descending limb) I bit confused about what its purpose is

To summarise, the Loop of Henle's main role is water reabsorption. The mechanism that makes this possible is called the countercurrent multiplier.

As you know, there is a descending limb and then an ascending limb. Weirdly, it's easiest to consider the ascending limb's role first.

The Ascending Limb

The ascending limb is permeable to sodium and potassium ions, yet impermeable to water. These two facts are very important. Sodium and potassium ions are pumped out by active transport. This reduces the water potential of the medulla because clearly there is now a high concentration of ions. This brings us on to...

The Descending Limb

The descending limb is the opposite to the ascending limb: it is permeable to water but impermeable to sodium and potassium ions. Now, considering the medulla has a low water potential for the reasons I just explained, water leaves down a water potential gradient into the medulla via osmosis.

This water is then reabsorbed by the surrounding capillaries: job done. Hope this helps!

Edit: By the way, it's worth remembering that animals that live in dry conditions (such as camels) have longer Loops of Henle than other organisms. This is because they need to reabsorb more water as they have to survive without drinking water for longer.

1) A typical receptor at a synapse is known as a ligand gated ion channel - it works very similarly to voltage gated sodium channels, but instead of being opened by depolarisation it's opened by binding a neurotransmitter - when neurotransmitter is released at a synapse it binds the receptor, opening the channels, and starting the process of depolarisation.

2) Endothelium lines blood and lymphatic vessels. Epithelium lines body cavities and structures. Endothelium is a specialised type of epithelium.

3) Argh first year physiology, where have you gone? I'll think about this and answer this evening. It's all about countercurrent multiplication I'm sure...

4) This is something that I've always accepted despite never really being more happy with an answer than "Physics says so". My basic understanding is that myelin insulates the nerve, preventing leakage of sodium/charge/something and that the diffusion along the short distances between nodes of Ranvier is quick enough for this to function. At the nodes themselves there is a very high concentration of sodium channels, so a large amount of charge is created, allowing it to propogate that far, whereas in unmyelinated nerves the channels are spread evenly and the charge quickly escapes meaning it has to open and close channels all the way along the nerve to propagate slowly, rather than jumping between nodes.

thanks for the reply! i'm still not happy with 4) but I suppose ill just accept it. xD btw, for the receptor question i mean things pressure receptors on skin, not the receptors on the postsynaptic membrane, sorry probably should of been clearer