Acetylcholine and its effect on depolarization of a cell

[donkeykong1]

I'm going over the neuroscience section in examcrackers and have a hard time figuring out what acetylcholine affects in terms of the depolarization of a cell. If it inhibits the opening of potassium cation channels wouldn't that cause a more probable depolarization? Doesn't a rapid depolarization occur when inhibiting chlorine anion channels as well?


I'm having a hard time considering the following entry wikepedia has on Acetylcholine. Maybe I'm not understanding what tonically-active currents are?

"Acetylcholine also has other effects on neurons. One effect is to cause a slow depolarization[citation needed] by blocking a tonically-active K+ current, which increases neuronal excitability."


source: http://en.wikipedia.org/wiki/Acetylcholine

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[Charles_Carmichael]

Acetylcholine opens ligand-gated channels (nicotinic receptors) at the neuromuscular junction (NMJ). This channel is permeable to both an influx (flowing into the cell) of Na+ and an efflux (flowing out of the cell) of K+. Both flow down their respective electrochemical gradients. Since the gradient for Na+ is generally larger, this generates depolarizations of the membrane (not full-blown action potentials). I'm not sure what you're talking about when you say ACh inhibits the opening of K+ channels. The channel it opens at the NMJ is permeable to both Na+ and K+.

I don't think a rapid depolarization would occur if Cl- channels are blocked. I think it would be a slower depolarization, but I'm not entirely sure on that. The reason depolarization occurs when Cl- channels are blocked is because it would prevent the movement of negative charge into the cell (or if you want to think of it another way, it's like moving positive charge out of the cell, thus making the membrane potential even more negative).

Tonically-active means that the channel is constantly open. These are the K+ leak channels that you may have heard about. These channels are the reason why cells are constantly leaking K+ out of themselves. Normally, this efflux of K+ means that positive charge is moving out of the cell and thus, the membrane potential is prevented from becoming more positive. However, if these leak channels are blocked, it would prevent the loss of positive charge. Since the K+ cannot leak out of the cell, positive charge builds up inside the cell. This is what leads to the depolarization of the membrane potential. And it's slow because, normally, the leakage of K+ occurs at a low rate. So, there's a slow build up of positive charge inside the cell when the leak channels are closed.

Hope this helps.

 

[sonnybobiche]

ACh has different effects on different cells. In the somatic nervous system, it generally serves to depolarize post-synaptic cells (skeletal muscle). In the autonomic, it generally depolarizes neurons, but hyperpolarizes smooth and cardiac muscle (think of the activity of the vagus nerve.)
The effect really depends on what the post-synaptic receptors do in response to the neurotransmitter. It is not the NT itself that makes anything happen.