The reply is in..

Had the reply from Prof today. Just when I'm about to re-revise the topic again..

Somehow it's frightening to study in the library. There's this gloomy, stressful, heavy aura that surrounds everyone. Bottles of water, cookies, sweets, files, books, laptops, jackets.. it seems that they're planning to camp for days. Ha.. well, I happen to be one of them..


Dear Prof,

I would appreciate it if you could look over my understanding about neurones and membrane resting potential.

Resting potential:
At resting, the membrane potential is at -71mV, which is near E(K) which is -75mV. (this is correct.) This anomaly is caused by two reasons:
1. all background (leak) K+ channels open, allowing an efflux of K+ down its concentration gradient. By itself, this causes the membrane potential to be at -75mV. However,
2. some Na+ background channels are open, allowing Na+ to move down it's concentration gradient. This cancels some of the potential generated by the movement of K+ out of the cell. (Yes, it is correct.)

At the same time, the membrane is not permeable to anions such as proteins and this causes a separation of charge across the membrane. Their interaction causes the generation of an electrical voltage . (Yes, that is correct.)

Also, the Na+/K+ pump actively pumps K+ into the cell and Na+ out to mantain the Na and K gradient directly, and thus maintaining indirectly the the resting membrane potential.

So, this takes place in all animal cells? (Yes.) And the voltage-gated Na+ channels are not involve in the maintenance of resting potential? (No, they do not. And, not all cell types has Na+ channels. Different types of Na channels can exist in different regions of the neuron.)

Generation of an action potential:

When a stimulus reaches the receptor membrane of a recepter neuron, the Na+ channels open proportionately to the log of the intensity of the stimulus which makes it graded. It is then transmitted electrotonically to the spike-generating zone of the axon of the neuron.

At the spike-generating zone, if the volatge reaches the threshold level, the voltage-gated Na+ channels undergo a comformational change, causing the gate to be open. This causes an influx of Na+ down its electrochemical (NOT concentration) gradient. The increase in permeability to Na+ causes more Na+ channels to open by positive feedback mechanism (No. that is not correct. Once the Na channels in a region experience a threshold voltage they will all open.) As a result, the membrane potential is depolarised and reaches close to +55mV.

However, in the axon, the inactivation gates of Na+ channels close after 0.5 msec (responds to the positive membrane potential and starts to close ) . Repolarisation then begins as the membrane becomes less impermeable to Na+. (No, this is incorrect.) repolarization occurrs because of the opening of the voltage dependent (or delayed) K channel which facilitates the efflux of K.)

Voltage-gated K+ channels starts to open, causing K+ to leave the cell and restore the resting potential. But this is repolariztion!

At this stage, wouldn't there be an upset over the concentration of of both Na+ and K+? Yes, but only a very tiny amount of change. My answer would be that the Na+/K+ pump has to do it's part in restoring the resting potential. No, to restore the Na and K concentration gradients.

I"m sorry if it's lengthy but it's hard to talk about one thing and not involving the rest. (Its OK.)
Thank you for sparking my interest in the phenomenons of the human body.

Thanks,
Fairoz