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In the resting membrane potential of neurons the inside of membrane is kept negative and outside of membrane is kept positive by the utilization of energy through Na-K Atpase pump, While during action membrane potential through the opening of specific gated ion channels inside of membrane is made positive while outside is made negative, What is the significance of specific charges across the membrane, does these specific charges across the membrane play any role in the passage of neurotransmitters ? or these charges are just arbitrary ?does this can happen that during resting membrane potential the inside will become positive and outside will become negative and vice versa, I want to know that what is the significance of these specific charges across the membrane ..?

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    $\begingroup$ I'm not sure how far back your confusion goes. 1. A membrane potential is defined as the difference in charge across the memb. Do you understand that? 2. The charge diff is not just the NaK Pump, but leaky K channel. Do you know that? 3. Have you looked at youtube videos of action potentials leading to neurotransmitter release? What part confuses you? $\endgroup$ – Adrienne Jun 2 '14 at 19:09
  • $\begingroup$ I m not confused about the points that you have written above, I m clear about these points, I just want to know the significance of These specific charges across the membrane..? That why there is need to maintain negative membrane potential by using Atp..?What is Significance of Negative membrane potential? $\endgroup$ – katherinebridges Jun 3 '14 at 13:46
  • $\begingroup$ Just wanted to point out that the K channel open near the resting potential isn't "leaky". Like all other ion channels, it allows a passive flow of ions so it's not actively pumping but just allowing things to happen. If you like, you can call that "leakage" but then you'd also have the leaky sodium current, leaky calcium current etc. What's more, there are already things called "leak currents" which are currents caused by ions slowly permeating the membrane through other means than the main pump, exchanger or ion channel currents. $\endgroup$ – Michael Clerx Jun 3 '14 at 18:03
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I've tried to split this into the answer to three questions:

  1. Why does a membrane potential arise at all?
  2. Why does it happen to be around -80mV
  3. How does the cell use its membrane potential

(1) You've indicated that you know this one already, but I'd still like to point a few things out: The NaK pump not only creates a negative resting potential, it also seriously unbalances the Sodium and Potassium concentrations on each side of the cell. Even without any voltage, diffusion forces would cause an inward sodium current and an outward potassium current if you made holes in the membrane. The NaK pump (and the other active pumps/exchangers) are much slower than the passive currents through ion channels when they open.

(2) The resting potential is a dynamical equilibrium. This means that the potential you end up with is simply the one where all the opposing forces are equal. If you were designing a new cell from scratch you could put it at other values than around -80mV without too much trouble. In the cells we have, there's this region around -80mV where sodium and calcium channels are closed but a particular Potassium channel is wide open. Because there is much more Potassium inside the cell than out, diffusion drives Potassium out of the channel. However, because K+ has a positive charge the negative potential drives the K+ right back in. These two forces are able to balance each other out at around -80mV.

(3) A cell uses its membrane potential in two important ways: Firstly, when you trigger an action potential (AP) in one cell, it almost immediately triggers an AP in its neighbours. This allows rapid communication between neurons in your brain, but also allows for the coordinated contraction of your heart. Secondly, the action potential opens up Calcium channels which let Calcium flow into the cell. The free calcium concentration within a cell acts as a second messenger initiating all sorts of actions. For example, in muscles the increased Calcium triggers a contraction: muscle cells contract during their AP. In neurons, the presence of Ca2+ in a cell strongly increases the release of neurotransmitters.

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Answering the question: "Why is a negative membrane potential important?"

I don't think cells have strong feelings about being slightly negative inside its membrane. I'm guessing these are the driving factors:

  • the cell membrane is nonpolar, so charged particles can only pass via transport proteins. This makes ion gradients an easy way to maintain a source of (literally, in the physics sense) potential energy.
  • the cell needs to control its environment and send signals to other cells. The particle gradient (sodium, for example), allows it to accomplish these tasks: bring in glucose by co-transporting with sodium, send an action potential by triggering voltage-gated ion channels.
  • Therefore, the Na/K pump is a way to "charge" the ability of the cell to perform various types of work in the future.

To risk an analogy, a cell having a membrane potential is like you charging your cell in the morning. You aren't quite sure what exactly you'll be doing with your phone all day, but it will be many different tasks and all will be doable because you've got a controlled energy source.

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