A fascinating hypothesis is that the high intracellular potassium concentration (and low sodium) is a remnant of the very first proto-cells that evolved on Earth. These cells had neither ion-tight membranes nor membrane pumps, so their intracellular environment would resemble that of the surroundings. The early, most basic cellular machineries thus evolved under ionic conditions similar to those in the primordial pond, but as life spread to different environments, cells evolved sophisticated pumps and transporters that allowed the maintenance of an intracellular solution similar to that found where life originated. 

Ref: http://www.pnas.org/content/early/2012/02/08/1117774109

Hardly my specialty but in case I am correct, I say the following. Given the relative size of the extracellular space (sea) to a cell, the extracellular sodium concentration will essentially be constant regardless of the amount of the ion movement across the membrane. So for the same amount of sodium ions pumped in or out, a cell attains a larger electrical potential by pumping them out according to the Nernst equation.

A very good question. In general a cell’s membrane potential is determined relatively by various types of ion channels open at resting stage. In glial cells, at resting stage cell membrane is “relatively more permeable to potassium ion (K+) than any other ions”, as a result K+ accumulates inside the cells. However because of concentration gradient K+ also starts moving out of the cell. Finally two forces electrical (positive and negative charges) and chemical (Concentration gradient) balance each out and this potential is called equilibrium potential and in case of glial cells because of relative selectivity to potassium ions the membrane potential is close to K+ equilibrium potential.

But in neurons the resting membrane is selectively open for other ions also like sodium and this is where Nernst equation becomes valuable. And if you plot all the values you get a membrane potential where all the forces balance out each other and neurons membrane potential is set at negative 70 mV. From biological point of view, majority of the cell’s energy is spent on maintain membrane potential and from an evolutionary point of view cells choose the most efficient method to expend energy. Also equilibrium potential of sodium is positive imagine the energy required to keep the membrane potential so high above zero. Entry of sodium causes depolarization, increase in intracellular calcium which then can activate wide range of signaling pathways and cause release of neurotransmitters. And this non selective response may not be suitable for proper functioning of cells.

I hope I was able to answer your question. I recommend the book Principles of Neural Science by Kandel. The chapter membrane potential will help you answer the question better.

.....membrane relatively more permeable to potassium ion (K+)...in resting ..electrical gradient(-ve inside) ...K+ accumulates inside (not Na+).... written by gourab roy chowdhury is the right answer.

I think that non of the answers are appropriate... if I understand the question it is about WHY is more Na ions outside and K ions inside of cell...not how is that regulated...everyone that has degree in medical/biological sciences must know that mechanism because its ABC of cell functioning :) 

The answer is: evolution. Once the first cell evolved with certain channels of certain permeabilities, and certain ion pumps, the early history froze the fact and all excitable cells are now this way. But of course it is possible to think any other combination, and this alternative combination could have been the winner.  I used to give alternative ion mechanisms of fictional "extraterrestrial" cells in electrophysiology exams. In that way it is possible to differenciate between mere memory and reasoning.

A fascinating hypothesis is that the high intracellular potassium concentration (and low sodium) is a remnant of the very first proto-cells that evolved on Earth. These cells had neither ion-tight membranes nor membrane pumps, so their intracellular environment would resemble that of the surroundings. The early, most basic cellular machineries thus evolved under ionic conditions similar to those in the primordial pond, but as life spread to different environments, cells evolved sophisticated pumps and transporters that allowed the maintenance of an intracellular solution similar to that found where life originated. 

Ref: http://www.pnas.org/content/early/2012/02/08/1117774109

I thoroughly agree Gaurav Roy Chaudhuri  In case of marine alga which are sorruonded by sea water rich with Na ion concentration  K ions are accumulated more compare to Na ions

Dear Gaurav,

as far as I can decude from the header of your question, you are asking why there is more K than Na inside the neuron. The answer is: Na/K pump activity (driving extrusion of sodium) and higher permeability of the plasma membrane for K than Na (determining the resting membrane potential according to Nernst potential of K derived from the electrochemical gradient), as explained by William Cromwell and Gourav Roy Choudhury.  

However, if you were asking why it is K that drives the resting membrane potential, not Na, I would recommend diving into phylogenetics of voltage-gated Na/K/Ca channels and consider the reference cited by Michael Jakob Voldsgaard Clausen.

Could you eventually specify your question?

All the best,

Jochen

I completely agree with the answer of Roberto Etchenique, the fact that Na is chosen be pumped out the cell and not K, in my view, responds to the fact that this situation was the first one to appear on earth, it was advantegeous for some forms of protolife, and therefore "selected". However, other combinations could be possible.

In my view, the primary functional mechanism to be seen as advantageous was: the maintenance of cell (or proto-cell) volume. 

As to my knowledge (I might be wrong, im not an specialist in Na/K pumps nor in its evolution) one really curious thing, with profounds phyligenetic implications responds to the utilization of these pumps as a mechanism to mantain and regulate cell volume, 

In the begining of life protoanimals utilized such a mechanism for aiding in maintaining cellular volume, with the energetic costs associated in terms of ATP. Pumping out 3 cations (Na) and  2 in (K) helped (still it does) in driving out the water that otherwise will burst the cell by inward difussion. This situation indirectly generated an electric pontential in cell membranes that evolution could later shape into the nervous system.

Plants, on the other hand, "chose" for another much cheaper way to maintain cellular volume (both plants and animals share the common "problem" of diving out water form cells) they constructed a cell wall that prevents the burst of the cell. 

Not sure if this is also the reason why plants do not have a nervous system, or this responds only to hazard (to my knolwdge plants membranes are also electrically charged). 

Hope it helps, cheers,

David

Adding to David's answer: The hydration shell of K+ ions are bigger, and are better fit in Pottasium pumps found in bacteria. Na+ ions are smaller and thus excluded. The pore-size that can be formed by protein channels depends on the proximity of four oxygen atoms in the filter. The bigger the pore size, smaller is the repulsion of oxygen atoms. Hence the larger pore-size and consequently, a larger ion gets selected.   

http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2003/popular.html

osmotic pressure is the reson

Experimental evidence showed by MacKinnon and Peter Agre that  hydration shell  of K with four water molecles allows K ion  to pass the ion channel  by selective catalysed transport, however the  smaller hydrated shell of Na excluds the transfer.

I wrote my answer to this question, made a pdf, and then read David Tamayo's answer. My answer is essentially the same. I have attached it here.

All the above hypothesis are fascinating, but the question is how do we explain that only the exitable cells which are considered to be highly specialised have retained the membrane transport mechanisms just similar to the proto cells and not the other cells.

Well, all animal cells have the sodium/potassium pump. Not just excitable cells. All animal cells have a potential difference across the membrane (negative inside) because of a higher resting conductance to potassium than any other ion. The degree of negativity varies though, depending on the relative dominance of potassium conductance over other possible inward conductances. Excitable cells probably have the highest voltages across their membranes. The difference between excitable (muscle-skeletal, cardiac and smooth; nerve and some neuroendocrine cells like adrenal chromaffin and pancreatic beta cells) and non-excitable cells is the presence of either voltage-gated sodium channels or voltage-gated calcium channels which pass inward currents and allow them to develop action potentials - which are periods of brief positivity ( of the order of milliseconds)  in membrane potential.

Dear scientist friend Biology is not going to answer why for anything , in Biology you are just explaining how a phenomenon is going to be explained.

That is the meaning of experimental sciences.

you should put Why for philosophers.

thank you for attention

I think Dr. Michael Jakob Voldsgaard Clausen has very well answered your question. I would suggest you do some reading about these at least from the Net. Wikipedia has got fantastic articles on this. You will soon discover that you can sit and write a few books on this topic.

Other living organisms use a strategy different from animal cells and do not pump out sodium.  Plants, fungi and bacteria simply pump out protons.  This, for the same benefit as what is gained from the sodium/potassium pump.  It creates a charge difference, which is exploited to do work; mostly, the transport of molecules against their concentration gradient.   In plants, sucrose, which has a mass far greater than a proton, is moved into a cell with an already higher sucrose concentration because the proton, really the charge of the proton, is moving down its concentration gradient.  The positive charged proton enters the more negatively charged cell and the sucrose is moved up its concentration gradient.  It is  an electrical gradient powering the system. 

Animal cells do this with sodium ions.  Because they are moved up their concentration gradient and because more + charges are moved out of the cell, the cell again behaves like a battery (or capacitor), which again can be used to move uncharged molecules like glucose and amino acids up their concentration gradients. 

In my opinion, the answer as to why sodium is low in the cell has more to do with why it is high outside the cell.  The sodium must be transported up its concentration gradient in order to generate usable energy for transport and other functions.  If the concentration of sodium was low outside, it would simply be moving down its concentration gradient and no potential energy would be stored by the movement.  An old reason stated for why serum has more sodium was that animal live evolved in the sea.

What is most fascinating is the role of electricity in organisms.  The weight of all the electrons on and withing our planet weighs just a fraction of a gram.  Yet look at what just a fraction of the electrons can do in terms of work.

Normal drinking water rich in sodium and less in potassium, so obviously movement of ions is possible simply by diffusion from extracellular region to cell for sodium and vice versa for potassium.  Moreover most of the metabolic enzymes require sodium as cofactor compared to potassium.

Because of these reasons, intracellular potassium concentration high compared to sodium

extra cellular fluid has higher osmtic pressure than intra celluar ,sodium maintains it

I did not understand what Charles wanted to say when he wrote: "The weight of all the electrons on and withing our planet weighs just a fraction of a gram. Yet look at what just a fraction of the electrons can do in terms of work."  Electrons are really powerful but  the weight of all electrons in our planet is huge, since each electron has a mass of about 1/2000 of a hydrogen atom.  In a human body, the mass of all electrons is small but not alt all negligible: about 20 grams.

necessity....,must be necessity......

@ Gaurav Kumar ，Are you still working on this wonderful question?