Central Nervous System
+ve and –ve charges inside a cell is relatively small compared to the total number of charged species (ions and larger molecules) in the cell.
The membrane potential of excitable cells (skeletal and cardiac muscles and nerve cells) is of the order of -70 mV. The negative value means that there is an excess of negative charge inside the cell. In smooth muscle, a typical membrane potential is -55 mV and in erythrocytes it is about -9 mV. Some key pieces of information to help with the understanding of this topic are as follows:
- The distribution of K+and Na+ ions inside and outside cells is shown in the diagram below
- Nerve and muscle cells are very permeable to K+ions but, at rest, not very permeable to Na+ Erythrocytes are much closer to equal permeability to the two ions
- The negative charge on the inside of cells cannot be attributed to the distribution of Cl– Intracellular [Cl–] is low (20 mmoles/L) compared to extracellular [Cl–] (105 mmoles/L). The excess negative charge inside cells is mainly held on the ionisable side groups of amino acids in proteins and on phosphate groups. These large charged molecules cannot escape across cell membranes
The distribution of K+ and Na+ ions inside and outside cells
What is the effect of a rise in extracellular [K+] on the resting potential of cells?
Cells become depolarised, this means that the resting potential moves towards zero.
A rise in extracellular [K+] reduces the concentration gradient between the intracellular and extracellular compartments. The tendency for K+ ions to leave the cell would therefore be reduced and the membrane potential would be less negative.
Another way of looking at this is that the potassium equilibrium potential (EK) would be smaller.
A fall in extracellular [K+] would correspondingly hyperpolarise the cell and EK would be more negative.
The resting potential of a cell is primarily determined by the concentration gradient for K+ across the cell membrane and the permeability to K+ ions.