Introduction - electrical signalling in excitable cells

Animal cells (notably neurons, sensory cells and muscle cells) are made excitable in part through the operation of ion pumps that variously keep cytosolic concentrations of Na+, Cl- and Ca2+ low and cytosolic K+ concentration high. It should be noted that the cytosolic free concentration of Ca2+ is extremely low (0.1 (jtM in resting cells and about 10 (xM in excited cells) as compared to cytosolic concentrations of Na+, CP and K+ of about 10, 10 and 100 mM, respectively. The transmembrane potential (v tm) of animal cells is typically about —0.1 volt (V) (potential difference inside with respect to the outside), this being substantially due to internal constituents, selective membrane permeability and the operation of electrogenic ion pumps. Changes in the permeability of the cell membrane (plasma membrane, PM) to particular ions causes a change in v tm as described below. 

The transmembrane potential difference can be described by the Goldman equation that relates t tm to the permeabilities of the membrane to specific ions and the concentrations of such major ions on either side of the PM  

Let us suppose in the following illustrative example that the concentrations of Na+, CP and K+ inside the cell are 10, 10 and lOOmM, respectively and that the concentrations of Na+, CP and K+ outside the cell are 100, 100 and 10 mM, respectively. Let us further suppose that in the resting state t[ m is —0.05 V 

An increase in the permeability of the PM to sodium ions (Na+) permits Na+ to enter the cell down a concentration gradient with a consequent increase in the positive charge within the cell that opposes Na+ entry. At equilibrium there is no further net entry and v m approximates to the Nernst equilibrium potential (t]txj for Na+ given by the following equation (noting that. c =tlie charge on the ion (+1))  

Thus an increase in the permeability of the plasma membrane to Na+ (T a+) can be seen to have depolarized the cell t tm (i.e. made it more positive). 

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