BJ Notes Flashcards
What is Resting Membrane Potential? Explain its genesis.
Resting Membrane Potential is the potential difference that exist across the membrane of an excitable cell in its
resting state.
Normal Value:-In case of large nerve fibers and muscle fibers it is -90 mv i.e. inside of the membrane is
negative with respect to outside.
Genesis of R.M.P:
RMP basically results from accumulation of more negatively charge ions on the inner surface of cell membrane
and more positive ions on its outer surface. This unequal distribution of charges results from.
A)Diffusion Potentials created by conc. Difference of ions across the cell
membrane:-
1.Role of K+ ions: Normally the conc. Of K+ in ICF is 140 meq/liter and in
ECF, it is 4 meq/lit. Therefore k+ ions tend to diffuse out of the cell. At the
same time, outward diffusion of K+ ions will create a negativity inside the cell
which will oppose the outward movement of K+ ions. Thus a state of
equilibrium will be reached in which there will be no net ionic flow across the
membrane. The Potential developed at this stage is called ‘Equilibrium
potential’ or ‘Nernst Potential ’for K+ ions. Its value depends upon conc.
gradient and can be calculated by using Nernst equation as follows.
Ek+ (mv)= RT/F Z X log K+ conc. Inside/conc. outside
where R is gas constant, T absolute temperature, F is Faraday and z is
valency of ion.
(Simplified form E(K+)= ±61 log K+ conc. inside/K+ conc. outside)
Equilibrium potential for K+ ions is -94 mv which is very close to RMP
indicating that K+ diffusion potential contributes significantly to RMP. One
of the reasons for this is greater permeability of membrane to K+ ions
compared to Na+ ions.
2.Role of Na+ ions:-Concentration of Na+ ions in ECF is 142 meq/1 and in
ICF only 14 meq/1. Therefore Na+ ions tend to diffuse into the cell. However,
the resulting positivity in the cell will oppose the entry of more Na+ ions.
Nernst potential for Na+ ions is +61 mv which is far away from RMP indicating that Na+ diffusion Potential
contribution is very little towards RMP.
3.Role of Chloride ions:-Equilibrium potential for Cl ions is –70 mv, however their contribution to RMP is
insignificant because of their passive diffusion secondary to movement of Na+ and K+ ions. Diffusion potential
for K+ an Na+ together account for membrane potential of – 86 mv.
B)Selective Permeability of cell membrane:-Resting cell membrane is 50 to 100 times more permeable to K+
ions due to K+ leak channels. Besides, the cell membrane is impermeable to certain negatively charged ions in
the cell such as proteins, sulphates and organic phosphates. So they remain inside the cell and contribute to
the negativity.
The membrane potential which results from the sum total of above factors is given by Goldman’s constant field
equation which consider the conc. difference of ions as well as their membrane permeability.
C)Role of Na+ K+ pump:- It is a protein present in cell membrane which pumps 3 Na+ ions out of cell and 2
K+ ions into the cell. It contributes to RMP in 2 ways-
1.Indirectly,by maintaining conc. gradient of Na+ and K+ ions across cell membrane.
2.Directly,by acting as an electrogenic pump. It causes net loss of positive ions from cell and contributes -4 mv
to RMP.
Significance of RMP:
1. It forms the basis of action Potential which is necessary for impulse transmission.
2. A change in RMP of a receptor cell is called Receptor Potential essential for transmission of sensation.
3.Resting negativity also helps in absorption of positive ions like Na+ in renal epithelial cells.
