1. Definition of terms Potential : The voltage difference between two points. Membrane Potential :The voltage difference between inside and outside of the cell. Resting Membrane Potential : The steady potential across membrane of a cell. Current: Movement of charges. Resistance: Resistance to movement of charges. Graded Potential: Action Potential: Equilibrium Potential: The potential difference (mV) across a membrane that produces enough force to oppose the movement of an ion along its concentration gradient is called equilibrium potential. brief occurrences of depolarization or hyper-polarization of a specific arena of the membrane A sudden change in membrane potential to positive and back to negative.

2. - In excitable cells the potential difference between inside and outside of the cell during rest is called “resting membrane potential” (r.m.p.) Resting Membrane potential Excitable cells: - Nerve cells - Muscle cells - Some endocrine cells - Some immune cells - Some reproductive cells Change in r.m.p occurs in these excitable cells to perform a function. Usually this change indicate a signal. For example nerve cells produce signals and propagate them by undergoing changes in their r.m.p.

3. - During rest in nerve cell negativity of inside of the cell is maintained at (-60 to -90 mV). - This results from the difference between concentrations of Na + and K + inside and outside of the cells which produces a potential difference across the membrane. 140 mM K + 4 mM 14 mM Na + 142 mM Resting Membrane potential (Nerve Cell) Nerve cell __ Organic _ __ A nerve cell Membrane potential = -90 mV

4. Resting Membrane Potential __ FIXED _ __ ATP 2 K + 3 Na + Gated Na+ Channel Gated K+ Channel Non-gated K+ Channel Sodium potassium leak channels Na + = 142 mM Na + = 14 mM K + = 4 mM K + = 140 mM _ +

5. Distribution of Charges The resting membrane potential is due to small excess of negative ions inside of the cell and small excess of positive ions outside of the cell. The negative charges are attracted to the positive charges and they form a thin layer of negative inside and positive outside of the cell. The rest of intracellular and extracellular fluid remain neutral.

6. Magnitude of Resting Membrane potential The magnitude of resting membrane potential depends on : 1)The difference in ionic concentrations between inside and outside of the cell, 2) Cell membrane permeability towards various ions. Permeability of the cell membrane for various ions can change due to the presence of specific ion channels in the cell membrane.

7. Magnitude of resting membrane potential in Nerve cells - The magnitude of r.m.p. for large nerve cells is -90 mV negative inside. The factors that determine this potential are: 1- Na/K pump 2- Leakage of Na and K through the nerve cell membrane: a- Contribution of the K diffusion (Equilibrium) potential b- Contribution of Na diffusion (Equilibrium) potential In large nerve cells the diffusion potential caused only by K and Na diffusion is approximately -86 mV. The other -4 mV potential is due to the action of Na/K pump.

8. Diffusion through Na+ and K+ channels contribute to about - 86 mV. Na/K pump contributes about -4 mV. EMF (mV) = - 61 log CiCi CoCo Na14 Na142 K140 K4 EMF (mV) for K = - 61 log 140 4 = - 61 log 35 = -61 x 1.54 = - 94 mV EMF (mV) for Na = - 61 log 14 142 = - 61 log 0.1 = - 61 x (- 1) = + 61 mV Because there are 75 to 100 times more K leak channels than Na leak channels therefore the sum of equilibrium potential is -86 mV. The rest ( -4 mV) is contribution by Na/K pump. -90 mv

9. Calculation of diffusion potential when membrane is permeable to several different ions Diffusion potential depends on:1) The charge of permeable ions 2) Permeability of the membrane to the ions 3) Concentration of ions (inside and outside) C Na i x P Na EMF (mV) = - 61 log + C K i x P K + C Cl o x P Cl C Na o x P Na + C K o x P K + C Cl i x P Cl C Na i = concentration of Na inside C Na o = concentration of Na outside P Na = Permeability towards Na C K i = concentration of K inside C K o = concentration of K outside P K = Permeability towards Ka C Cl i = concentration of Cl inside C Cl o = concentration of Cl outside P Cl = Permeability towards Cla Goldman-Hodgkin-Katz Equation

10. Example: Na + = 14 K + = 140 Cl - = 5 Na + = 142, K + = 4 Cl - = 115 C Na i x P Na EMF (mV) = - 61 log + C K i x P K + C Cl o x P Cl C Na o x P Na + C K o x P K + C Cl i x P Cl (14 x 0.01) EMF (mV) = - 61 log + (140 x 1) + (115 x 0) (142 x 0.01) + (4 x 1)+ (5 x 0) 140.14 EMF (mV) = - 61 log 5.42 = - 61 log 25.85 = - 61 x 1.41 EMF (mV) = - 86.1 mV Permeability to Cl = 0 Permeability ratio K : Na 100 : 1

11. Changes in Membrane Potential In excitable cells the resting membrane potential can change. There are specific terms used to indicate these changes in membrane potential. Depolarization : When membrane potential becomes less negative (goes towards positive) Hyperpolarization : When membrane potential becomes more negative than the r.m.p. Repolarization : When membrane that has been depolarized or hyperpolarized goes back towards r.m.p. Overshoot : When membrane Potential becomes positive.

12. Graded Potential When a stimulus produces graded potential: 2- outside of the cell positive charges in the extracellular fluid flow towards the stimulus site. 1- inside of the cell positive ions in the intracellular fluid flow away from the stimulus site. These movement of ions creates local currents

13. Graded Potential Graded potentials are changes in membrane potential within a very small region of plasma membrane and die out within 1-2 mm of their site of origin. Occur either as depolarizing or hyperpolarizing Greater stimulus produces greater magnitude (Graded) Become weaker as they go away from the site of origin Successive stimuli can cause summation of potentials