Resting Membrane Potential
RESTING MEMBRANE POTENTIAL
Introduction:
- “Membrane Potential” –
- Potential of inner surface of membrane with respect to its outer surface.
- Produced by passive diffusion of ions.
- Hence, also “Diffusion potential”.
- “Resting Membrane Potential (RMP)”-
- Potential of unstimulated cell.
Properties of cell membrane:
- Selectively permeable.
- Except K+ ions.
- Are freely permeable
- Due to potassium leak channels.
- Less Na2+permeability.
EVENTS IN RMP GENESIS:
Step 1 –
- K+ concentration,
- Very high inside cells.
- Very low in ECF.
- K+difuses out of cell –
- Along concentration gradient.
Step 2 –
- K+ diffusion produces an intracellular negativity.
- Also due to non-diffusible protein anion in cells.
- This effect not nullified by Na2+ diffusion in opposite direction & Cl– diffusion in same direction.
- Due to Na2+membrane impermeabliltiy.
- & Very high ECF Cl- concentration.
Step 3 –
- Intracellular negativity opposes outward K+ diffusion.
Step 4 –
- At equilibrium inner side of cell is negatively charged.
- RMP is generated.
FACTORS IN GENESIS OF RMP:
1. Difference between ICF & ECF potassium concentration –
- Important factor influencing RMP genesis.
- K+efflux for RMP generation due to chemical/concentration gradient.
- I.e., Diffusion from higher concentration/inside of cells to lower concentration/outside cell.
b. If K+ concentration is increased in ECF fluid:
- Depolarization occurs.
Mechanism:
- Increased ECF K+ conc. decreases conc. gradient.
- Hence, K+ decreased movement from ICF to ECF.
- Thus, inside of membrane retains some (+) ve charge.
Resulting in,
- Decreased RMP magnitude.
- Increased positivity/decreased negativity (depolarization).
- Opens voltage-gated Na2+ channels.
- Yet, ineffective for AP generation.
c. If concentration of K+ is decreased in ECF:
- Hyperpolarization occurs.
Mechanism:
- Decreased ECF K+ conc. increases conc. gradient.
- Hence, increased K+ movement to ECF.
Results in,
- Increased RMP magnitude.
- Decreased positivity/increased membrane negativity (hyperpolarization)
d. Any ECF positivity,
- Hampers K+ emigration from ICF to ECF due to repellant force.
- Increases RMP magnitude.
3. Protien membrane impermeability.
4. Poor sodium membrane permeability.
- Sodium-potassium pump:
- Maintains low ICF Na+ conc.
- Maintains net movement of positive charge out of cell.
- Pumps 3Na2+ out of cell & 2K+ into cell.
CALCULATION OF RMP:
- RMP value is calculated on basis of Nernst equation.
- Also referred “Nernst potential/Equilibrium potential/Diffusion potential of K+.
Effect of Na2+ & Cl– on RMP:
- Na2+ & Cl– diffuse into cells.
- Yet minimal effect observed.
- Each ion derives membrane potential on its own Nernst potential.
- Referred as “Equilibrium potential of membrane”.
- This is calculated by Goldman’s equation
Variables in Goldman’s equation:
- PK, PNa & PCl – Permeability of K+, Na2+& Cl–.
Condition 1:
- If only one ion (e.g., K+) is permeable –
- I.e., PNa & PCI are zero (Cl– & Na2+are not permeable).
- Then, Goldman equation reduces to Nernst equation.
Condition 2:
- If permeability of one ion far exceeds that of others –
- Equilibrium potential of membrane will approach Nernst potential of highly permeable ion.
IMPORTANT METRICS:
- Values of equilibrium potential/isoelectric potential/Nernst potential:
- Na2+= +60 mV.
- K+ =-90mV.
- Cl– =-70 mV.
- RMP in neurons is -70 mV
- Exactly same as Cl– equilibrium potential.
- Value close to K+ equilibrium potential.
RMP FOR VARIOUS EXCITABLE TISSUES:
- Neuron: -70 mV
- Skeletal muscle & Ventricle (cardiac muscle): -90 mV
- SA node: -30 to -40mV
- Smooth muscle: -30 to -50 mV
- Inner ear hair cell: -150 mV
- RBC: -10 mV
- Thyroid gland: -50mV
- Hair cells baseline membrane potential: – 60mV
Note:
- Though RMP values are different,
- Generation mechanism is same.
- i.e., Due to K+ concentration difference in ICF & ECF.
Exam Question
RESTING MEMBRANE POTENTIAL
- RMP is principally due to K+ concentration/chemical difference in ECF & ICF.
- In neurons, RMP is usually about -70 mV.
- Value close to K+ equilibrium potential.
- Exactly same as Cl– equilibrium potential.
- Due to negligible effect of other ions diffusing through membrane.
- Values of equilibrium potential/isoelectric potential/Nernst potential:
- Na2+– (+60 mV).
- K+– (-90mV).
- Cl–– (-70 mV)
CALCULATION OF RMP
- RMP magnitude calculated from Nernst equation/”Nernst potential/Equilibrium potential/Diffusion potential of K+.
Effect of Na2+ & Cl– on RMP:
- Each ion derives membrane potential on its own Nernst potential.
- Referred as “Equilibrium potential of membrane”.
- Calculated by Goldman’s equation
FACTORS IN GENESIS OF RMP:
1. Difference between ICF & ECF potassium concentration –
- Important factor influencing RMP genesis.
a. If increased ECF K+ concentration:
- Depolarization occurs.
Mechainsm:
- Decreases K+ movement to ECF.
- Decreases RMP magnitude.
- Increases positivity/decreases negativity (depolarization).
- Opens voltage-gated Na2+ channels.
b. If decreased ECF K+ concentration:
- Hyperpolarization occurs.
- Increases RMP magnitude.
- Decreases positivity/increases negativity of membrane (hyperpolarization)
2. Poor sodium membrane permeability:
- Sodium-potassium pump:
- Pumps 3Na2+ out of cell & 2K+ into cell.
RMP FOR VARIOUS EXCITABLE TISSUES:
- Neuron: -70 mV
- Ventricle (cardiac muscle) : -90 mV
- Smooth muscle: -30 to -50 mV
- Hair cells baseline membrane potential: – 60mV
Don’t Forget to Solve all the previous Year Question asked on Resting Membrane Potential
