Resting Membrane Potential

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 Kdiffusion.

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, Kdecreased 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 Kmovement 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 Kmovement 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

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