Membrane Potentials & Resting Membrane Potential

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Membrane potentials and resting membrane potential are important topics for the NEET PG exam as they form the basic foundation for understanding cellular physiology, and a thorough understanding of these concepts is crucial for the study of various physiological systems.

In addition to that, understanding membrane potentials and resting membrane potential is essential for integrating and linking the different physiological systems, including the nervous, cardiovascular, and renal systems.

Read this blog further and get a quick overview of this important physiology topic for NEET PG/NExT exam preparation.

Membrane Potential

Any cell inside the membrane is negatively charged compared to the exterior. If exterior ECF is taken to be zero, then the inside of the membrane has – ve charge. There are excess anions in the ICF in the form of protein & phosphates.

To be Remembered: When we talk of any potential of any cells, we are essentially talking about the inside of the membrane. RBCs, Epithelial cells: The potential is less negative, somewhere between -8 and -20 mV. Smooth muscle cells have a membrane potential of -35 to -45 mV. SA nodal cells have a membrane potential of -55 to -65 mV. Nerve has a membrane potential of -70 mV. Skeletal muscle & Purkinje fiber in heart has a membrane potential of -90mV.

Trans-membrane Electrical Gradient 

Important Information

• Hair cells are surrounded by Endolymph which is having excess of K+ Due to which its potential becomes + 80 mv, known as endocochlear or endolymphatic potential

WHAT IS RESTING MEMBRANE POTENTIAL?

Due to varying concentrations inside and outside the cell, the resting membrane potential is the outcome. The resting membrane potential is primarily determined by the difference in the quantity of positively charged potassium ions (K+) inside and outside the cell.Due to a net movement with the concentration gradient, K+ ions build up inside the cell when the membrane is at rest. By elevating the concentration of cations in the extracellular fluid relative to the cytoplasm outside the cell, the negative resting membrane potential is established and sustained. The cell membrane's greater permeability to potassium ion movement than to sodium ion transport results in the negative charge present within the cell.The potassium and sodium cations can diffuse down their concentration gradients because the cell has leaky channels that allow them to do so.

The number of potassium leakage channels in neurons is significantly higher than that of sodium leakage channels. Potassium therefore diffuses out of the cell considerably more quickly than sodium seeps in. The cell's interior is negatively charged in comparison to the cell's exterior because more cations are exiting the cell than are entering. Once the resting potential is established, the sodium potassium pump contributes to its maintenance. Remember that for every ATP expended by sodium potassium pumps, two K+ ions are brought into the cell and three Na+ ions are removed. The internal charge of the cell remains negative compared to the extracellular fluid because more cations are released from the cell than are taken in. It should be noted that chloride ions (Cl-) are attracted to negatively charged proteins in the cytoplasm and tend to accumulate outside the cell.

Origin & development of the RMP (- 90 mv)

• Equilibrium potential for an individual ion: It means only that ion is allowed to move & other ions are not allowed to move.
• It is calculated by the Nernst Equation:

EMF (mV) = ±61×log(c1/c2)

C₁ & C₂ → Concentration of that individual ion on either side of the membrane.

• Na+(141/14) 

• Equilibrium potential for sodium = + 61 mV + because sodium is a cation & by initial concentration Na is going to come in & carry positive charges.
• Equilibrium potential for K+ = - 96 mV
• (K⁺ is more on the inside, low on the outside. So from initial concentration it will go from inside to outside, carry +ve charges to the outside & excessive negative charges to the inside. So charge on the membrane in equilibrium will be –ve.

• If only Na⁺ moves & reaches equilibrium, charge on the membrane would be + 61 mV.
• If only K+ moves & reaches equilibrium, charge on the membrane would be – 96 mV.
• Charge on RMP is –90 mV which means K⁺ ion has contributed maximum.
• Therefore, K+ diffusion contributes maximally to the development of RMP because the membrane is much more permeable to potassium.
• When we allow Na⁺ and K⁺ to move simultaneously K+ movement will be so great that it will be the main contributing force, the most determining force for the development of the RMP.  
• Ep for Cl^- = -89 mV  (closest to the RMP)
  • It means when the membrane is at RMP, Cl^- will remain in equilibrium.
  • Least movement will be of Cl^- when the membrane is resting at the RMP.
• When the membrane potential is closer to the equilibrium potential for an ion, like –90 & –89, then if we allow the ion movement, ions will not move much & remain at equilibrium.
• When the membrane potential is far away from the equilibrium potential of that ion, then the ion movement will be great.
• When all 3 ions are allowed to move & reach equilibrium simultaneously, there will be two determining factors governing their movements:

1. Concentration gradient of those individual ions
2. Relative membrane permeability

• Goldman’s Constant Field Equation
  • It is also called Goldman – Hodgkin – Katz Equation or Hodgkin – Huxley Equation 
    • We take the concentration gradient of three ions and their membrane permeabilities. When we put these in the equation, we get a value of –86 mV.
    • When 3 ions move simultaneously & reach equilibrium, then the charge on the membrane will be –86 mV. In this, potassium will be the greatest contributor. The remaining –4 mV will be contributed by Na⁺ - K⁺ pump. It is an electrogenic pump & contributes –4 mV to the development of RMP.

Important Information

Goldman's constant field/ Goldman's Hodgkin Katz Equation 

Considers 2 Factors 

• Concentration gradient of individual ions 
• Membrane permeability

Nerve 

• RMP of a nerve = – 70mV
  • Equilibrium potential for Na⁺ = + 60 mV
  • Equilibrium potential for K⁺ = –90 mV
  • Equilibrium potential for Cl- = -70 mV
• Na⁺ concentration will remain the same in the extracellular fluid but in the intracellular, the nerve will have a different value for Na⁺ and muscle will have a different volume of ICF sodium.
• RMP of nerve is –70 mV & Equilibrium potential for Cl^- is also –70 mV which means when the nerve is at RMP:
  • EP for Ca⁺⁺ = +129 mV (highest)
  • Ca⁺⁺ is a divalent ion. It brings double +ve charge 
  • EP for H⁺ = – 23 mV
  • EP for HCO₃^– = –24 mV

Also Read: CARBOHYDRATE & Amino Acid Metabolism DISORDERS

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