Membrane Potentials & Resting Membrane Potential

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 exam preparation.

What is Membrane Potential?

Any cell inside the membrane is negatively charged compared to the exterior. If the exterior ECF is taken to be zero, then the inside of the membrane has a negative charge. Excess anions in the ICF are protein and phosphates.

When we talk of the potential of any cell, we are essentially talking about the inside of the membrane. RBCs and epithelial cells: The potential is less negative, 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. The nerve has a membrane potential of -70 mV. Skeletal muscle and Purkinje fiber in the heart have a membrane potential of -90 mV.

Trans-membrane Electrical Gradient

If skeletal muscle is—90 mV on the inside and exterior and is taken to be 0, it is a relative voltage. So, the trans-membrane electrical gradient will be 90 mV.  There is one cell in the body with a transmembrane voltage gradient of 150 mV, and that is the highest for any cell in the body. It is the hair cell in the labyrinth. It is the highest because inside the membrane is –70 mV, but outside is not zero. The hair cell in the labyrinth is surrounded by a very exceptional ECF called Endolymph. Endolymph has excess K⁺ ions.

It is an ECF & K⁺ should be low but here there are excess K⁺ ions over & above the ECF. So its voltage is not zero but something above zero because of these +ve charges. It has got +80 mV. It is called Endocochlear potential or Endolymphatic potential.

150 mV is the highest electrical gradient in the body which is for the hair cell. It makes it the most excitable cell in the body. Hair cells are surrounded by Endolymph, which is an excess of K+. Due to this its potential becomes + 80 mv, known as endocochlear or endolymphatic potential.

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?

 The difference in electrical potential across the plasma membrane when a cell is not stimulated or when it is relaxed is known as its resting potential. Traditionally, the difference between an electrical cell's internal and exterior potential has been stated as a percentage.

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×logc1c2

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

Na+14114 

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 and 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 and reaches equilibrium, the charge on the membrane would be + 61 mV. If only K+ moves and reaches equilibrium, the charge on the membrane would be – 96 mV. The charge on RMP is –90 mV, which means the K+ ion has contributed the 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 Cl- will remain in equilibrium when the membrane is at RMP. 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 the Goldman—Hodgkin—Katz Equation or the 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.

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

To study this topic in detail and practice related MCQs, download the PrepLadder app and get access to engaging video lectures and comprehensive study notes. 

Frequently Asked Questions

Q: Which of the following cells has the highest transmembrane electrical gradient?

Ans. Hair cell

Q: What are the 2 factors considered by Goldman's constant field/ Goldman's Hodgkin-Katz Equation?

Answer: The two factors are Considered. They are

• Concentration gradient of individual ions 
• Membrane permeability

Q: There is a cation. Its ECF concentration is 100 & ICF is 10. Calculate its equilibrium potential.

Ans. It is a cation & because of its concentration gradient, it will initially move from outside to inside. It will carry the positive charges to the inside & then eventually, it will reach equilibrium. When it reaches equilibrium, the charge on the membrane will be +. After putting the values in the equation, the equilibrium potential for this cation will be +61 mV.

Q: If there was an anion & its concentrations were 100 on the outside & 10 on the inside. Calculate the equilibrium potential for this anion.

Ans. – 61 mV Because all other things will remain the same. Since this is an anion & by its initial concentration, it will go from outside to inside & carry the negative charges to the inside. So, when it reaches equilibrium, the charge on the membrane will be –ve.

To study this topic in detail and practice related MCQs, download the PrepLadder app and get access to engaging video lectures and comprehensive study notes.