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Resting Membrane Potential Simulation: Section 2

The Main Factors Affecting Resting Membrane Potentials

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Introduction to the Main Affecting Factors

Two factors primarily influence the measured value of the resting membrane potential of excitable cells.

  1. Electrochemical gradients for potassium (K+), sodium (Na+), and chloride (Cl).
    • Concentration (chemical) gradients.
    • Electrical gradients.
  2. Relative permeabilities of the cell membrane to these same ions.

Electrochemical Gradients

Concentration Gradients

A concentration gradient for an ion occurs when the number of that particular ion differs on either side of a cell membrane. It is commonly referred to as a chemical gradient because it is concerned with the quantity of particles, irrespective of their charge.

In excitable cells, sodium and chloride ions are more concentrated outside the cell membrane, while potassium ions and large anions, such as phosphates and proteins, are more concentrated inside the cell membrane.

The differences in concentration cause the ions to move through the cell membrane along their respective gradients from areas of higher concentration to regions of lower concentration. As these charged particles shift positions, it alters the value of the resting membrane potential.

Ion Concentration Gradients

These ion concentration gradients result from different physiological processes.

  • The Na+/K+ pump actively maintains the respective concentration gradients for Na+ and K+.
  • Cl ions are passively distributed based on charge differences across the membrane resulting from the distribution of Na+ and K+.
  • Large intracellular anions (phosphates and proteins) are produced inside the cell and accumulate there due to their size, which prevents them from passing through the cell membrane.

Review Questions.

Question 1: What is an ion concentration gradient?

Question 2: What is another name for ion concentration gradients, and why is this name applied?

Question 3: Which ions have a higher concentration outside the cell membrane?

Question 4: Which ions have a higher concentration inside the cell membrane?

Question 5: How do the ion concentration gradients affect the resting membrane potential?

Question 6: What maintains the gradients for Na+ and K+?

Question 7: How is the gradient for Cl ions established?

Question 8: Why do large anions like phosphates and proteins accumulate inside the membrane?

Electrical Gradients

An electrical gradient develops when there is a difference in the electrical charge between the interior and exterior of a cell membrane. This imbalance develops when ions move across the cell membrane along their concentration gradient, causing particles with the same charge to accumulate on the opposite side of the membrane.

The increase in charge on the opposite side of the membrane repels similarly charged ions from entering. As ions continue to move across the cell membrane, the repelling force of the electrical gradient increases. When the opposing gradients reach an equilibrium, ions move back and forth across the cell membrane in equal proportions.

Concentration and Electrical Gradients at Equilibrium

concentration and electrical gradients for sodium, potassium, and chloride

Review Questions.

Question 1: What is an electrical gradient?

Question 2: How is an electrical gradient produced?

Question 3: How does the electrical gradient affect the movement of ions down their concentration gradient?

More About Concentration Gradient and Electrical Gradient Interactions

Let us investigate the interaction between concentration and electrical gradients using K+ ions. We will focus on K+ because the cell membranes of excitable cells are highly permeable to this ion, which significantly affects the resting membrane potential.

The following diagram shows a cell membrane consisting of a lipid bilayer without channels or other embedded proteins. A higher concentration of K+ ions exits inside the membrane than outside. This distribution produces an outward K+ concentration gradient.

Adding negatively charged particles (anions) eliminates the charge on either side of the membrane. Each is neutral because the number of anions equals the number of cations. Therefore, there is no electrical gradient, and the membrane potential is 0 mV.

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Review Questions.

Question 1: What conditions make both sides of the membrane neutral?

Question 2: Why is there no electrical gradient?

Question 3: Why is the membrane equilibrium potential 0 mV?

Adding potassium leak channels to the membrane provides a pathway for K+ ions to exit the cell along their concentration gradient while the anions remain in place. The outward flow of K+ ions makes the cell’s external environment positively charged and the internal environment negatively charged, creating an inward electrical gradient.

As K+ ions continue their outward flow, the charges on either side of the membrane increase. As a result, the growing inward electrical gradient creates an opposing force that makes it increasingly difficult for potassium ions (K+) to move across the cell membrane. The outer positive charge increasingly repels the K+ ions, while the inner negative charge increasingly attracts them.

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Review Questions.

Question 1: What pathway allows the K+ ions to cross the membrane?

Question 2: What causes the K+ ions to move outside the cell membrane?

Question 3: Does a concentration gradient exist for the anions? If so, what direction is the gradient?

Question 4: What prevents the anions from crossing the membrane?

Question 5: What causes an electrical gradient to develop for the K+ ions?

Question 6: In what direction is the electrical gradient for K+?

Question 7: What causes the magnitude of the electrical potential to grow?

An ion, like K+, attains an equilibrium state when the force of its electrical gradient equals the force of its opposing concentration gradient.

The electrical potential at this stage is the ion’s equilibrium potential, and its magnitude is directly proportional to the initial ion concentration. The larger the initial concentration gradient, the greater the equilibrium potential to offset the flow of ions through the membrane.

Although ions continue to move across the membrane at equilibrium, the exchange remains balanced, with no net gain on either side.

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Review Questions.

Question 1: At what point is an equilibrium achieved between an ion’s concentration and electrical gradients?

Question 2: What is an ion’s equilibrium potential?

Question 3: Do ions stop moving across the membrane once equilibrium is attained?

Ion Membrane Permeability

The concentration gradient is the force that drives ions across the cell membrane. However, the quantity of open ion channels in the membrane determines its permeability.

The permeability of a cell membrane to ions is comparable to how floodgates control water flow through a dam. In a dam, the water level represents the ion’s concentration gradient, with a higher concentration on one side and a lower concentration on the other. The dam’s floodgates are analogous to the ion channels in the cell membrane. When the floodgates are closed, water cannot flow through, just as ions cannot pass through a cell membrane when ion channels are absent. When the floodgates open, water flows from the side of higher concentration to the side of lower concentration, similar to ions moving down their concentration gradient through open leak channels. The amount of water that can flow through the dam is determined by the number of open floodgates, just as the number of open ion channels influences the rate of ion movement across the cell membrane.

Diagram comparing dam floodgates to membrane channel proteins

Below is a list of ions and their relative membrane permeability values. Because excitable cell membranes are most permeable to K+, this ion’s permeability value is 1. The Na+ and Cl values indicate how permeable these ions are relative to K+ (PK).

Membrane Permeability
Ion Relative Permeability
Potassium
(K+)
PK =
1
Chloride
(Cl)
PCl =
0.3 – 0.5
Sodium
(Na+)
PNa =
0.05 – 0.01
Comparison of potassium, sodium, and chloride membrane permeabilities diagram

Review Questions.

Question 1: How much less permeable is the membrane to Na+ ions than to K+ ions? Determine your answer using PK / PNa.

Question 2: How much less permeable is the membrane to Cl ions than to K+ ions? Determine your answer using PK / PCl.

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