Resting Membrane Potential Simulation
– Section 2 –
Main Affecting Factors
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Introduction to the Affecting Factors
Two factors primarily determine the resting membrane potential of excitable cells:
- the concentration gradients of potassium (K+), sodium (Na+), and chloride (Cl–) ions,
- and the permeability of the cell membrane to these ions.
Concentration Gradients
What are 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 also commonly referred to as a chemical gradient because it concerns the quantity of particles, irrespective of their charge.
In excitable cells, sodium and chloride ions are more concentrated outside the cell membrane.
In contrast, 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.
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How Do Concentration Gradients Form?
The concentration gradients for Na+, K+, and Cl– ions result from different physiological processes.
1. The Na+/K+ pump (ATPase) causes Na+ ions to accumulate outside the membrane and K+ ions inside the membrane.
2. Cl– ions passively accumulate outside the cell because they are drawn to the positive charge outside the membrane when an excitable cell is at rest.
3. 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.
Analysis Questions
1. What is an ion concentration gradient?
An ion concentration gradient occurs when the number of particles on the outside and inside of the cell membrane differ.
2. What is another name for ion concentration gradients, and why is this name applied?
Ion concentration gradients are also known as chemical gradients because the gradients are based on differences in the number of particles rather than differences in charge.
3. Which ions have a higher concentration outside the cell membrane?
Na+ and Cl– have higher concentrations outside the cell membrane.
4. Which ions have a higher concentration inside the cell membrane?
K+ and large anions (phosphates and proteins) have higher concentrations outside the cell membrane.
5. How do the ion concentration gradients affect the resting membrane potential?
The ions move through the cell membrane along their respective gradients from areas of higher concentration to regions of lower concentration. As the charged particles shift positions, the value of the resting membrane potential changes.
6. What maintains the gradients for Na+ and K+?
The sodium-potassium pump (Na+/K+ ATPase) actively maintains the gradients for Na+ and K+ ions.
7. How is the gradient for Cl– ions established?
Cl– ions are passively distributed based on charge differences across the membrane resulting from the distribution of Na+ and K+.
8. Why do large anions like phosphates and proteins accumulate inside the membrane?
They are produced inside the cell and are too large to pass through the membrane.
Membrane Permeability
What Determines Cell Membrane Permeability?
The permeability of a membrane is determined by the number of open ion channels.
The permeability of a cell membrane to ions is comparable to how floodgates control water flow through a dam. The number of open floodgates determines the amount of water that can flow through the dam, just as the number of open ion channels influences the rate of ion movement across the cell membrane.
Water flows through the floodgates due to the force of gravity that acts on the water behind the dam. It flows from a higher level on one side of the dam to a lower level on the other.
Similarly, ions flow through open channels in the cell membrane in response to their concentration gradient. Ions move from areas of higher concentration to areas of lower concentration.
Cell Membrane Permeability Values
The associated slider activity displays a table of K+, Cl–, and Na+ ions and their corresponding membrane permeability values.
The cell membrane of excitable cells is most permeable to K+, which is why its permeability value is the highest of the three ions.
- PK = 1.
This high permeability value indicates that the cell membrane has many open (leak) channels through which K+ can pass.
The Na+ and Cl– membrane permeability values are relatively lower than K+, meaning there are fewer open channels for these ions.
- PCl = 0.3 – 0.5.
- PNa = 0.01 – 0.05.
Permeability Value
1
Potassium (K+)
Sodium (Na+)
Permeability Value
0.01 – 0.05
Permeability Value
0.3 – 0.5
Chloride (Cl–)
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Analysis Questions
1. How much less permeable is the membrane to Na+ ions than to K+ ions? Determine your answer using PK / PNa.
The membrane is about 20 to 100 times less permeable to Na+ ions than to K+ ions.
2. How much less permeable is the membrane to Cl– ions than to K+ ions? Determine your answer using PK / PCl.
The membrane is about 2 to 3 times less permeable to Cl– ions than to K+ ions.
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References
Click here to view the reference list.
1. Advances in Physiology Education – “Generation of resting membrane potential.”
2. Clinical Journal of the American Society of Nephrology – “A Critically Swift Response: Insulin-Stimulated Potassium and Glucose Transport in Skeletal Muscle.”
3. European Society of Cardiology – “Hyperkalemia, the sodium-potassium pump and the heart.”
4. Gatorade Sports Science Institute – Hyponatremia in Athletes.”
5. Hindawi Case Reports in Medicine – “Acute Ascending Muscle Weakness Secondary to Medication-Induced Hyperkalemia.”
6. Lab Exchange – “Membrane bilayers are permeable to small, uncharged molecules.”
7. Lippincott Learning – “Concentration and Volume: Understanding Sodium and Water in the Body.”
8. Journal of the Association of Physicians in India – “Severe Muscle Weakness due to Hyperkalemia.”
9. McGill Medicine and Health Sciences – “Resting Membrane Potential.”
10. Medical Neuroscience – “Action Potential.”
11. Michigan State University Library – “Foundations of Neuroscience.”
12. N. I. H. National Library of Medicine – “The Electrophysiology of Hypo- and Hyperkalemia.”
13. N. I. H. National Library of Medicine – “Extracellular Potassium Homeostasis: Insights from Hypokalemic Periodic Paralysis.”
14. N. I. H. National Library of Medicine – “Getting Across the Cell Membrane: An Overview for Small Molecules, Peptides, and Proteins.”
15. N. I. H. National Library of Medicine – “Hyperkalemia.”
16. N. I. H. National Library of Medicine – “Hyponatremia and the Brain.”
17. N. I. H. National Library of Medicine – “Ion Channels and the Electrical Properties of Membranes.”
18. N. I. H. National Library of Medicine – “Physiology, Resting Potential.”
19. N. I. H. National Library of Medicine – “Physiology, Sodium Potassium Pump.”
20. N. I. H. National Library of Medicine – “Principles of Membrane Transport.”
21. N. I. H. National Library of Medicine – “Structure of the Plasma Membrane.”
22. N. I. H. National Library of Medicine – “Transport of Small Molecules.”
23. Nursing 2020 Critical Care – “Electrolytes Series: Potassium.”
24. Nursing 2020 Critical Care – “Electrolytes Series: Sodium and Chloride.”
25. ResearchGate – “Implications of active lifestyles and environmental factors for water needs and consequences of failure to meet those needs.”
26. Saudi Journal of Kidney Diseases and Transplantation – “Hyperkalemia Revisited.”
27. ScienceDirect – “Development of models of active ion transport for whole-cell modeling: cardiac sodium-potassium pump as a case study.”
28. ScienceDirect – “Membrane Potential.”
29. ScienceDirect – “Skeletal Muscle Physiology.”
30. University of New Mexico – “Ionic gradients, membrane potential and ionic currents.”
31. University of St. Andrews – “Neurosims.”
32. University of St. Andrews – “The Origin of the Resting Membrane Potential.”