Resting Membrane Potential Simulation: Section 1
Overview of Resting Membrane Potentials and Cell Membranes
What Is A Resting Membrane Potential?
A resting membrane potential (RMP) is the electrical potential across the cell (plasma) membrane of an excitable cell, such as a neuron or muscle cell, when it is not generating electrochemical impulses in response to stimuli.
How Membrane Potentials Are Measured.
The resting membrane potential can be measured by placing a reference electrode outside the cell membrane and a recording electrode inside the membrane. The charge difference between the two electrodes, the membrane potential, is measured in millivolts (mV) using a voltmeter.
When there is a difference in the charges between the inside and outside of the cell membrane, the measured membrane potential will be greater or less than 0 mV. Typically, the resting membrane potential of excitable cells is around -70 mV (ranging from -60 mV to 90 mV), indicating that the inside of the cell membrane is relatively negative compared to its outside. This charge imbalance results from an uneven distribution of ions across the cell membrane, mainly involving potassium (K+), sodium (Na+), and chloride (Cl–).
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Question 1: What is an excitable cell?
Excitable cells produce electrochemical impulses in response to stimuli.
Question 2: What are two types of excitable cells?
Nerve cells (neurons) and muscle cells are types of excitable cells.
Question 3: Where are the electrodes placed when measuring the resting membrane potential of an excitable cell?
A reference electrode is placed outside the cell membrane, and a recording electrode is placed inside the membrane.
Question 4: What does a 0 mV membrane potential indicate?
No charge difference can be detected between the inside and outside of the cell membrane.
Question 5: What does a – mV membrane potential indicate?
It indicates that a charge difference exists between the two sides of the membrane, with the inside being relatively negative to the outside.
Question 6: What does a + mV membrane potential indicate?
The inside of the cell membrane is negatively charged compared to the outside.
Introduction to Cell Membranes.
Before proceeding with this simulation, let us take some time to review the cell membrane structures and functions that are most relevant to resting membrane potentials.
Basic Membrane Components.
All cells in the body are surrounded by a cell or plasma membrane primarily composed of lipids and proteins. The cell membrane’s basic framework consists of a bilayer of phospholipid molecules. Interspersed among the phospholipids are cholesterol molecules, which help make the membrane more fluid. Several types of proteins are also associated with the cell membrane. They are either intrinsic (embedded) or extrinsic (attached) to the membrane and assist with membrane transport, cell communication, and cell recognition. Many membrane lipids and proteins have attached carbohydrates, which play a role in cell recognition.
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The Role of Phospholipid Bilayer.
Each phospholipid molecule has a polar head and two neutral fatty acid (lipid) tails. The polar heads of the phospholipids are hydrophilic, meaning they are attracted to the polar water molecules surrounding the membrane. In contrast, the phospholipids’ neutral tails are hydrophobic (repelled by water) and drawn to each other in the middle of the membrane.
The phospholipid bilayer acts as a semipermeable barrier that allows certain substances to pass through the cell membrane while preventing others from doing so. Small nonpolar molecules, like oxygen and other gases, and small polar molecules, like water, can diffuse through the lipid barrier. However, ions such as K+ and large polar molecules like glucose cannot pass through this barrier.
|Small nonpolar molecules
|Small polar molecules
|Ions (cations and anions)
|Large polar molecules
|Carbon dioxide (CO2)
|Glucose (other sugars)
As a result, the lipid bilayer separates the cell cytoplasm from the surrounding environment, creating extracellular (ECF) and intracellular fluid (ICF) compartments.
Question 1: What molecules form the framework of a cell (plasma) membrane?
Question 2: What are the chemical components of these molecules?
Each phospholipid has a polar head and two neutral fatty acid (lipid) tails.
Question 3: What term best describes the arrangement of these molecules?
They are arranged in a bilayer.
Question 4: Why do the heads of these molecules face the surrounding watery environment?
The polar heads of the phospholipids are attracted to the water molecules in the surrounding environment.
Question 5: Why do the tails of these molecules face each other?
The neutral fatty acid tails are attracted to each other while being repelled by the surrounding water molecules.
Question 6: What types of substances can freely pass through the membrane framework?
Small nonpolar molecules, such as O2, and small polar molecules, such as H2O.
Question 7: What types of substances can not freely pass through the membrane framework?
Ions, such as K+ or Na+, and large polar molecules, such as glucose and amino acids.
Question 8: What are the names of the watery compartments formed by the semipermeable membrane?
The watery environment outside the cell membrane is called extracellular fluid, and the environment inside the membrane is called intracellular fluid.
The Role of Integral Proteins.
Integral proteins, called membrane transport proteins, play a significant role in the proper function of excitable cells. They span the width of the cell membrane and function as gatekeepers, allowing only particular ions in and out of the cell. There are two main types of membrane transport proteins: channel and carrier proteins.
Channel proteins allow ions to move by diffusion through the cell membrane along their concentration gradients. Some channels, called leak channels, are continuously open, while others are gated and open only when stimulated. Voltage-gated channels open in response to changes in membrane potential, and ligand-gated channels open when chemicals bind. Other neurons have channels that respond to stimuli such as stretch, changes in temperature, or pressure.
Carrier proteins differ from channel proteins because they are only open to one side of the membrane at a time. They have binding sites that attach only to particular substances. Once the carrier protein has bonded with a specific substance, the protein alters its shape to transport the substance from one side of the membrane to the other.
The most relevant carrier protein to the resting membrane potential is the sodium-potassium pump or Na+/K+ ATPase. Each pump protein moves 3 Na+ ions out of the cell while simultaneously moving 2 K+ ions into the cell. The sodium-potassium pump moves the ions by changing conformation (shape) using the energy stored in a molecule of ATP (adenosine triphosphate).
While facing the cell’s interior (cytoplasmic side), the pump protein has a high affinity for Na+ and binds with three Na+ ions and an ATP molecule. The pump protein then acts as an enzyme and hydrolyzes the ATP to ADP + P, allowing a low-energy phosphate group to bond.
The pump protein then changes conformation to face the cell’s exterior, where its affinity for Na+ decreases and its affinity for K+ increases. As a result, the three Na+ ions detach and enter the extracellular fluid, and two K+ ions attach.
The attachment of the K+ ions causes the phosphate to detach, and the pump protein changes conformation to face the cell’s interior again. The repositioning of the pump protein alters its ion affinities, causing it to release the two K+ ions and bind three Na+ ions, which allows the active transport process to start again.
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The sodium-potassium pump functions continuously to maintain a higher concentration of Na+ ions outside the cell membrane and a higher concentration of K+ ions inside. This results in opposing concentration gradients for these two ions. The membrane’s open channels permit these ions to move down their respective gradients, contributing to a separation of charges across the membrane that produces the resting membrane potential.
Question 1: What are the two general types of channel proteins?
Leak (or continuously open) channels and gated channels.
Question 2: What force causes substances to cross the membrane through channel proteins?
Substances move by diffusion down their respective concentration gradients.
Question 3: What is a carrier protein?
Carrier proteins are integral proteins that bind to substances and move them across the membrane by changing their shape.
Question 4: What carrier protein is most relevant to the resting membrane potential?
The sodium-potassium pump or Na+/K+ ATPase is the most relevant carrier protein.
Question 5: In what ratio and direction does this protein move ions?
The pump moves 3 Na+ out of the cell and 2 K+ ions into the cell.
Question 6: Why is the ion exchange process considered active, and what molecule is involved?
The pumping process is active because it requires the energy stored in ATP or adenosine triphosphate.
Question 7: How does the protein affect ion concentration gradients?
It creates a high concentration gradient of Na+ ions outside the cell and a high concentration of K+ ions inside the cell.
Question 8: How do the ion concentration gradients affect the resting membrane potential?
When these ions move down their gradients through open channels, a separation of charges develops across the membrane, producing the resting membrane potential.
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References and Attributions
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.”
26. Saudi Journal of Kidney Diseases and Transplantation – “Hyperkalemia Revisited.”
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.”