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M04.04.02 ION CHANNELS
The cell membrane potential, a fundamental concept in physiology, refers to the voltage difference between the inside and outside of a cell. This voltage is not random; it’s a carefully maintained electrical gradient crucial for various cellular processes like communication, nutrient transport, and muscle contraction.
Understanding the Players:
| Component | Description |
|---|---|
| Phospholipid Bilayer | The cell membrane’s foundation, a double layer of fat molecules with charged heads and uncharged tails. |
| Ion Channels | Specialized protein pores that selectively allow specific ions (charged atoms) to pass through the membrane. |
| Sodium-Potassium Pump | An active transport system that uses cellular energy (ATP) to pump sodium (Na+) out and potassium (K+) into the cell. |
| Leak Channels | Channels that allow passive diffusion of some ions across the membrane down their concentration gradient. |
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Points to Remember:
- Resting Membrane Potential: In a resting cell, the inside is typically negative compared to the outside. This negativity arises due to:
- Unequal Distribution of Ions: K+ is more concentrated inside the cell, while Na+ is more concentrated outside.
- Selective Permeability: Leak channels allow K+ to freely diffuse out, creating a negative charge inside. The sodium-potassium pump actively maintains this gradient by pumping K+ in and Na+ out.
- Negative Charges on Proteins: Certain proteins inside the cell also contribute to the negative charge.
- Magnitude of the Resting Potential: The resting potential typically ranges from -70 mV to -90 mV. This value depends on the balance between ion concentrations, leak channels, and the sodium-potassium pump activity.
- Generating Action Potentials: Stimuli like neurotransmitters or hormones can cause voltage-gated ion channels to open. This allows a rapid influx of Na+, leading to a brief reversal of the membrane potential (depolarization). The repolarization and hyperpolarization phases follow, restoring the resting potential. This sequence of events forms an action potential, the electrical signal used for communication between cells, particularly nerve and muscle cells.
- Clinical Significance:
- Disruptions in cell membrane potential can contribute to various diseases. For example, abnormal ion channel activity is implicated in epilepsy and cardiac arrhythmias.
- Understanding membrane potential is crucial for interpreting electrophysiological tests like electrocardiograms (ECGs) and electroencephalograms (EEGs).
Bibliography:
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th ed.). Garland Science.
- Boron, W. F., & Boulpaep, E. L. (2016). Medical physiology: A cellular and molecular approach (2nd ed.). Elsevier Saunders.
- Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S. A., & Hudspeth, A. J. (2013). Principles of neural science (5th ed.). McGraw-Hill Education.