hyperpolarization
Summary
Hyperpolarization is when a cell's membrane potential becomes more negative than its resting potential, moving further from the threshold needed for action potential generation. This typically occurs due to K+ efflux or Cl- influx, making the cell less excitable and serving as an inhibitory mechanism in neural and cardiac tissues.
Detail
Hyperpolarization represents a critical mechanism for cellular inhibition and signal modulation. During hyperpolarization, the membrane potential becomes more negative than the typical resting potential (usually around -70mV in neurons), often reaching -80 to -90mV or more negative. This occurs through several mechanisms: opening of voltage-gated or ligand-gated K+ channels (causing K+ efflux), opening of Cl- channels (causing Cl- influx), or activation of electrogenic pumps like Na+/K+-ATPase. In neurons, hyperpolarization serves as an inhibitory mechanism, making it harder to reach the threshold potential (-55mV) required for action potential generation. This is seen with inhibitory neurotransmitters like GABA (opening Cl- channels) and glycine. In cardiac physiology, hyperpolarization occurs during phase 4 of the cardiac action potential and is crucial for the refractory period. The afterhyperpolarization phase following an action potential involves Ca2+-activated K+ channels and helps prevent tetanic contractions in muscle. Pathologically, excessive hyperpolarization can lead to conduction blocks, while insufficient hyperpolarization may cause hyperexcitability states like epilepsy.
Sources
- Guyton and Hall Textbook of Medical Physiology
- Kandel's Principles of Neural Science
- Lippincott's Illustrated Reviews: Physiology
- First Aid for the USMLE Step 1
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