positive feedback
Summary
Positive feedback is a physiological control mechanism where the response amplifies or enhances the original stimulus, creating a self-reinforcing cycle. Unlike negative feedback (which maintains homeostasis), positive feedback drives processes to completion or rapid change. Classic examples include oxytocin during labor, blood clotting cascade, and action potential depolarization.
Detail
Positive feedback mechanisms amplify the initial stimulus rather than opposing it, creating an escalating response that continues until the stimulus is removed or the process reaches completion. In obstetrics, oxytocin release during labor exemplifies this: cervical stretching stimulates oxytocin release, which increases uterine contractions, leading to more cervical stretching and further oxytocin release until delivery occurs. The coagulation cascade demonstrates positive feedback through amplification loops where activated clotting factors activate more factors (e.g., Factor XIa activating Factor IX, Factor VIIIa amplifying Factor X activation). Neurologically, sodium channel opening during action potentials creates positive feedback - initial depolarization opens voltage-gated Na+ channels, causing more depolarization and opening more channels until the threshold is reached. Unlike negative feedback systems that maintain homeostasis, positive feedback is typically brief and ends when the stimulus is removed or maximum response is achieved. Pathologically, positive feedback can create dangerous cycles like septic shock (where inflammatory mediators trigger more inflammation) or malignant hyperthermia.
Sources
- Guyton and Hall Textbook of Medical Physiology
- Boron and Boulpaep Medical Physiology
- Ganong's Review of Medical Physiology
- Costanzo Physiology
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