Positive Feedback Mechanisms in Physiology

Positive Feedback

  • Positive feedback amplifies the original stimulus, unlike negative feedback which reverses it.

Uterine Contractions During Childbirth

  • Overview: A classic example of positive feedback in human physiology.
  • Anatomy:
    • Uterus: Contains the myometrium (muscle of the uterus).
    • Cervix: The opening that must dilate for the baby to pass through.
    • Vaginal Canal (Birth Canal): The passageway for the baby's delivery.
    • Pituitary Gland (specifically the neurohypophysis, or posterior pituitary): Releases oxytocin.
  • Process (Weeks 38-40):
    • Baby's head pushes against the cervix.
    • Cervix stretches, sending a signal to the neurohypophysis.
    • Neurohypophysis releases oxytocin into the bloodstream.
    • Oxytocin stimulates the myometrium to contract, pushing the baby further towards the cervix.
    • This further stretching of the cervix sends more signals to the neurohypophysis, releasing more oxytocin. The cycle repeats and intensifies.
  • Positive Feedback Loop: The stretch of the cervix is amplified, causing stronger and more frequent uterine contractions.
  • Termination: The loop continues until the baby is delivered and no longer pushing on the cervix.
  • Uterine contractions diminish dramatically once the baby is delivered through the vaginal canal.

Blood Clotting

  • Overview: Another example of positive feedback, triggered by a hemorrhage.
  • Initiation:
    • Hemorrhage occurs, exposing collagen in the lacerated blood vessel wall.
    • Collagen activates inactive factor 12, converting it to active factor 12.
  • Cascade:
    • Active factor 12 converts inactive factor 11 to active factor 11.
    • This is a simplified view of a complex cascade of events.
  • Thrombin's Role: Active factor 11 eventually leads to the production of thrombin.
  • Fibrinogen to Fibrin: Thrombin converts inactive fibrinogen to active fibrin.
  • Fibrinogen (inactive) → Fibrin (active)
  • Fibrin is crucial for clot formation.
  • Positive Feedback Loop:
    • Thrombin not only converts fibrinogen to fibrin but also positively influences the conversion of inactive factor 11 to active factor 11.
    • More thrombin leads to more active factor 11, resulting in more fibrin and ensuring clot formation.

Nerve Cell Depolarization

  • Neuron Structure:
    • Axon: Elongated structure where depolarization events spread.
    • Cell Body: Where the axon extends from; depolarization travels from the cell body to the axon terminals.
  • Depolarization Process:
    • Sodium influx into the axon.
    • Sodium influx opens voltage-gated sodium channels.
    • Opening of voltage-gated sodium channels
    • More sodium enters the axon, propagating the electrical impulse along the axon.
  • Positive Feedback:
    • The initial sodium influx leads to more and more sodium influx.
      Na^+ influx opens Na^+ channels allowing more Na^+ to enter the cell.