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.