Positive Feedback Mechanisms in Physiology

These flashcards review the main ideas and steps of positive feedback in uterine contractions, blood clotting, and neuronal signaling, contrasting them with negative feedback.

What is the key difference between positive and negative feedback?

Negative feedback reverses the direction of the original stimulus, while positive feedback amplifies it.

Which hormone released from the posterior pituitary drives uterine contractions during childbirth?

Oxytocin

During childbirth, what initial stimulus triggers the positive-feedback loop?

Stretching of the cervix by the baby’s head

Describe the sequence of events in the uterine positive-feedback loop.

Cervix stretch → signal to neurohypophysis → oxytocin release → myometrium contracts → baby pushed further onto cervix → increased stretch → more oxytocin, and the cycle repeats until delivery.

When do uterine contractions dramatically diminish in the childbirth feedback loop?

After the baby is delivered and no longer stretches the cervix

What plasma protein is first activated when collagen is exposed during a hemorrhage?

Factor XII (inactive factor 12 → active factor 12)

How does active factor 12 initiate a positive-feedback cascade in blood clotting?

It converts inactive factor 11 to active factor 11, which ultimately leads to thrombin formation.

What does thrombin convert, and why is this important?

Thrombin converts fibrinogen (inactive) into fibrin, forming the structural mesh of a blood clot.

Explain thrombin’s positive-feedback role in the clotting cascade.

Thrombin further promotes activation of factor 11, accelerating its own production and fibrin formation.

Why are clotting factors stored in inactive forms in the bloodstream?

To prevent unwanted clotting that could block vessels or damage organs.

Which part of a neuron carries the action potential away from the cell body?

The axon

What ion’s influx initiates the neuronal positive-feedback loop of depolarization?

Sodium (Na⁺)

How does sodium influx create a positive-feedback mechanism along the axon?

Incoming Na⁺ opens nearby voltage-gated Na⁺ channels, permitting more Na⁺ entry and propagating the depolarization.

Give one commonality among the childbirth, clotting, and neuronal examples of positive feedback.

Each involves an initial trigger that intensifies its own effect through a self-reinforcing loop until a specific endpoint is reached.