Questions: Positive Feedback Mechanisms

Question 1

A patient receiving synthetic oxytocin (Pitocin) to induce labor begins with a low dose. The contractions intensify rapidly and soon exceed safe levels, requiring dose reduction. What does this pattern demonstrate about positive feedback?

• A. Pitocin overwhelmed the negative feedback systems that normally regulate uterine contractions.
• B. Even a small initial stimulus can engage and amplify through the positive feedback loop, making careful dose titration necessary to avoid runaway escalation.
• C. Positive feedback was not involved — the dose itself directly caused the strong contractions through pharmacological action.
• D. The patient's uterus was insensitive to normal feedback inhibition due to hormonal imbalance.

Answer: (B)

This is positive feedback in action: oxytocin → contractions → cervical stretch → more oxytocin → stronger contractions → more stretch. Once the loop is engaged, even a small initial push amplifies through the cycle. The clinical consequence is that Pitocin doses must be titrated carefully precisely because positive feedback is inherently self-amplifying — a dose that initially seems too small can trigger escalating contractions within minutes. This is why labor induction requires continuous monitoring, not a set-and-forget dose.

Question 2

Which of the following correctly describes the positive feedback loop during the rising phase of an action potential?

• A. Membrane depolarization opens K⁺ channels, K⁺ influx further depolarizes the membrane, opening more K⁺ channels.
• B. Membrane depolarization opens voltage-gated Na⁺ channels, Na⁺ influx further depolarizes the membrane, which opens more Na⁺ channels, driving further depolarization.
• C. Na⁺ influx depolarizes the membrane, which opens Ca²⁺ channels that independently amplify the signal.
• D. Depolarization causes Na⁺ channel inactivation, which paradoxically increases Na⁺ conductance further.

Answer: (B)

The rising phase of the action potential is a classic positive feedback loop: depolarization opens voltage-gated Na⁺ channels → Na⁺ rushes in (down its electrochemical gradient) → membrane depolarizes further → more Na⁺ channels open → more Na⁺ influx → faster depolarization. This regenerative process is what produces the rapid, explosive upstroke. The loop terminates not by any internal brake but by Na⁺ channel inactivation (a built-in molecular timer) — an external termination event relative to the loop itself.

Question 3

Positive feedback mechanisms are inherently pathological in physiology because they typically lead to uncontrolled escalation.

• T. True
• F. False

Answer: False

Positive feedback is not inherently pathological — it is a tool the body uses for specific purposes that require rapid, committed, all-or-nothing responses. Childbirth, blood clotting, and action potential generation all depend on positive feedback working correctly. Each has a natural termination event (delivery of the baby, wound sealing + anticoagulant factors, Na⁺ channel inactivation) that stops the loop. The body uses positive feedback sparingly and always pairs it with an external off-switch. Pathology arises not from positive feedback itself but from failure of the termination mechanism.

Question 4

Each physiological positive feedback loop contains a built-in internal mechanism that automatically slows and stops the loop once a threshold is reached.

• T. True
• F. False

Answer: False

This is the defining difference between positive feedback and negative feedback. Negative feedback has an internal off-switch: the response itself counteracts the original stimulus, pulling the system back toward equilibrium. Positive feedback has no such internal brake — the response amplifies the stimulus, which amplifies the response further. Every physiological positive feedback loop terminates through an EXTERNAL event that removes the original stimulus or interrupts the loop from outside: delivery of the baby removes cervical stretch, wound sealing limits platelet recruitment, and Na⁺ channel inactivation is a separate molecular mechanism not part of the depolarization-Na⁺-influx loop.

Question 5

Why does positive feedback require an external termination event? Use one physiological example to illustrate what would happen if that termination event failed.

Model answer: Positive feedback amplifies the original stimulus rather than opposing it, so the loop has no internal mechanism to reduce itself — the stronger the response, the stronger the next cycle. Without an external termination event, the loop would escalate indefinitely. In blood clotting: exposed collagen activates platelets, which release signals recruiting more platelets, in a self-amplifying cascade. The termination events are physical coverage of the wound and anticoagulant factors (antithrombin, protein C) that limit clot growth. If these fail — as in disseminated intravascular coagulation (DIC) — the positive feedback runs systemically, forming clots throughout the vasculature, consuming clotting factors, and paradoxically causing both clotting and bleeding simultaneously.

The childbirth example is equally instructive: if the baby were not delivered (e.g., fetal malposition preventing descent), the oxytocin loop would continue escalating, eventually leading to uterine rupture — which is why obstructed labor is a medical emergency requiring intervention.