Questions: Negative Feedback Mechanisms

Question 1

Blood glucose rises sharply after a large meal. Tracing the negative feedback loop, which sequence correctly describes what happens next?

• A. Glucagon is secreted → liver releases stored glucose → blood glucose rises further, completing the loop
• B. Insulin is secreted → cells increase glucose uptake → blood glucose falls → insulin secretion diminishes
• C. Insulin is secreted at a constant high rate until blood glucose returns exactly to the set point, then stops abruptly
• D. The pancreas adjusts the set point upward to accommodate the new glucose level, reducing the need for correction

Answer: (B)

This traces the classic negative feedback loop: the deviation (high glucose) triggers a response (insulin secretion) that opposes the deviation (drives glucose into cells), which reduces the original signal (glucose falls), which in turn reduces the stimulus for the response (insulin secretion diminishes). Option A describes a positive feedback loop. Option C is wrong because negative feedback is self-limiting — insulin tapers off as glucose approaches the set point, not in an abrupt stop. Option D misunderstands homeostasis — the set point is defended, not adjusted to match the disturbance.

Question 2

Why is the word 'negative' used in 'negative feedback,' and which statement best captures its meaning?

• A. The feedback signal directly inhibits ('negates') the effector organ's activity
• B. The feedback has a net harmful or negative effect on the organism if unregulated
• C. The response produced by the effector opposes and counters the direction of the original change
• D. The output of the system is numerically smaller than the input that triggered it

Answer: (C)

In negative feedback, 'negative' means the system's response negates — works against — the initial deviation. If a variable rises, the response pushes it down; if it falls, the response pushes it up. This has nothing to do with harm (Option B) or inhibition per se (Option A — the effector could be activated or inhibited depending on the deviation direction). Option D is vague and not the formal meaning. The term comes from control theory, where 'negative' feedback refers to subtraction of the output from the input to produce a corrective error signal.

Question 3

A negative feedback loop is self-limiting: the corrective response it generates reduces the very signal that triggered the response in the first place.

• T. True
• F. False

Answer: True

This is the defining property of negative feedback. In the blood glucose example, insulin lowers blood glucose, which reduces the pancreatic beta cell's stimulus for insulin secretion. The correction eats away at its own trigger. This self-limiting quality is what gives negative feedback its inherent stability — it cannot run away with itself the way positive feedback can.

Question 4

Negative feedback mechanisms restore a regulated physiological variable to exactly its set point value after a disturbance.

• T. True
• F. False

Answer: False

Negative feedback produces oscillations around the set point, not perfect restoration to a single exact value. The thermostat analogy makes this clear: a room thermostat set at 20°C will cause the temperature to drift slightly below 20°C before the furnace re-activates, then slightly above before it cuts off — cycling around the target. Blood glucose, blood pressure, and body temperature all fluctuate within a narrow physiological range rather than locking onto a precise number. Perfect set-point restoration is a common misconception.

Question 5

Why is positive feedback inherently unstable while negative feedback is inherently stable, and what does each require to eventually terminate?

Model answer: In negative feedback, the effector's output opposes the triggering deviation, so as the system corrects, the stimulus for the response diminishes — the loop is self-terminating. In positive feedback, the output amplifies the original change, so the response grows rather than diminishes — it cannot terminate itself. Positive feedback requires an external mechanism to stop it: blood clotting terminates when the damaged vessel is sealed; uterine contractions in labor terminate with delivery; action potential upstroke terminates through Na⁺ channel inactivation.

The stability difference explains the physiological use case for each: negative feedback governs ongoing regulation (blood pressure, glucose, temperature) because it naturally settles into equilibrium. Positive feedback is reserved for rapid, all-or-nothing events that need to reach completion quickly and cannot be allowed to reverse midway — which is why every positive feedback loop in physiology has a built-in shut-off trigger.