Negative and Positive Feedback

Spec Mapping — OCR H420 Module 5.1.1 — Communication and homeostasis, content statements covering negative feedback as the principal mechanism of homeostatic control and positive feedback as a less common amplifying mechanism, with named physiological examples of each (refer to the official OCR H420 specification document for exact wording).

Homeostatic control relies on feedback: the output of a system is fed back to its input so that the system can correct itself. In mammals, feedback comes in two contrasting forms. Negative feedback reverses a change and is the dominant mechanism of homeostasis, keeping variables like body temperature and blood glucose close to a set point. Positive feedback amplifies a change and drives a process to completion — it is far less common, but plays crucial roles in childbirth, blood clotting and the generation of action potentials. This lesson explains both systems in the detail required for OCR A-Level Biology A specification module 5.1.1(d)–(e).

The conceptual heritage matters here. Claude Bernard's nineteenth-century insistence on the constancy of the milieu intérieur and Walter Cannon's twentieth-century coining of the word homeostasis both relied implicitly on negative-feedback control as their mechanism, although neither used the term feedback in the engineering sense. The mathematical theory of feedback control came later, from cyberneticists such as Norbert Wiener in the 1940s, and is paraphrased here only to make clear that the biology you are learning maps directly onto a wider scientific framework that runs from thermostats to climate models. Positive feedback in physiology has a different historical lineage — Alan Hodgkin and Andrew Huxley's celebrated 1952 paper on the squid giant axon described the regenerative Na⁺ cycle that creates an action potential, demonstrating that nature deploys positive feedback wherever a rapid, decisive, all-or-nothing switch is needed (paraphrased).

Key Definitions:

• Negative feedback — a control mechanism in which a change in a variable triggers a response that reverses the change, restoring the variable to its set point.
• Positive feedback — a control mechanism in which a change in a variable triggers a response that amplifies the change, moving the variable further from its starting value.
• Set point — the normal or target value of a controlled variable.

---

The Generic Feedback Loop

Every feedback loop in a mammal has the same basic architecture: receptors detect a change; a coordinator (often a region of the brain or an endocrine gland) interprets the signal; effectors carry out the response; and the result is detected again by the receptors, closing the loop.

flowchart LR
    A[Stimulus: change in variable] --> B[Receptor]
    B --> C[Coordinator / central control]
    C --> D[Effector]
    D --> E[Response]
    E -.feedback.-> A

If the response reverses the stimulus, the feedback is negative. If the response reinforces the stimulus, it is positive.

---

Negative Feedback

Negative feedback is the basis of almost all homeostatic control in mammals. It keeps variables oscillating gently around a set point rather than drifting uncontrollably.

Example 1 — Temperature Regulation

1. Core body temperature rises above 37 °C.
2. Thermoreceptors in the hypothalamus detect the rise.
3. The hypothalamus sends impulses to effectors: sweat glands begin to secrete, and arterioles in the skin dilate.
4. Evaporation of sweat and increased heat loss cool the body.
5. The thermoreceptors detect that core temperature is returning to normal and reduce their signalling — the response switches off.

Example 2 — Blood Glucose Regulation

1. Blood glucose rises after a meal.
2. β cells in the pancreatic islets of Langerhans detect the rise.
3. They release insulin into the bloodstream.
4. Insulin causes liver and muscle cells to take up glucose and store it as glycogen.
5. Blood glucose falls; insulin secretion decreases.

Features of a Negative Feedback System

• The response is always opposite in direction to the change.
• It is self-limiting — as the variable returns to normal, the response weakens and then stops.
• It produces oscillation around a set point. No feedback system responds instantaneously, so the variable typically overshoots slightly, is corrected, overshoots in the other direction, and so on, settling into small oscillations around the set point.
• Multiple negative feedback loops often work simultaneously (e.g., insulin and glucagon together regulate blood glucose).

Set Points and Ranges

The set point is not a single fixed number but a narrow range around which the variable oscillates. For human core temperature, the set point is ~37 °C with daily fluctuations of ±0.5 °C. For blood glucose, fasting levels sit around 4–6 mmol dm⁻³ and rise to around 7–8 mmol dm⁻³ after a meal.

flowchart TB
    A[Rising variable] --> B[Receptor detects increase]
    B --> C[Coordinator signals effector]
    C --> D[Effector reduces variable]
    D --> E[Variable returns to set point]
    F[Falling variable] --> G[Receptor detects decrease]
    G --> H[Coordinator signals opposing effector]
    H --> I[Effector increases variable]
    I --> E

---

Positive Feedback

Positive feedback is rare in mammals because it is inherently destabilising — a small change is amplified until the system reaches a new end-point. It is used when rapid, all-or-nothing responses are required.

Example 1 — Oxytocin and Childbirth

1. Stretch receptors in the cervix detect distension as the foetus presses against it.
2. Signals travel to the posterior pituitary, which releases oxytocin into the blood.
3. Oxytocin binds to receptors on uterine smooth muscle, causing contraction.
4. Contraction pushes the foetus harder against the cervix, stretching it further.
5. More oxytocin is released, causing stronger contractions — the cycle accelerates until the baby is delivered and the stimulus ceases.

Example 2 — Action Potentials

1. A stimulus causes a small depolarisation of a neurone's membrane.
2. If the threshold is reached (~−55 mV), voltage-gated Na⁺ channels open.
3. Na⁺ flows in, further depolarising the membrane.
4. This opens more Na⁺ channels, causing more Na⁺ influx — a runaway positive feedback loop.
5. Depolarisation peaks at about +40 mV, after which the Na⁺ channels inactivate and the membrane repolarises.

Positive feedback here ensures that the action potential is all-or-nothing: once threshold is reached, the depolarisation runs to completion.

Example 3 — Blood Clotting

1. Platelets encounter damaged endothelium and become activated.
2. Activated platelets release chemicals (e.g., thromboxane) that recruit and activate more platelets.
3. A clotting cascade is triggered, each step activating more enzyme, rapidly building a fibrin mesh.
4. Clot formation is arrested only when the damage is sealed and inhibitors take over.

Features of a Positive Feedback System

• The response reinforces the change.
• It is self-amplifying, accelerating until something outside the loop brings it to an end (e.g., birth, Na⁺ channel inactivation, sealing of damaged tissue).
• It is not used in typical homeostasis because it drives variables away from normal.

---

Negative Versus Positive Feedback

Feature	Negative feedback	Positive feedback
Effect on change	Reverses it	Reinforces it
Role in homeostasis	Central — keeps variables stable	Unusual — used only where amplification is needed
Outcome	Returns to set point	Moves further from starting point
Examples	Body temperature, blood glucose, blood water potential	Childbirth, action potentials, blood clotting
End of response	Self-limiting as variable normalises	Ends only when an external factor stops it

---

Worked Example — Mapping a Feedback Loop

A dehydrated runner experiences a rise in blood solute concentration.

1. Receptors: Osmoreceptors in the hypothalamus detect increased solute concentration (decreased water potential).
2. Coordinator: Hypothalamus signals the posterior pituitary.
3. Effector: Posterior pituitary releases ADH into the blood. ADH acts on collecting duct cells of the kidney to insert aquaporins.
4. Response: More water is reabsorbed from the filtrate; less urine is produced.
5. Feedback: Blood water potential rises, osmoreceptor signalling decreases, ADH secretion is reduced.

This is a classic negative feedback loop — the change in blood water potential is reversed by the response.

---

Exam Tip

When describing any feedback loop, structure your answer using the five steps: stimulus → receptor → coordinator → effector → response → feedback. OCR mark schemes almost always give a mark for correctly identifying each component, and candidates who use a clear framework score much more reliably than those who write general paragraphs.

---

Common Exam Mistakes

• Describing negative feedback as "producing a negative effect". Negative here means "opposite in direction", not "bad".
• Forgetting that positive feedback is normal in certain contexts. Childbirth is physiological, not pathological.
• Confusing oscillation with failure. Small oscillations around the set point are normal and unavoidable in a real biological system.
• Describing the response as instant. There is always a delay between the stimulus and the response; this is why homeostatic variables fluctuate rather than sitting perfectly at the set point.
• Omitting the feedback step. Without feedback, the system cannot switch off and would overshoot wildly.
• Saying hormones "cause" feedback. Hormones are the means by which the signal is carried; feedback is the whole loop.

---

Quick Recap

• Feedback loops consist of a stimulus, receptors, a coordinator, effectors, and a response.
• Negative feedback reverses the original change and is the basis of homeostasis (e.g., temperature, glucose, water potential).
• Positive feedback amplifies the change and is used only in specific, short-lived situations (e.g., childbirth, action potentials, blood clotting).
• Homeostatic variables oscillate gently around a set point rather than sitting perfectly at a fixed value.

---

Visualising a Negative Feedback Loop — Body Temperature

The thermoregulation loop is the OCR canonical example of negative feedback in action. When core temperature rises, the loop opposes the rise; when core temperature falls (e.g. in cold air), an exactly analogous loop runs through the heat-gain centre, triggering vasoconstriction, piloerection and shivering. The detailed mechanism is the focus of the next lesson.

---

Visualising a Positive Feedback Loop — Action Potential Initiation

flowchart LR
    A[Stimulus depolarises<br/>membrane to threshold] --> B[Voltage-gated Na+ channels open]
    B --> C[Na+ enters cell]
    C --> D[Membrane depolarises further]
    D --> B
    D --> E[Peak at ~+40 mV]
    E --> F[Na+ channels inactivate]
    F --> G[K+ channels open<br/>repolarisation]