Homeostasis & Negative Feedback
Homeostasis is the maintenance of a relatively constant internal environment, ensuring optimum conditions for the body to function. In humans it depends on two systems, the nervous system and the endocrine system, and it operates through the stimulus–response pathway. The stages of the pathway are given letter codes that are used in the diagrams throughout this topic:
- Stimulus (S) - a change in a variable in the internal or external environment that the organism can detect.
- Receptor (R1) - the cells or tissues that detect the change, then generate a nerve impulse (electrochemical) or a hormone (chemical) in response.
- Transmission (T) - the relay of that information to an effector, via nerves (nervous) and/or hormones (hormonal).
- Effector organ (E) - a muscle or gland that carries out the response, by movement or secretion.
- Response (R2) - the action that restores conditions toward normal.
In summary: stimulus (S) → receptor (R1) → transmission (T) → effector organ (E) → response (R2).
The stimulus–response model with its letter codes. The response feeds back on the stimulus (dashed arrow), removing it - this is negative feedback (N).
SENSORY RECEPTORS
Sensory receptors are the dendrites of sensory neurons, each specialised for receiving one particular type of stimulus. They respond to a stimulus's intensity, location and duration. Each kind of change has its own class of receptor:
| Receptor | Detects | Location |
|---|---|---|
| Photoreceptor | light | eyes |
| Mechanoreceptor | pressure, touch, stretch, sound | various (sound: ears) |
| Thermoreceptor | temperature changes | head (and skin) |
| Chemoreceptor | taste and smell; blood O₂, CO₂, H⁺ | various |
| Nociceptor | tissue damage — interpreted by the brain as pain | various |
| Baroreceptor | blood pressure | aortic arch, carotid sinuses |
| Osmoreceptor | solute concentration of body fluids | hypothalamus |
| Proprioceptor | body position | muscles, tendons, ligaments, joints |
EFFECTORS
Effectors in animals are usually muscles or glands; their responses therefore always involve movement or secretion. Plants also respond to stimuli: many grow toward the light, and some flowers close at sunset.
NEGATIVE FEEDBACK
Most homeostatic control uses negative feedback: the response diminishes or reverses the original stimulus, bringing the controlled factor back toward its set point. ('Negative' here means opposing, not harmful.) For example, when the hand touches a hot object, the response - moving the hand away - removes the stimulus. Negative feedback has four defining properties:
- It results in control of the factor.
- It is self-regulating - no outside intervention is required.
- It causes the controlled factor to fluctuate about a mean (a pre-set level, in the same way a thermostat holds an oven near its set temperature).
- A system controlled in this way is called a homeostatic control mechanism.
Negative feedback drawn as a loop: a stimulus moves the factor away from its normal level; the receptor, transmission and effector produce a response that opposes the change, and negative feedback returns the factor to normal.
The controlled factor is never perfectly constant; negative feedback keeps it fluctuating about the set point.
For example: blood glucose rises after a meal (S) → the pancreas detects the rise (R1) and releases insulin (T) → target cells take up glucose (E) → blood glucose falls back to normal (R2, N).
POSITIVE FEEDBACK
Occasionally the response reinforces or increases the initial stimulus; this is positive feedback. When a newborn suckles at the breast, milk is released, and a hormone is then released that stimulates the release of more milk. Positive feedback amplifies a change rather than stabilising it, which is why it is rare in homeostasis.
| Feature | Negative feedback | Positive feedback |
|---|---|---|
| Effect on stimulus | inhibits or reverses it | reinforces or increases it |
| Outcome | control — fluctuates about a mean | amplification toward an end-point |
| How common | the basis of almost all homeostasis | rare |
| Example | glucose ↑ → insulin → glucose falls | suckling → milk release → more milk |
In humans the transmission step always involves the endocrine and/or nervous system: regulation requires communication between organs and systems, by nerves, by hormones, or by both. The next two notes examine each messenger system in turn.