Overview
This lecture explains the concepts of negative and positive feedback loops in biological pathways, highlighting differences, examples, and their roles in homeostasis.
Negative Feedback Loops
- A negative feedback loop occurs when the response decreases or removes the original stimulus.
- In negative feedback, the response "feeds back" to slow or stop the initial stimulus.
- Example: When overheated (stimulus), sweating (response) cools the body and removes the heat stimulus.
- Negative feedback loops are often involved in maintaining homeostasis (internal stability).
- Not all negative feedback loops are homeostatic; some do not regulate extracellular fluid or internal conditions.
- Most homeostatic pathways use negative feedback to return to a set point.
Positive Feedback Loops
- A positive feedback loop occurs when the response increases the original stimulus, amplifying the effect.
- Positive feedback is usually destabilizing and can be dangerous if unchecked.
- Example: In birth, uterine contractions (response) increase cervical stretching (stimulus), which leads to stronger contractions, repeating until birth.
- Example: Blood clotting, where clotting chemicals stimulate more chemical release, increasing clotting until bleeding stops.
- Positive feedback loops typically have a natural endpoint to stop the cycle (e.g., delivery of a baby, cessation of bleeding).
Key Terms & Definitions
- Negative Feedback Loop — A process where the response decreases or eliminates the original stimulus.
- Homeostasis — The maintenance of a stable internal environment within certain physiological limits.
- Positive Feedback Loop — A process where the response increases the original stimulus, leading to greater change.
- Stimulus — Any change or signal in the environment that elicits a response.
- Response — The action or change that occurs as a result of a stimulus.
Action Items / Next Steps
- Practice identifying and diagramming negative and positive feedback loops in assigned lab exercises.
- Review examples of feedback loops for future classes and assignments.