14.03 Negative Feedback (Homeostatic Control)

1. Key Concepts and Terms

• Homeostasis: The maintenance of a relatively constant internal environment for optimal cellular function.
• Negative Feedback: A regulatory process where a change in a physiological factor (e.g., blood glucose) triggers responses to bring it back to a normal range.
• Positive Feedback: A process in which a change in a physiological factor leads to further change in the same direction, generally not involved in maintaining homeostasis due to its destabilizing effects.
• Set Point: The ideal value of a physiological factor that homeostasis aims to maintain.
• Receptor: Specialized cells or tissues that detect changes (stimuli) in the environment and send information to a central control.
• Effector: Muscles or glands that perform actions to correct physiological changes and bring factors closer to the set point.
• Stimulus: A detectable change in the internal or external environment.
• Corrective Action: Responses triggered to restore conditions near the set point.

2. Negative Feedback Control Mechanism

Stages in Negative Feedback:

1. Stimulus Detection:
   • Stimuli are changes in internal or external environments, such as fluctuations in blood glucose or temperature.
   • Receptors detect these changes and send sensory information (input) to the control center, usually located in the brain or spinal cord.
2. Coordination Systems:
   • Two systems facilitate coordination and transmission of information across the body:
     • Nervous System:
       • Made up of the brain, spinal cord, and nerves, with specialized cells called neurons.
       • It transmits electrical impulses rapidly for immediate responses.
     • Endocrine System:
       • Composed of various glands that release hormones, which act as chemical messengers carried by the bloodstream for longer-term regulation.
3. Response by Effectors:
   • Effectors (muscles and glands) receive output signals from the control center and perform corrective actions, working to bring physiological factors closer to the set point.
   • Examples of Effectors:
     • Muscles: Generate heat through shivering when body temperature drops.
     • Glands: Release insulin to lower high blood glucose levels.
4. Continuous Monitoring and Feedback:
   • Receptors provide continuous feedback to the control center, allowing for ongoing adjustments to keep the factor near the set point.
   • This results in minor fluctuations around the ideal value rather than a constant level.

3. Examples of Homeostatic Regulation

• Blood Glucose Control:
  • High blood glucose prompts insulin release, lowering blood glucose.
  • As glucose returns to normal, insulin secretion ceases due to negative feedback.
  • Stages in blood glucose regulation:
    • High blood glucose → Insulin release → Blood glucose falls → Insulin release stops.
• Body Temperature Control:
  • Receptors in the skin and hypothalamus detect temperature changes.
  • Effectors (e.g., sweat glands for cooling or muscles for shivering) respond, keeping body temperature within an optimal range (around 36.4°C to 37.6°C).

4. Importance of Regulating the Internal Environment

• Ensures stable conditions for enzyme activity, metabolic reactions, and overall cell health.
• Tissue fluid around cells must maintain optimal temperature, water potential, glucose concentration, and pH to support cellular functions efficiently.

5. Differences Between Negative and Positive Feedback

• Negative Feedback:
  • Works to maintain stability by counteracting deviations from the set point, helping stabilize physiological factors.
• Positive Feedback:
  • Amplifies changes, moving the factor further from the set point.
  • An example includes increased CO₂ intake leading to higher breathing rates in CO₂-rich environments, further increasing CO₂ levels in the blood.
  • Positive feedback destabilizes conditions and is rarely used in homeostasis.

6. Review Questions for Understanding

• a. Output:
  • The actions carried out by effectors in response to the input to correct or stabilize the factor.
• b. Describe the immediate environment of a typical cell in a mammal.
  • The cell is surrounded by tissue fluid that supplies essential nutrients, oxygen, and conditions required for function.
• c. Explain why it is crucial to regulate the internal environment.
  • Maintaining a stable internal environment supports enzyme function, osmotic balance, and efficient metabolism.
• d. Explain how stimuli, receptors, coordination systems, and effectors work together in homeostasis.
  • Stimuli trigger responses by causing a change in a factor;
  • Receptors detect this change and send it as input to the control center;
  • Coordination systems process the information, and effectors perform actions to restore stability.
• e. Differentiate between input and output in a homeostatic mechanism.
  • Input: Information received from receptors about changes in a physiological factor, sent to the control center.