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Biology (Year 12) - Homeostasis

Ben Whitten

What is a feedback mechanism? A feedback mechanism is triggered when a stimulus is detected by a receptor, then the information about the stimulus is processed and a message is conveyed to an effector, which carries out a physiological response to the stimulus.

The two types of feedback include:

  • Negative feedback that leads to homeostasis and counteracts the stimulus

  • Positive feedback that reinforces the deviation from the optimal state, and is less common

Feedback mechanisms are physiological processes that respond to small disturbances to keep internal conditions and concentrations of substances within narrow limits for optimal function.

What is negative feedback? A homeostatic process that responds by changing the direction of a stimulus is a negative feedback loop. The initial stimulus is a deviation of a factor away from the norm, such as temperature or water balance, and a negative feedback loop will always reduce the stimulus. The six components of a negative feedback loop include:

  1. Stimulus: A change in one of the internal or external environmental factors is detected by a receptor, and the change is a deviation from the normal/optimal value.

  2. Receptor: The cell or tissue that detects the stimulus (change in the environment); may be external or internal

  3. Modulator (coordinating centre): The structure that receives messages from receptors via sensory neurons, coordinates a response then sends an instruction to an effector via motor neurons

  4. Effector: A muscle or gland that receives the message from the modulator and carries out the response

  5. Response: The action of the effector that counteracts the stimulus

  6. Negative feedback: A message that counteracts the stimulus. It returns the value back to its optimal or normal value.

What is positive feedback? If a response reinforces the original stimulus, the mechanism is called positive feedback. This process increases the output of the system which furthers the deviation from the optimal or normal value. Advantages Positive feedback is rare, but necessary during some developmental processes. For example, the development of frogs and toads is controlled by the hormone thyroxine. Just before the tadpole undergoes metamorphosis into an adult frog, negative feedback is changed into positive feedback and the concentration of the thyroxine increases and triggers metamorphosis. Disadvantages Positive feedback can be very dangerous in terms of homeostasis however. When human body temperature rises during a fever, a new and higher set point for temperature can be established and the person may suffer from heatstroke, cell impairment and eventual death.

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