Cell signalling pathways are commonly regulated by two distinct feedback mechanisms – negative and positive feedback loops

• Examples of intercellular communication within organisms include autocrine, paracrine and endocrine signalling
  

• Homeostatic processes are controlled by negative feedback loops and so these systems occur more commonly within the body

Negative Feedback

Negative feedback involves a response that is the reverse of the change detected (it functions to reduce the change)

• A signalling molecule will induce a response that is the opposite of the initial stimulus – this promotes equilibrium
  

• Examples of processes that utilise negative feedback loops include:
  

  • Thermoregulation – if body temperature changes, the hormone thyroxin induces changes to restore normal levels

  • Blood sugar regulation – insulin lowers blood glucose when levels are high ; glucagon raises blood glucose when levels are low

  • Osmoregulation – ADH is secreted to retain water when dehydrated and its release is inhibited when the body is hydrated

Positive Feedback

Positive feedback involves a response that reinforces the change detected (it functions to amplify the change)

• A signalling molecule induces an effect that is the same as the initial stimulus – this promotes further change

• Positive feedback loops will continue to amplify the initial stimulus until the signal is removed

• Examples of processes that utilise positive feedback loops include:
  

  • Blood clotting – platelets release clotting factors which cause more platelets to aggregate at the site of injury
    

  • Cell division – The activation of cyclin-dependent kinases (CDKs) may trigger the production of more cyclins