• The majority of homeostatic control mechanisms in organisms use?negative feedback?to maintain?homeostatic balance?(ie. to keep certain physiological factors, such as blood glucose concentration,?within certain limits)
• Negative feedback control loops involve:
  • A?receptor?(or sensor) – to?detect?a?stimulus?that is involved with a condition / physiological factor
  • A?coordination system?(nervous system and endocrine system) – to?transfer information?between different parts of the body
  • An?effector?(muscles and glands) – to?carry out a response

   

• Outcome of a negative feedback loop:
  • The factor/stimulus is?continuously monitored
  • If there is an increase in the factor, the body responds to make the factor decrease
  • If there is a decrease in the factor, the body responds to make the factor increase
  • The system is?restored to its original level

   

A negative feedback control loop

• Negative feedback loops help maintain a normal range or balance within an organism
  • They?reduce the initial effect of the stimulus

   

• ?Receptors?detect any deviations from the normal range (stimuli) which results in a?corrective mechanism?to return the factor back to its normal range
• In a negative feedback loop there are usually?two corrective mechanisms:
  • One for when the factor becomes?too low
  • One for when the factor becomes?too high

   

• The corrective mechanisms may involve the nervous system or the endocrine system
• The?magnitude?of the correction required to bring a factor back within its normal range is monitored and?regulated?by negative feedback
  • As the factor gets closer to its normal value the level of correction reduces

   

Two corrective mechanisms are involved in the negative feedback loop

• In the exam you may be asked to identify whether a particular system is a negative or positive feedback loop
• Negative?feedback loops?reduce the effect of the original stimulus?to keep a factor within a normal range
  • There are two?corrective?mechanisms
  • One mechanism causes the factor to decrease
  • Another mechanism causes the factor to increase

   

• In?positive?feedback loops, the original stimulus produces a response that causes the factor to deviate even more from the normal range
  • They?enhance the effect of the original stimulus

   

Positive feedback loop in bone repair

• The repair of broken bones is carried out via a positive feedback loop involving special cells called osteoblasts and osteoclasts
• The osteoblasts secrete a hormone called osteocalcin
  • Osteocalcin is a?protein

   

• They secrete the osteocalcin in an?inactive?form
• The osteoclasts secrete acid which lowers the pH and the?acidic conditions?cause the inactive form of the protein osteocalcin to change into the active form of osteocalcin
  • The low pH alters the hydrogen and ionic bonds in the protein which changes the tertiary structure

   

• The active form of osteocalcin binds to a receptor on beta (β) cells in the pancreas which stimulates them to release insulin
• Osteoblast cells possess?insulin receptors?which when stimulated causes them to release more inactive osteocalcin
• The osteoblast cells?enhance the effect of the original stimulus?(insulin) - positive feedback

A positive feedback loop