Feedback Mechanisms

Feedback Mechanisms and Homeostasis

Feedback mechanisms come in two types, negative feedback and positive feedback.  Both involve these major components:  stimulus, detector, control center, and effector.  Here are the definitions for each of these as they relate to homeostasis…

  • Stimulus:  A change in an environmental variable that is headed outside of the homeostatic range.
  • Detector:  This is some form of receptor that detects the stimulus and sends the information to a control center (sometimes in the same organ).
  • Control center (also known as integration center):  This is an organ or group of cells that acts as controller.  The control center knows what the homeostatic range is and it is capable of stimulating effectors that can change the environmental variable.
  • Effector:  This is a cell, organ, or organs that can alter the environmental variable taking it either up or down.  There are usually at least two effectors for each environmental variable, one that can cause an increase, and another that can cause a decrease (often there are more than two).

 

Negative feedback:  In negative feedback the objective is to stay within homeostatic range.  If any variable starts to head outside of homeostatic range a negative feedback mechanism will be triggered and the variable will be pushed back into homeostatic range…  pushing that variable back to homeostasis is the “negative” in negative feedback.  Here’s an example…

 

Example of negative feedback:  “Standing up quickly and seeing spots”

Have you ever quickly gotten up from a laying position and noticed that you felt a bit lightheaded, maybe saw some spots in your vision, or even fainted?  The reason this happens is that while laying down our heart and blood vessels do not need to work as hard to maintain blood pressure, so the vessels relax a bit and the heart may slow a bit.  When we stand quickly the heart and blood vessels are still working “slowly” and as a result there is a quick drop in blood pressure (that is what causes the the lightheaded feeling).  Most of the time we don’t pass out though, and we might feel our heart pounding for a little while and then come back to normal.  here is how this happens using the terms for feedback components…

  • Stimulus:  blood pressure is dropping below homeostatic range.
  • Receptor:  baroreceptors in our major blood vessels (these are basically pressure receptors that detect blood pressure) send signals to the cardiac control center in the brain letting it know that blood pressure is going down.
  • Control center:  Cardiac control center in the medulla oblongata (a structure in the brain).  This area in the brain monitors information about blood pressure.
  • Effector:  The cardiac control center sends signals to the heart telling it to increase heart rate.  That increase in heart rate brings blood pressure back into the homeostatic range.  This higher rate will be maintained until the blood vessels contract a bit (it takes them a bit longer to respond).

 

Positive feedback mechanisms:  Positive feedback mechanisms often push a given environmental variable outside of the homeostatic range and because of this positive feedback mechanisms are potentially dangerous.  However, normal positive feedback mechanisms are very important.  The job of these normal positive feedback mechanisms is to accomplish something important that need to happen.  Once that thing is done we return to negative feedback…  In this way normal positive feedback mechanisms are really part of larger negative feedback mechanisms.  Here’s an example…

 

Positive feedback example:  “The positive in childbirth”

When a woman is in active labor there is a very important positive feedback mechanism at work.  This positive feedback mechanism’s job is to make sure that the baby is completely born, that the afterbirth is also pushed out, and that the woman does not blood to death from the walls of her uterus.  Here is how it works again using the terms of feedback mechanisms.

  • Stimulus:  The cervix (opening of the uterus) is being stretched.
  • Receptor:  Stretch receptors on the wall of the cervix detect that the cervix is being stretched and send signals to the hypothalamus (collection of neurons in the brain) letting it know that the cervix is being stretched.
  • Control center:  The hypothalamus gets those signals from the stretch receptors and in response it causes the release of oxytocin (a hormone that causes the uterus to contract).
  • Effector:  The uterine wall receives the oxytocin signal and smooth muscles in the wall of the uterus squeeze the baby towards the cervix, stretching it further.
  • From here we return to the starting stimulus (which is stronger now), until the baby is completely pushed outside of mom.  Once that is completed the high levels of oxytocin cause the uterus to continue to contract and push out the afterbirth and keeps on contracting for some time to help staunch the bleeding.
  • There is no longer stretching of the cervix now and slowly the oxytocin levels drop and uterine contractions decrease over time.  The woman’s body returns to homeostasis.

From this story you can see the importance of this particular positive feedback loop.  If the baby was only half birthed it would be a bad day for mom and baby.  Positive feedback mechanisms work to accomplish important tasks that must be completed.

 

Here are just two more examples with short blurbs as to the purpose of each.  Notice that in each case the person could die without the positive feedback loop:

  • Blood clotting:  this involves a positive feedback mechanism activating platelets and fibrinogen to stop bleeding.  The bleeding must be stopped!!!
  • Fever:  this involves a positive feedback mechanism that raises body temperature well outside of homeostatic range in order to help kill an infection.  The infection must be killed!!!!

 

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