BIO 5406 Notes, 1/18/05

 

 

CONTROL SYSTEM THEORY

 

 

I.  Control Systems.                                                       [Hadley, pp. 7-12]

 

     A.  Control systems are designed to maintain homeostasis.

 

          1.  Homeostasis  =  Maintaining a constant internal environment in spite

               of changes in the external environment.

 

     B.  Objective is to maintain a controlled variable within the range of a

           set point.

 

     C.  Five elements of a control system.

 

          1.  Sensor.

 

               a.  Monitors the status of some controlled variable.

 

               b.  Example:

 

          2.  Afferent pathway.

 

               a.  Carries information from the receptor to the control center.

 

               b.  Example:

 

          3.  Control center.

 

               a.  Compares the current status of the controlled variable to the set

                    point.

 

               b.  Makes a decision as to what action should be taken.

 

               c.  Example:

 

          4.  Efferent pathway.

 

               a.  Carries information from the integrating center to the effector.

 

               b.  Example:

 

          5.  Effector.

 

               a.  Produces changes in the controlled variable in response to the

                    commands of the control center.

 

               b.  Example:

 

          6.  Summary (figure^a).

 

     D.  Endocrinology example.

 

          1.  Sensor  ---->

 

          2.  Afferent pathway  ---->

 

          3.  Control center  ---->

 

          4.  Efferent pathway  ---->

 

          5.  Effector  ---->

 

     ^aWidmaier, E.P., Raff, H., and Strang, K.T.  Human Physiology:  The Mechanisms of

       Body Function, 9th ed., 2004.  Links are password-protected.
 

 

II.  Control System Diagrams.                                     [Engelberg paper]

 

     A.  Meaning of arrows.

 

          1.  A        >  B   If A increases, then B will increase.

                                    If A decreases, then B will decrease.

 

          2.  A  ----->  B   If A increases, then B will decrease.

                                    If A decreases, then B will increase.

 

     B.  Example from Engelberg^b (figure).

 

     C.  Negative feedback loops.

 

          1.  When a change in the controlled variable triggers a response that

               opposes the change (fig. 1.4, figure^c).

 

          2.  Maintains homeostasis.

 

          3.  Odd number of broken arrows (figure).

 

          4.  Examples of negative feedback loops in endocrinology:

 

 

 

 

 

 

 

 

 

     C.  Positive feedback loops.

 

          1.  When a change in the controlled variable triggers a response that

               sustains or accelerates the change (fig. 1.4).

 

         2.  Even number of broken arrows.

 

         3.  Brief or destructive.

 

         4.  Examples of positive feedback loops in endocrinology.

 

 

 

 

 

 

     ^bEngelberg, J.  Integrative physiology:  On mapping the organism.  Physiologist 26: 142-144,

      1983.

     ^cFox, S.I.  Human Physiology, 7th ed., 2002.  Links are password-protected.