Make your own free website on Tripod.com

Physiology I
Section 4
Syllabi

Physiological Control Theory

Suggested Reading:  Guyton Chapters 1 & 73

Key Words

Homeostasis:  The maintenance of static or constant conditions in the internal environment.  Essentially all of the organs and tissues of the body perform functions that help to maintain these constant conditions.  Guyton Page 4

Control System:  Operate within cells to control intracellular function as well as all extracellular functions.  They operate within the organs to control functions of the individual parts of the organs.  Other systems operate throughout the entire body to control the interrelations between the organs.  Guyton Page 6

Regulated Variable:  In the control system this reflects the item being regulated. In a heating system analogy the regulated variable is the actual room temp. I am assuming that this is the same as controlled variable.

Sensor:  Detect the state of the body or the sate of the surroundings.  Guyton Page 5

Afferent Pathway:  A pathway that leads toward the central nervous system in neurophysiology. It is the sensory signal. In control theory it is the sensory feedback that goes back to the integrating center.

Integrating Center:  This is the Command center in control theory that processes the info and determines what action needs to be taken.

Efferent Pathway:  Pathway going away from the CNS as the motor signal. It is the forcing function of control theory.

Effector:  To be covered in class

Negative Feedback:  Most control systems of the body act by this type of feedback.  In general, if some factor becomes excessive or deficient, a control system initiates negative feedback, which consists f a series of changes that return the factor toward a certain mean value, thus maintaining homeostatis.   Guyton Page 7

Positive Feedback:  The initiating stimulus causes more of the same.  Does not usually lead to stability but to instability and often death.  Positive feedback is better known as a vicious circle, but a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body, and a vicious circle fails to develop.  Positive feedback can be useful though in such things as Blood Clotting, contractions during childbirth and generation of nerve signals.  Guyton Page 8

Stimulus:  To be covered in class

Stability:  To be covered in class

Learning Objectives

Define the terms "homeostasis" and "control system":  Homeostasis:  The maintenance of static or constant conditions in the internal environment.  Essentially all of the organs and tissues of the body perform functions that help to maintain these constant conditions.  Guyton Page 4    Control System:  Operate within cells to control intracellular function as well as all extracellular functions.  They operate within the organs to control functions of the individual parts of the organs.  Other systems operate throughout the entire body to control the interrelations between the organs.  Guyton Page 6

Identify and discuss the general principles of physiological control systems:  Control theory gives one mathematical and graphical shorthand to describe systems in a way to compare different control systems. It can describe the system, predict its behavior under certain conditions, and it can act as a model system to form hypothesis on and test or clinically can be used as a diagnostic tool. Control Theory begins with a command signal that goes through a controller, which produces a forcing function, which has an effect on the control of the system, which also, has an effect from outside disturbances. The control system then yields a controlled variable. In a heating system analogy the command signal would be the thermostat, the controller would be the furnace which puts out heat which is the forcing function. Heat goes into a control system, which is the room or house. The controlled variable is the actual room temp. The disturbances would be any heat or cold from another source like the cold outside. This is known as an Open Loop System. If you turn up the thermometer, the furnace will produce heat and room temp will increase. In a more advanced system you can add a feedback feature to have more control on the amount of heat the forcing function produces. The feedback takes the controlled variable and senses it with a sensor, which has input into the controller and then the new control signal of the room temp is the result. This method uses a desired value of a SetPoint. If you set the set point to 68degrees and the actual room temp is 50 degrees then the sensor or the feedback goes back to the integration center and compares the set point with the actual value. The error signal will be an 18 degree difference so the furnace kicks on and the temp increases.

Discuss the advantages/disadvantages of viewing physiological mechanisms as a complex multi-level control system operating simultaneously at the cellular, tissue, organ, organ system, and organismal levels:  This theory gives you another set of tools to look at the way the body controls different areas, whether it be BP or temp or whatever. This gives you a mathematical and graphical look at the physiological systems. Disadvantages are from my point of view, yet another theory to remember!! I donít find this particularly helpful.

Identify the primary importance of cell-to-cell communication in homeostatic control mechanisms:  To be covered in class

name and describe the components of a typical reflex arc:  To be covered in class

Define and describe the various levels of control mechanisms, their feedback, and their stability:  There are crude systems that are very slow and methodical in their function but are very stable and reliable. The opposite end of the spectrum is a fine-tuned and rapid system that works very fast as a controller and becomes faster and more responsive but is then also more capable of getting out of control and being very unstable.

  1. ON/OFF System: Simple switch control. If it gets too hot, turn the stove off, too cold, turn the stove on. This system is slow to respond and overshoots the temp then allows the temp to drift back past the set point and then kicks on and overshoots again. There are large variations in the pattern of on and off.
  2. Proportional Control: or Linear. This output varies with the magnitude of control signal. So if it is very cold out the heating system will produce lots of heat per minute, rapidly but if its only a little cool out will put out only a little heat. The intensity is a measure of how far it is from set point. The intensity of response is dependent on the difference in the size of the signal. This curve will change much faster and stay closer to the set point.
  3. Rate or velocity control: The output changes as a function of change in input. Senses the rate of change in different stimuli. The response will be faster and the response to changes will be faster so the curve will look very close to the set point.
  4. Acceleration: Senses the rate of change of the rate of change of the input.

With each system the response is sped up and you see less oscillation but then more and more instability is introduced. Speed and stability are inversely proportional.

Describe the components of the Thermoregulatory System which play a role in it's control of body temperature.  Characterize it's organization and efficacy as a control system:  The temperature of the body is regulated almost entirely by nervous feedback mechanism, and almost all these operate through temperature-regulating centers located in the hypothalamus.  For these feedback mechanisms to operate, there must also exist temperature detectors to determine when the body temperature becomes either too ho or too cold.
Anterior Hypothalamus-Preoptic area:  (Central Temperature Sensory Signals)  Has the capability of serving as a thermostatic body temperature control center.  Using a thermode, the anterior hypothalamic-preoptic area has been found to contain large numbers of heat-sensitive neurons as well as about one third as many cold-sensitive neurons.  These neurons are believed to function as temperature sensors for controlling body temperature.  When the preoptic area is heated, the skin everywhere over the body immediately breaks out into a profuse sweat while at the same time the skin blood vessels over the entire body become greatly vasodialated.
Skin and Deep Body Tissues:  (Peripheral Temperature Sensory Signals)  Both the skin and deep body tissue receptors mainly concern themselves with detecting cool and cold instead of warm temperatures.  They are more concerned with preventing hypothermia than hyperthermia.
Posterior Hypothalamus:  Here the signals form the preoptic area and the signals from the body periphery are combined to control the heat-producing and heat-conserving reactions of the body.  

Homeostasis:  The maintenance of static or constant conditions in the internal environment.  Essentially all of the organs and tissues of the body perform functions that help to maintain these constant conditions.  Guyton Page 4

Control System:  Operate within cells to control intracellular function as well as all extracellular functions.  They operate within the organs to control functions of the individual parts of the organs.  Other systems operate throughout the entire body to control the interrelations between the organs.  Guyton Page 916 & 917

 

Return To The MNA 2001 Homepage
Last Updated 04/10/00 12:26:59 PM