Instructions: Begin by diagramming the general scheme for a negative feedback control system:
(stimulus) > RECEPTOR > CONTROL CENTER > EFFECTOR > (response)
Leave this scheme up and then show Figure 6–16 from Section 6-8 in Fundamentals of A & P. Use the general scheme to conceptualize the information in the text figure.
Do not forget to add and label the feedback path that “feeds” the corrective action “back” to the disturbance. That is, a departure from the normal range of the physiological variable that is being controlled. In this example, that variable is the concentration of ionized calcium in the plasma.
For factors that increase calcium (panel a), ask students to systematically match up the real-world components in the projected figure with the theoretical scheme above, beginning with the disturbance. Students may have a hard time acknowledging that the receptor and control center are both located in the same place (parathyroid gland cell).
Repeat the exercise for panel b. To keep the discussion realistic, point out that it does not come into play until calcium ion levels have risen to levels not often reached in healthy people.