Homeostasis I Negative and Positive Feedback Mechanism I Feedforward Mechanism I General Physiology I

Homeostasis I Negative and Positive Feedback Mechanism I Feedforward Mechanism I General Physiology I

The slide will be about :
1. Definition of homeostasis
2. What is internal environment ?
3. Why ECF is considered as an internal environment for cell ?
4. Homeostatic mechanism
5. Components of homeostatic mechanism
6. Feedback mechanism
7. Negative feedback mechanism
8. Positive feedback mechanism
9. Feedforward mechanism

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1. Homeostasis- Definition Homoios means like and stasis means standing still Homeostasis is defined as the maintenance of nearly constant conditions in the internal environment. Walter Cannon coined the term ‘Homeostasis’ Homoeostasis is defined as the self regulatory capacity to maintain steady states in the body, essential for keeping the individual alive despite of changes in the external environment.

2. Claude Bernard coined the term milieu intérieu which means internal environment. Internal environment is the fluid that surrounds the cells. Internal environment is the extracellular fluid (plasma + interstitial fluid). All the cells live in the same environment that is the ECF. What is internal environment? Why ECF is considered as internal environment for the cell ? ECF bathes the cells continuously. ECF has two components- plasma and interstitial fluid Plasma delivers nutrients, O 2 , growth factors, hormones etc. from the parent organ to the recipient site.

3. There will be movement of these substances from the plasma into the interstitial fluid and from the interstitial fluid into the intracellular fluid (ICF). Similarly, there will movement of waste products (CO 2 , urea, uric acid) from the ICF into the interstitial fluid and then back to the plasma. Plasma then takes these substances to the organ concerned for their removal. Hence, any change in composition of ECF will cause change in intracellular fluid composition.

4. Disturbance in external environment Disturbance in ECF Detected by sensors Activation of homoeostatic mechanisms to reverse the change

5. Homeostatic mechanisms Feedback mechanism Feedforward mechanism

6. Components of homeostatic mechanisms Disturbance in external environment Disturbance in internal environment Variation in regulated variable Detected by sensors Effector response Effector Stimulate control center

7. Components of homeostatic mechanisms 1. Regulated variable- kept within the certain range, sensed by a sensor 2. Sensor – measures the value of regulated variable 3. Control center – control the output of effector response 4. Effector – generates the effector response 5. Controlled variable – manipulated to achieve regulation of regulated variable

8. Feedback mechanisms Feedback mechanisms occurs in response to variation in a variable to bring the variable to its set point. There are two types of feedback mechanisms- Negative feedback Positive feedback

9. External environment Internal environment Disturbance in external environment Disturbance in internal environment Variation in regulated variable Detected by sensors Detected by sensors Variation in regulated variable Stimulate control center Effector Effector Effector response Effector response Stimulate control center

10. Negative feedback mechanism Negative feedback mechanism is a series of changes that return the regulated variable toward its mean value or within its normal range by a negative response to the initiating stimulus (change in regulated variable). Error reduction

11. Variation in MAP (regulated variable ) Detected by baroreceptors (sensors) Stimulate medulla (control center) Impulse go to heart, blood vessels (effector) Change in heart rate, arteriolar diameter, venomotor tone (effector response) External environment Internal environment Disturbance in external environment Disturbance in internal environment

12. Homeostatically regulated variables Regulated variable Normal range Senser Control center Effectors Effector response Arterial PO 2 75-100 mmHg Chemoreceptors (carotid bodies & aortic body) Medulla Diaphragm & respiratory muscles Change breathing sequence & tidal volume Mean Arterial pressure 93 mmHg Baroreceptors (aortic arch, carotid sinus) Medulla Heart & blood vessels Change in HR, arteriolar diameter, venomotor tone Blood glucose concentration 70-110 mg/dl Fed state- ATP sensitive K+ channel Fasting state- chemosensors Pancreas, Hypothalamus Liver, adipose tissue, skeletal muscle Alter glucose metabolism Blood osmolarity 280-296 mosM /kg Osmoreceptors in hypothalamus Hypothalamus Kidneys Alter water reabsorption

13. Gain Gain determines the efficiency of negative feedback mechanism. Greater the value of gain, more efficient will be the mechanism. Gain = correction applied/ residual error For e.g. The MAP raised from its set point of 90 mmHg to 120mmHg during exercise. However, baroreceptor reflex brings it down to 100mmHg. Find the gain of baroreceptor reflex. Solution- Correction applied = 120-100 mmHg = 20 mmHg Residual error = 100-90 mmHg = 10 mmHg Gain = correction applied/ residual error = 20/10 = 2

14. Find the Gain A cold exposure which is expected to bring the body temperature from 37˚C to 30˚C, actually brings it down to only 36.5˚C. Calculate the Gain of the thermoregulatory mechanism. Write you answer in comment section

15. Positive feedback mechanism Positive feedback mechanism is a series of changes that increase the variation in regulated variable by positive response to the initiating stimulus. Error amplification It is a vicious cycle which terminates when the initiating stimulus is removed.

16. Why positive feedback mechanism is considered a homeostatic mechanism ? The positive feedback itself is part of an overall negative feedback process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feedback process for maintenance of normal blood volume. Another example is parturition reflex which is performed to restore the nongravid physiology.

17. Examples of Positive feedback mechanism 1. Enzyme cascade hypothesis of blood coagulation 2. Parturition reflex 3. Hodgkin’s cycle 4. Activation of digestive enzymes 5. LH surge

18. Blood coagulation Injury to blood vessel Activation of clotting factors Activated clotting factors act on other inactivated enzyme This will continue till the permanent clot is formed and hence the stimulus is itself terminated.

19. Matured fetus cause cervix distension Afferent signals from uterus goes to posterior pituitary Posterior pituitary secrete oxytocin Oxytocin cause myometrium contraction Parturition reflex This will continue till birth of child and hence the stimulus is terminated itself.

20. Feedforward mechanism Feedforward or anticipatory control mechanisms permit the body to predict a change in the physiology of the organism and initiate a response that can reduce the movement of a regulated variable out of its normal range. Thus, feedforward mechanisms may help minimize the effects of a perturbation and can help maintain homeostasis. For example, anticipatory increases in breathing frequency will reduce the time course of the response to exercise-induced hypoxia

21. Feedback Vs Feedforward mechanism Feedback mechanism Feedforward mechanism Generate response after the variation is detected Generate response anticipatory to variation Certain time lag between variation and response No time lag between variation and response