CONTROL OF RESPIRATION

1. CONTROL OF RESPIRATION Lecture - 7 Dr. Zahoor Ali Shaikh

2. Control Of Respiration • Respiratory process is involuntary process, but under voluntary control as we can stop breathing. • Respiratory center is in the brain stem. It causes rhythmic breathing pattern of inspiration and expiration. • Inspiratory and Expiratory muscles are skeletal muscles and contract only when stimulated by their nerve supply.

3. Neural Control Of Respiration • We will discuss 1. Center that generate inspiration and expiration. 2. Factors that regulate rate and depth of respiration .

4. Respiratory Center • In Medulla - Inspiratory center - Expiratory center These are neuronal cells that provide output to respiratory muscles for inspiration and expiration. • In Pons - Pneumotaxic center – upper pons - Apneustic center – lower pons Pontine Center influence the output from medullary centers.

6. Respiratory Center • Inspiratory and Expiratory neurons in the medullary center. • We are breathing rhythmically in and out during quiet breathing because of alternate contraction and relaxation of inspiratory muscles [diaphragm and External-intercostal muscles] supplied by phrenic nerve [C-3,4,5] and intercostal nerves .

7. Respiratory Center • Order comes from medullary center to spinal cord motor neuron cell bodies [anterior horn cells]. • When these motor neurons are activated, they stimulate the inspiratory muscles leading to inspiration. • When these neurons are not firing, the inspiratory muscles relax and expiration takes place.

9. Respiratory Center Medullary respiratory center • It has two neuronal groups: 1. Dorsal Respiratory Group [DRG] – Inspiratory neuron. 2. Ventral Respiratory Group [VRG] – Expiratory neuron.

10. Respiratory Center • Dorsal Respiratory Group [DRG] • It consist of mostly inspiratory neurons, when DRG fire, inspiration takes place, when they stop firing, expiration takes place. • DRG has important connection with VRG. • Ventral Respiratory Group [VRG] • It is composed of both inspiratory and expiratory neurons. • VRG remain inactive during normal quiet breathing.

11. Respiratory Center • VRG plays role during forceful breathing that is during active expiration [remember normal expiration is passive]. • Only during active expiration, expiratory neuron fire from VRG and stimulate expiratory muscles.

13. Respiratory Center Generation of respiratory rhythm • Before it was thought that DRG generates the respiratory rhythm. • Now it is believed that rhythm is generated by Pre – Botzinger Complex. It displays pace-maker activity causing self induced action potential. • It is located near the respiratory center.

14. Respiratory Center Pontine Center ‘Pneumotaxic & Apneustic Centers’ • Pneumotaxic Center [Upper pons] • It sends message to DRG neurons to stop inspiration, so that expiration can take place. • Apneustic center [Lower pons] • It causes deep inspiration when Pneumotaxic center is damaged, Apneusis occurs [Deep Inspiration] as Apneustic center is free to act in absence of Pneumotaxic center. • Apneusis is seen in brain damage [Pneumotaxic center damage].

15. Respiratory Center‘Summary’ • Inspiratory center [DRG] – Inspiration • Expiratory center [VRG] – used during forced Expiration • Pneumotaxic center – acts on inspiratory center to stop inspiration therefore regulates inspiration and expiration. • Apneustic center – causes Apneusis [deep inspiration] when Pneumotaxic center is damaged.

17. Hering – Breuer Reflex • When tidal volume is large, more than 1 liter e.g. during exercise, then Hering Breuer Reflex is triggered to prevent over inflation of the lungs. • How ? • There are pulmonary receptors in the lungs, they are stretched by large tidal volume. • Action Potential from stretched receptor go via afferent X cranial nerve ( vagus ) to medullary center and inhibit inspiratory neuron. • This negative feedback mechanism helps to cut inspiration before lungs are over inflated.

18. Chemical Control Of Breathing • Chemical factors which affect the ventilation are -PO2 -PCO2 -H+ ion • Their effect is mediated via respiratory chemoreceptor. • We will study chemoreceptors first .

19. CHEMORECEPTORS • There are two types of Chemoreceptors 1. Peripheral Chemoreceptors 2. Central Chemoreceptors Peripheral Chemoreceptors • Peripheral Chemoreceptors are Carotid bodies & Aortic bodies. Carotid Bodies • Carotid body is present near the carotid artery bifurcation on each side. • They contain cells which can sense the level of PO2, PCO2 , H+ ion.

20. Peripheral Chemoreceptors Carotid bodies [cont] • Carotid body sends impulse to respiratory center in medulla via IX cranial nerve [glassophyrangeal]. Aortic bodies • These receptors are situated in the aortic arch . • They also sense the O2, CO2, and H+ ion changes in the blood. • Aortic body sends impulse to respiratory center in medulla via X cranial nerve [vagus].

22. Central Chemoreceptors • They are located in the medulla near the respiratory center . • These central chemoreceptors monitor the effect of PO2, PCO2 , andH+ ion. • This H+ ion is generated by CO2 in the Extra Cellular Fluid [ECF] of the brain which surrounds the central chemoreceptors. • When CO2 increases, we get: CO2 + H2O  H+ + HCO3- • Increased H+ directly stimulates the central chemoreceptors.

23. Effect of PO2, PCO2 , and H+ ion On Peripheral & Central Chemoreceptors Effect On Peripheral Chemoreceptors • Decreased PO2 in the arterial blood – stimulates peripheral chemoreceptors when arterial PO2 falls below 60mm Hg (strong effect). • Increased PCO2 in the arterial blood – weakly stimulates peripheral chemoreceptors. • Increased H+ ion in the arterial blood – stimulates peripheral chemoreceptors.

24. Effect of PO2, PCO2 , and H+ ion On Peripheral & Central Chemoreceptors Effect On Central Chemoreceptors • Decreased PO2 in the arterial blood – depresses the central chemoreceptors when arterial PO2 falls below 60mm Hg. • Increased PCO2 in the arterial blood and [increased H+ in the brain ECF] – strongly stimulates central chemoreceptors. It is dominant control of ventilation.

25. Effect of PO2, PCO2 , and H+ ion On Peripheral & Central Chemoreceptors • IMPORTANT - PCO2 level more than 70 -80mmHg directly depresses the central chemoreceptors and respiratory center.

28. ‘Summary’ • Decreased PO2, increased PCO2, increased H+ ion concentration in arterial blood stimulates Peripheral Chemoreceptors. Most important stimulating factor is decreased PO2 on peripheral chemoreceptors. • Increased PCO2 in the arterial blood and increased H+ ion in the brain ECF strongly stimulates the central chemoreceptors and dominant control of ventilation. -Decreased PO2 in the arterial blood – depresses the central chemoreceptors.

29. What Happens When We Hold The Breath Voluntarily? • When we hold breath, there is increased CO2 and increased H+ ion in the ECF of brain. • It stimulates the central chemoreceptors – which stimulates respiratory center in medulla, therefore, we have to break the breath. • During this period of holding, PO2 does not fall below 60mmHg to cause stimulation of peripheral chemoreceptors, therefore, it is central effect.

30. What You Should Know From This Lecture • Neural Control of Respiration • Name of Respiratory centers in the Brain stem • Role of Inspiratory, Expiratory, Pneumotaxic and Apneustic centers in control of breathing • Pre-Botzinger complex [Pace-maker for respiration] • Hering Breuer Reflex • Chemical Control of Breathing • Peripheral Chemoreceptors • Central Chemoreceptors • Effect of decreased PO2, increased PCO2 and H+ on Peripheral and Central Chemoreceptors

31. Thank you