RESPIRATORY SYSTEM (CONTROL OF RESPIRATION) Dr. Mohammed Sharique Ahmed Quadri Assistant Prof. physiology Al maarefa college 1

Objectives To understand how the respiratory centers control breathing to maintain homeostasis. To examine how PCO 2, pH, PO 2, and other factors affect ventilation. To understand the relationship between breathing and blood pH.

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. 5

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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. 8

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

Respiratory Center  Dorsal Respiratory Group [DRG] It consist of mostly inspiratory neurons, when DRG fire(inspiratory ramp signals), inspiration takes place, when they stop firing, expiration takes place. Inspiratory “ramp” signals begins weakly and increases steadily in a ramp manner for about 2 sec,(initiating inspiration) then it ceases abruptly for 3 sec(leads to expiration). DRG has important connection with VRG. 10

Respiratory Center  Ventral Respiratory Group [VRG] It is composed of both inspiratory and expiratory neurons. These neurons remain totally inactive during normal quite breathing Appears to function mainly during forced expiration, stimulating the internal intercostals and abdominal muscles to contract 12

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. 13

Pontine Respiratory Centers A pneumotaxic center, located dorsally in upper Pons. The primary effect is to control the “switch-off” point of the inspiratory ramp signals from DRG of neurons The function of the pneumotaxic center is primarily to limit inspiration

Pontine Respiratory Centers The apneustic area in the lower Pons sends stimulatory impulses to the inspiratory area that activate it and prolong inhalation. It causes deep inspiration when Pneumotaxic center is damaged, Apneusis occurs [Deep Inspiration] Thus pontine centers modify inspiration and allow for smooth transitions between inspiration and expiration. 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. 16

Overall Control of Activity of Respiratory Centre

Chemical Control of Respiration Although the basic rhythm of breathing is established by the respiratory centers, it is modified by input from the central and peripheral chemoreceptors. They respond to changes in the PCO2, pH, and PO2 of arterial blood, which are the most important factors that alter ventilation. The ultimate goal is to maintain proper concentration of O2 and CO2 in the body

CHEMORECEPTORS There are two types of Chemoreceptors 1. Peripheral Chemoreceptors 2. Central Chemoreceptors 19

1- Central Chemoreceptors Pathway Central chemoreceptors Lying just beneath ventral surface of medulla Sensitive to H + generated from CO 2 within the CSF. Sends signals directly to the respiratory centers. Most sensitive to change in PCO 2,H + conc., but not to PO 2

Under normal conditions, ~75-85% of respiratory drive is due to stimulation of central chemoreceptors by CO 2 Central chemoreceptors are directly stimulated only by H + But H + can not cross blood brain barrier while CO 2 can So, how central chemoreceptors are stimulated by an increase in arterial PCO 2 ? 1- Central Chemoreceptors Pathway Central chemoreceptors

Central chemoreceptors are stimulated by an INCREASE in H+ & PCO2 Medullary respiratory center Increase ventilation Decrease arterial Pco2 4 th ventricle

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 O 2, CO 2, and H+ ion changes in the blood. Aortic body sends impulse to respiratory center in medulla via X cranial nerve [vagus]. 23

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2- Peripheral Chemoreceptor Pathway Stimulation of Peripheral chemoreceptors The carotid & aortic bodies are sensitive to fall in PO 2, an increase in PCO 2 or H + concentration They maximally stimulated when PO 2 decreases below 60mm Hg They detect changes in dissolved O 2 but not in the O 2 that is bound to Hb (e.g. in anaemia there is normal PO 2 but reduced content of O 2 bound to Hb)

Summary of the Effect of ↑arterial PCO2 on ventilation

Effect of a decreased arterial PO 2

Effects of increased arterial H + that are not due to altered co 2 on ventilation relieves + cannot penetrate BBB + no + Arterial non CO 2 -H + Medullary respiratory center Increase ventilation decrease arterial pco 2 Central chemoreceptor Peripheral chemoreceptor acidosis decrease co 2 -H +

Summary of Chemical Pathways stimulating Ventilation

II- Neurogenic Reflexes Hering-Breuer Reflex J- receptor reflex Reflex Cough & sneezing Reflexes Other influences (mediated via hypothalamus)

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 stretch receptors in the bronchioles, 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. 31

Neurogenic Reflexes 2- J-receptor Reflex Pulmonary emboli or oedema →juxtapulmonary-capillaries receptors →vagal afferent to respiratory centre → rapid shallow respiration These receptors are responsible for the sensation of air hunger (Dyspnea; shortness of breath)

Neurogenic Reflexes 3- Cough, Sneezing reflexes Dust, smoking, irritant substances → stimulation of irritant receptors in upper airways →afferent signals via vagus (Upper airways, {larynx, cough}) or trigeminal or olfactory (nose, sneezing) → respiratory centre → deep inspiration followed by forced expiration against closed glottis →opening of glottis →forceful outflow of air

Neurogenic Reflexes Other Influences from higher centres hypothalamus & limbic system Temperature: Increases respiratory rate EMOTIONS : Increases respiratory rate

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‘Summary’ PERIPHERAL CHEMO RECEPTORS ARE Most SENSITIVE TO decreased Po 2. central chemoreceptors are most sensitive to Increased increased H+ ion in the brain ECF.strongly stimulates the and dominant control of ventilation. -Decreased P O 2 in the arterial blood – depresses the central chemoreceptors. 36

References Human physiology by Lauralee Sherwood, seventh edition Text book physiology by Guyton &Hall,11 th edition Text book of physiology by Linda.s contanzo,third edition