CONTROL OF RESPIRATION Lecture - 7 Dr. Zahoor Ali Shaikh

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.

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

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.

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 .

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.

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

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.

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.

Generation of respiratory rhythm 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.

‘Pneumotaxic & Apneustic Centers’ 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].

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.

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.

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 .

Peripheral 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.

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].

Central Chemoreceptors They are located in the medulla near the respiratory center . These central chemoreceptors monitor the effect of PO2, PCO2 , and H+ 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.

Effect On Peripheral Chemoreceptors 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.

Effect On Central Chemoreceptors 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.

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.

‘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.

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.

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

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