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CONTROL OF RESPIRATION

Lecture - 7Dr. Zahoor Ali Shaikh

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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.

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Neural Control Of RespirationWe will discuss 1. Center that generate inspiration and

expiration. 2. Factors that regulate rate and depth of

respiration .

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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.

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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 .

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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.

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Respiratory Center

Medullary respiratory center • It has two neuronal groups: 1. Dorsal Respiratory Group [DRG] –

Inspiratory neuron. 2. Ventral Respiratory Group [VRG] –

Expiratory neuron.

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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.

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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.

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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.

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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].

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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.

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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.

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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 .

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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.

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Peripheral ChemoreceptorsCarotid 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. • Carotid body sends impulse to respiratory center in

medulla via X cranial nerve [vagus].

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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.

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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.

• Increased PCO2 in the arterial blood – weakly stimulates peripheral chemoreceptors.

• Increased H+ ion in the arterial blood – stimulates peripheral chemoreceptors.

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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.

IMPORTANT - PCO2 level more than 70 -80mmHg directly depresses the central chemoreceptors and respiratory center.

• Increased H+ ion in the arterial blood – can not cross blood brain barrier, therefore, does not affect central chemoreceptor.

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‘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 CO2 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.

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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.

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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