REGULATION OF RESPIRATION

PONTO MEDULLARY RESPIRATORY CENTERS

ALL ARE PAIRED & INTERCONNECTED

RESPIRATORY CENTERS:-

PNEUMOTAXIC CENTER:

• Location: Upper Pons
• Absence causes APNEUSTIC BREATHING (Esp when the vagi are cut)
• Curtails inspiratory activity & thus can increase the rate of respiration

APNEUSTIC CENTER:

• Location: Lower Pons
• Stimulates the Inspiratory Center and increases Inspiration
• Gets feed back from Vagi & other Centers.

RESPIRATORY CONTROL ORGANIZATION:MODERN CONCEPT

• All the respiratory centers are termed as the BULBOPONTINE RESPIRATORY NEURONAL COMPLEX
• There is an inspiratory ramp generator called Respiratory Control Pattern Generator: Pre Bottzinger Complex
• The Inspiratory Off switch(IOS) is fine tuned by PTC & the chemoreceptor drive.
• Both Neural & Chemical controls are well coordinated.

PERIPHERAL INFLUENCES ON RESPIRATORY CONTROL

LUNG OR PULMONARY RECEPTORS:

• Receptors in and around the lungs.

CHEMORECEPTORS

• Peripheral Chemoreceptors
• Central Chemoreceptors.

PERIPHERAL INFLUENCES

• The four influences from the lungs are:
• Pulmonary stretch receptors
• Lung irritant receptors
• J receptors
• Proprioceptors
• Along with the chemoreceptors, these receptors send information to the respiratory centers.

HERING BREUER(HB) REFLEX

• It is a ‘Volume’ reflex.
• Receptors are located in between the smooth muscles of the small airways.
• These receptors are unmyelinated nerve endings.
• They are stimulated by the change of shape of the Airways.

• Excessive deflation of the lungs causes Inspiration.
• This reflex prevents Atelectasis.
• Atelectasis is the collapse of the lungs.
• This reflex also opens up collapsed portions of the lung.

CHEMICAL CONTROL:THE THREE MAIN ‘CHEMICALS’

OXYGEN

PO2 levels in blood.

CARBON DIOXIDE:

PCO2 levels in blood.

HYDROGEN ION:

Concentration in blood.

CO2 & [H+] act centrally while the Oxygen levels act on the peripheral chemoreceptors.

RESPIRATORY CHEMORECEPTORS

• CENTRAL:
• CHEMORECEPTOR ZONE:
• BILATERAL
• LOCATED IN THE MEDULLA
• JUST BENEATH IT’S VENTRAL SURFACE
• HIGHLY SENSITIVE TO PCO2 AND [H+]
• FUNCTIONS BY STIMULATING THE RESPIRATORY CENTERS:
• DRG,VRG & PTC.

CENTRAL CHEMORECEPTORS

• PRIMARY STIMULUS:
• [H+]
• PERHAPS THE ONLY IMPORTANT DIRECT STIMULUS FOR THE CENTRAL CHEMORECEPTOR CELLS (MEDULLARY CHEMORECEPTORS)
• But these ions do not cross the Blood Brain Barrier
• So, the blood PCO2 level has more effect as CO2 readily crosses the BBB.

STIMULATION BY CARBONDIOXIDE

• Is not direct.
• Even the indirect effect of CO2 is most potent. Why?
• Because CO2 easily crosses the BBB.
• Once it is across the BBB,
• CO2 + H2O -- H2CO3 -- H+ + HCO3-
• These increased H+ ions in the brain stimulate the medullary chemoreceptors.

QUANTITATIVE EFFECT OF H+ IONS

• The stimulatory effect of H+ ions increases in the first few hours.
• It then decreases in the next 1 to 2 days.
• It comes down to about 1/5th the initial effect.
• This is due to Renal readjustment of [H+] in the circulating blood.
• The kidneys increase blood HCO3.
• This bicarbonate binds with the free H+ ions in the blood & decreases their concentration.
• Bicarbonate also diffuses slowly past the BBB and decreases the H+ ions in the brain.
• Therefore the effect of H+ ions is:
• POTENT: Acutely
• WEAK : Chronically.

EFFECT OF CO2

• Change in PCO2 between 35 to 75mmHg causes peak increase in alveolar ventilation.
• 
  
• Change in the normal range causes less than tenth of change in alveolar ventilation.

EFFECT OF OXYGEN

• The partial pressure of Oxygen has no effect on the medullary chemoreceptors.
• 
  
• It only has an effect on the peripheral chemoreceptors.

PERIPHERAL CHEMORECEPTORS

• There are two pairs of chemoreceptors:
• Aortic Bodies: located at the arch of aorta.
• Carotid bodies: located at the branching of the common carotid arteries.
• Their functions are:
• To detect changes in the PO2
• To transmit nervous signals to the Respiratory Centers.
• These bodies have two types of special cells called glomus cells.
• The type 2 glomus cells have special ion channels sensitive to PO2.
• They fire the nerve endings and send signals via:
• Aortic bodies: Vagi.
• Carotid bodies: Hering nerve & Glossopharyngeal nerve.
• Both these bodies receive their own special blood supply through minute arteries, directly from the trunk.
• Their blood flow is roughly 20 times their own weight.
• THEY ARE ALL THE TIME EXPOSED ONLY TO ARTERIAL BLOOD.
• Decreased PO2 stimulates these chemoreceptors strongly.

ARTERIAL PO2 & IMPULSES IN AORTIC BODY

Decreased PO2 especially between 60 and 30mm Hg strongly stimulates the carotid bodies.

This is the range wherein the Hb saturation decreases

EFFECT OF PO2

• When PCO2 & [H+] are kept constantly normal,
• There is no effect if the PO2 is >100mmHg
• If it falls below 100mmHg, ventilation doubles upto 60 mmHg.
• It increases upto 5 times at very low PO2 levels

CO2 & H+

• They also stimulate the peripheral chemoreceptors.
• But their effects on the central or medullary chemoreceptors are more powerful.
• PCO2 stimulates the peripheral chemoreceptors 5 times as rapidly as it stimulates the central ones.
• So this is responsible for the rapid response to CO2 at the onset of exercise.