PONTO MEDULLARY RESPIRATORY CENTERS
ALL ARE PAIRED & INTERCONNECTED
- Location: Upper Pons
- Absence causes APNEUSTIC BREATHING (Esp when the vagi are cut)
- Curtails inspiratory activity & thus can increase the rate of respiration
- 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.
- Peripheral Chemoreceptors
- Central Chemoreceptors.
- The four influences from the lungs are:
- Pulmonary stretch receptors
- Lung irritant receptors
- J receptors
- 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’
PO2 levels in blood.
PCO2 levels in blood.
Concentration in blood.
CO2 & [H+] act centrally while the Oxygen levels act on the peripheral chemoreceptors.
- CHEMORECEPTOR ZONE:
- 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.
- PRIMARY STIMULUS:
- 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.
- 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.