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.

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