- Brain stem (Pons & Medulla) ; Main Centre
- Cortex — Voluntary control centre.
– Voluntary Hyper ventilation.
– Breath holding
- Other parts of Brain
– Limbic system (Rage & Fear)
- Pre- Botzinger complex located on either side of medulla
- DRGN & VRGN in medulla project to the pre- Botzinger
- Pre-Botzinger neurons discharge rhythmically and produce
rhythmical discharges in Phrenic nerves (respiration).
Respiratory Center in Brain Stem:
Commonly called “Respiratory Center”.
- Medullary respiratory center
– Dorsal respiratory group (DRG) of neurons
– Ventral respiratory group (VRG) of neurons
Dorsal Respiratory Center:
Site: Dorsal region of Medulla
Nucleus Tractus solitarius
Mechanism: – Intrinsic periodic firing
– Insp. ramp signals
(Inhibited by Pneumotaxic Center)
Ventral Resp. Group:
Site: 5 mm Ant & Lat. to Dorsal respiratory group of neurons.
in Nucleus ambiguus & Retro ambiguus
Role: – Quiescent during quiet breathing
– Expiration during exercise
Apneustic Center: in lower pons
– Excitation of inspiratory area.
– Prolong Insp. ramp.
by “Switching off” Insp. ramp.
Chemical Control of Respiration
To maintain proper concentration of O2, CO2 & H+ in the tissues.
- Central chemoreceptors (chemo sensitive area)
– Direct chemical control of Resp. center activity
- Peripheral Chemoreceptors
– Indirect chemical control of Resp. center activity
Chemosensitive Area of Resp. Centre:
• Located bilaterally less than 1 mm beneath the ventral surface of medulla.
• Highly sensitive to changes in bl. PCO2 or H+ conc.
• Excites Medullary Resp. Centre.
Stimulation of Chemosensitive Area.
Primary stimulus/only direct stimulus for the neurons of chemo sensitive area.
Changes in H+ conc. in blood actually have less effect in stimulating chemo sensitive neurons than do changes in PCO2.
because: • H+ & HCO3- cannot easily cross Blood brain barrier.
• Normal pH of cerebrospinal fluid is 7.32 Less buffering power
• Less Proteins in CSF
• CO2 is Lipid-soluble & diffuses easily
thus changes in CSF pH for given change in PCO2 is greater than that in blood.
• Acute Effect:-
Stimulates chemo sensitive neurons indirectly;
it has very little direct effect.
“Since CO2 is lipid soluble, PCO2 in blood, immediately leads to PCO2 in CSF & brain tissue (i.e interstitial fluid or medulla)”.
thus – H+ conc. promptly rises in CSF.
• Chronic Effect:-
Decreased stimulatory effect of CO2 after 1-2 days, due to : –
i. Renal adjustment of H+
ii. Very slow diffusion of HCO3-
• No direct effect on Resp. center
Hb-O2 buffer system maintains almost exactly
normal amount of O2 even when pulmonary
PO2 changes from 60 mmHg to 1000 mmHg.
Location: outside the brain;
In carotid bodies – Largest number
In aortic bodies – Sizable number
In other arteries of thorax
Few in abdominal region
Respond to: decrease in arterial PO2
increase in arterial pH & in arterial PCO2
Role of Art. PO2:
• No direct effect on resp. center.
• Acts through carotid bodies.
the sensitivity of receptors begin at 500 mmHg
max. sensitivity of receptors between 60-30 mmHg
Role of Art. PCO2/pH.
• Indirect effect is less important i.e. PCO2 & H+ conc.
• Stimulate central chemo directly.
• Direct effect is 7 times more powerful.
• Indirect effect is 5 times rapid.
(Important at the onset of exercise)
i.e. through carotid bodies.
In human carotid but not aortic bodies respond to fall in Art. pH