Regulation of Respiration

Respiratory Center

Consists of:

• Brain stem (Pons & Medulla) ; Main Centre
• Cortex — Voluntary control centre.

– Voluntary Hyper ventilation.

– Breath holding

• Other parts of Brain

– Hypothalamus

– Limbic system (Rage & Fear)

• Pre- Botzinger complex located on either side of medulla
• DRGN & VRGN in medulla project to the pre- Botzinger

pacemaker neurons.

• Pre-Botzinger neurons discharge rhythmically and produce

rhythmical discharges in Phrenic nerves (respiration).

Respiratory Center in Brain Stem:

Commonly called “Respiratory Center”.

Composed of:

1. Medullary respiratory center

–        Dorsal respiratory group (DRG) of neurons

–        Ventral respiratory group (VRG) of neurons

2.Apneustic center

3.Pneumotaxic center

Dorsal Respiratory Center:

Site: Dorsal region of Medulla

Nucleus Tractus solitarius

Role: Inspiration

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.

Pneumotaxic Center:

Inhibits Inspiration

by “Switching off” Insp. ramp.

Chemical Control of Respiration

Objective:

To maintain proper concentration of O2, CO2 & H+ in the tissues.

Chemoreceptors:

• • 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.

H+ Ions:

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.

CO2:

•         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-

O2:

•         No direct effect on Resp. center

Reason:-

Hb-O2 buffer system maintains almost exactly

normal amount of O2 even when pulmonary

PO2 changes from 60 mmHg to 1000 mmHg.

Peripheral Chemoreceptors:

Location: outside the brain;

In carotid bodies – Largest number

In aortic bodies   – Sizable number

In other arteries of thorax

Few in abdominal region

Sensitivity:

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