Q3 LONG ANSWER 10M ★ High-yield

Describe the nervous and chemical regulation of respiration. Add a note on the Hering–Breuer reflex.

Physiology · 1st Professional — Pre-Clinical

Respiration is regulated by (A) Nervous and (B) Chemical mechanisms, so as to keep arterial PO2, PCO2 and pH within narrow limits.

(i) Medullary centres

Centre	Site	Function
Dorsal respiratory group (DRG)	Nucleus tractus solitarius	Generates basic rhythm; drives inspiration; "ramp" discharge
Ventral respiratory group (VRG)	Nucleus ambiguus and retroambiguus	Inactive in quiet breathing; drives forced inspiration and active expiration

(ii) Pontine centres

Centre	Site	Function
Pneumotaxic	Upper pons (nucleus parabrachialis)	Limits inspiration; strong signal → ↑ respiratory rate
Apneustic	Lower pons	Prolongs inspiration; normally overridden

Voluntary control — cerebral cortex → corticospinal tract → respiratory motor neurones (breath-holding, speech, singing)

Feature	Central	Peripheral
Site	Ventral surface of medulla	Carotid bodies (bifurcation of common carotid), aortic bodies
Nerve	—	Carotid → glossopharyngeal (IX); Aortic → vagus (X)
Main stimulus	H+ in CSF (derived from CO2)	↓ PaO2 below 60 mmHg
Also responds to	—	↑ PCO2, ↑ H+
Share of CO2 response	80%	20%
Speed	Slow	Rapid

Role of CO₂ — the most powerful normal stimulus

↑ PaCO2 → CO2 crosses BBB freely (H+ does not) → CO2 + H2O → H2CO3 in CSF → → H+ + HCO3− → Stimulates central chemoreceptors → ↑ Ventilation

A rise of only 1–2 mmHg in PaCO2 can double alveolar ventilation
Hypoxia acts only through peripheral chemoreceptors, and only below PaO2 60 mmHg → an emergency drive, not a routine regulator

An inflation reflex that protects the lung from over-distension.

Component	Detail
Receptors	Slowly adapting stretch receptors in smooth muscle of bronchi and bronchioles
Afferent	Vagus nerve
Centre	DRG in medulla
Threshold	Tidal volume > 1–1.5 L
Response	Inhibits inspiration; prolongs expiration

Not active in quiet breathing in adults — threshold volume never reached
Important in the newborn, in exercise, and during mechanical ventilation
Deflation reflex — converse; stimulates inspiration when lung collapsed; basis of periodic sighing

J-receptors (juxtacapillary) → stimulated by pulmonary congestion → rapid shallow breathing, dyspnoea
Irritant receptors → cough, bronchoconstriction
Baroreceptor reflex → ↑ BP → reflex hypoventilation
Proprioceptors in joints and muscles → hyperpnoea of exercise

– Neurogenic — collateral impulses from motor cortex to respiratory centre (feed-forward)

– Proprioceptors in moving joints and muscles

– ↑ body temperature

– ↑ K+ released from exercising muscle

– Lactic acidosis — only in severe exercise, beyond the anaerobic threshold

Glomus (type I) cells — contain dopamine; the actual chemosensors
Sustentacular (type II) cells — supporting
Highest blood flow per gram of any tissue in the body (2000 mL/100 g/min)
Hence they sense arterial PO2, not O2 content → not stimulated in anaemia or CO poisoning

Chronic type II respiratory failure (COPD) — central chemoreceptors become insensitive to CO2 → respiration maintained by hypoxic drive. High-flow O2 abolishes this drive → apnoea. Hence controlled O2 therapy
Opioids and anaesthetics → depress medullary centres → blunt CO2 response → hypoventilation
Voluntary hyperventilation → washes out CO2 → apnoea, tetany

CLINICAL PEARL

Note: CO2 is the normal drive to breathing; O2 is the emergency drive. This reverses in chronic CO2 retainers.

All questions in Respiratory System