Q.Given below are the characteristics of some modified respiratory movement. Identify them. a. Spasmodic contraction of muscles of expiration and forceful expulsion of air through nose and mouth. b. An inspiration followed by many short convulsive expiration accompanied by facial expression.

Breathing rate and depth run automatically, without conscious control, set by respiratory centres — groups of neurons in the pons and medulla of the brainstem — organised into a dorsal (inspiratory) group, a ventro-lateral (inspiratory and expiratory) group in the medulla, a pneumotaxic centre in the pons (limiting inspiration, active in sleep) and an apneustic centre in the medulla that opposes it. These centres respond mainly to chemical cues: rising blood CO2 acts on them directly, while falling blood O2 is sensed by O2 sensors in the aortic arch and relayed by nerve impulse. Once triggered, the centres send signals via the phrenic nerves (to the diaphragm) and intercostal nerves (to the intercostal muscles) to produce a breath.

Breathing rhythm is additionally fine-tuned by the Hering-Breuer reflex: as the lungs expand during inspiration, stretch receptors trigger and send impulses along the vagus nerve to the expiratory centre, which inhibits the inspiratory centre and lets expiration follow; as the lungs deflate, the receptors fall silent and inspiration can begin again — this loop also protects the lungs from over-inflating. Voluntary control is possible too, through connections to the cerebral cortex (letting a person hold their breath to keep out water or irritants), but is limited because rising blood CO2 eventually forces breathing to resume regardless.

Some breathing movements deliberately depart from this quiet rhythm — modified respiratory movements used to clear the airway or express emotion, some reflexive and some voluntary. Sneezing is a spasmodic contraction of the expiratory muscles forcefully expelling air through the nose and mouth together; laughing is a single inspiration followed by many short convulsive expirations with a characteristic facial expression (crying/sobbing is a similar pattern with a different expression and trigger); coughing, yawning and hiccupping are other everyday examples.