Reflex control of respiratory activity

Information about ventilatory mechanics, and also about chemical stimuli and temperature, is transmitted via afferent fibers running in the olfactory, trigeminal, glossopharyngeal, and vagal nerves. This provokes various reflexes, such as termination of inspiration (Hering-Breuer reflex), sneezing, aspiration, coughing, or vomiting, and behavioral reactions, such as sniffing and inhibition of motor activity during heavy working load.

Afferents from the glossopharyngeal and vagal nerves terminate in a specific relay nucleus known as the nucleus of the solitary tract (NTS) which contains the target neurons of afferents from 'pulmonary stretch' receptors (Hering-Breuer reflex). However, there are also direct projections of pulmonary afferents to pontine regions.

Reflexes from the upper respiratory tract (irritant receptors and mechanoreceptors) may cause bronchoconstriction, laryngospasm, and coughing. They can provoke serious complications during endotracheal intubation in sensitive individuals. Nociceptive receptors (J receptors) are activated by tissue damage, accumulation of interstitial fluid, and release of mediators resulting in rapid shallow breathing. This mechanism may cause dyspnea in pulmonary vascular congestion, lung edema, and pulmonary embolism.

Other sensory inputs (e.g. information of hyperinflation or deflation of the lungs from pulmonary stretch receptors) activate a variety of respiratory and non-respiratory NTS neurons for rapid adaptation (physiological mechanisms to turn off inspiration). Other NTS neurons also receive afferent inputs from arterial chemoreceptors and baroreceptors which can modulate the central respiratory network (e.g. respiratory depression during arterial hypertension).

The specific connectivity of all these relay neurons with the respiratory network is still unknown, but their axonal projections seem to be widespread. Chemical control of respiration

The ultimate goal of respiration is to maintain adequate concentrations of oxygen, CO 2, and hydrogen ions in the body fluid. Excess CO2 or hydrogen ions directly affect the respiratory center, whereas mild hypoxia only acts through the peripheral chemoreceptors. Changes in blood CO 2 play the greatest role in respiratory control (Nunn 1.993.) (see Fig 1).

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