Hering Breur reflex is mediated by
**Core Concept**
The Hering Breuer reflex is a physiological reflex that helps prevent lung over-expansion during inspiration, thereby maintaining a stable intrathoracic pressure and preventing barotrauma. This reflex involves the stimulation of stretch receptors in the bronchi and bronchioles, which send signals to the brainstem to inhibit further inspiration.
**Why the Correct Answer is Right**
The Hering Breuer reflex is mediated by the slowly adapting stretch receptors (SARs) located in the bronchi and bronchioles. These receptors are activated when the lung expands and stretch the bronchial walls. The SARs send signals to the cranial nerve X (vagus nerve) via the bronchial plexus, which in turn transmits these signals to the medulla oblongata. The medulla then sends an inhibitory signal to the inspiratory neurons, thereby preventing further inspiration and preventing lung over-expansion.
**Why Each Wrong Option is Incorrect**
**Option A:** The Hering Breuer reflex is not primarily mediated by the rapidly adapting stretch receptors (RARs), which are activated by the rapid expansion of the lung during the initial phases of inspiration. While RARs do play a role in the regulation of breathing, they are not the primary mediators of the Hering Breuer reflex.
**Option B:** The Hering Breuer reflex is not primarily mediated by the chemoreceptors located in the carotid and aortic bodies. While these chemoreceptors do play a role in the regulation of breathing in response to changes in blood pH, CO2, and O2 levels, they are not directly involved in the Hering Breuer reflex.
**Option C:** The Hering Breuer reflex is not primarily mediated by the mechanoreceptors located in the parietal pleura. While these mechanoreceptors do play a role in the detection of changes in intrathoracic pressure, they are not the primary mediators of the Hering Breuer reflex.
**Option D:** The Hering Breuer reflex is not primarily mediated by the vagus nerve's preganglionic fibers. While the vagus nerve does play a role in the transmission of signals from the stretch receptors to the brainstem, the Hering Breuer reflex is primarily mediated by the slowly adapting stretch receptors themselves.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Hering Breuer reflex is a critical mechanism that helps prevent lung over-expansion and barotrauma, especially during mechanical ventilation. Clinicians should be aware of the importance of this reflex in the management of patients with respiratory failure.
**Correct Answer: C. The Hering Breuer reflex is primarily mediated by the mechanoreceptors located in the parietal pleura.**