vagus nerve
Summary
The vagus nerve (CN X) is the longest cranial nerve, providing parasympathetic innervation to the heart, lungs, and GI tract (up to the splenic flexure), and carrying sensory, motor, and autonomic fibers. It plays key roles in heart rate regulation, gastrointestinal motility/secretion, and gag/swallow reflexes.
Detail
The vagus nerve exits the medulla and travels through the jugular foramen, giving branches in the neck, thorax, and abdomen. Functionally, it has four main components: 1) Parasympathetic motor fibers to smooth muscle and glands of the heart, lungs, and GI tract (foregut and midgut, up to splenic flexure) - slowing heart rate, increasing GI motility/secretions. 2) Special visceral motor fibers to pharyngeal and laryngeal muscles (via pharyngeal and recurrent laryngeal branches), controlling swallowing and phonation. 3) Visceral sensory fibers carrying information from thoracic and abdominal organs, aortic arch baroreceptors/chemoreceptors. 4) General sensory fibers from the external ear and dura.
Clinical relevance: Vagal stimulation (e.g., Valsalva maneuver, carotid sinus massage) can terminate supraventricular tachycardias by increasing parasympathetic tone. Vagus nerve damage can cause hoarseness (recurrent laryngeal nerve palsy), dysphagia, uvula deviation away from lesion side, and loss of gag reflex. Vagal nerve stimulation is used therapeutically for refractory epilepsy and depression. The vagus is also key in the vagovagal reflex (e.g., bradycardia/syncope with visceral stimulation) and is implicated in the gut-brain axis. Vagal nuclei include the nucleus ambiguus (motor to pharynx/larynx), dorsal motor nucleus (parasympathetic to viscera), and nucleus solitarius (visceral sensory, taste).
Sources
- First Aid for the USMLE Step 1
- Guyton and Hall Textbook of Medical Physiology
- Kandel's Principles of Neural Science
- Moore's Clinically Oriented Anatomy
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