Correct Answer: D. Auricular branch of vagus
The cough reflex triggered by scratching the external acoustic canal (EAC) is mediated by the auricular branch of the vagus nerve (Arnold's nerve), a sensory branch that provides innervation to the posteroinferior part of the EAC and the auricle. When the EAC is mechanically stimulated, afferent impulses travel via Arnold's nerve to the medulla, where they synapse in the nucleus tractus solitarius. This triggers a reflex arc that activates the vagal motor nucleus, sending efferent signals back through the vagus nerve to the laryngeal and pharyngeal muscles, producing the characteristic cough. This is clinically relevant in Indian ENT practice—patients with cerumen impaction, otitis externa, or post-auricular surgery often experience this reflex cough. The phenomenon is sometimes called "Arnold's reflex" or "auricular cough reflex" and is a normal physiological response. Understanding this anatomy is crucial for explaining unexpected cough to patients during otoscopic examination or cerumen removal procedures.
Why the other options are wrong
A. Auriculotemporal nerve — The auriculotemporal nerve (a branch of the mandibular division of the trigeminal nerve) provides sensory innervation to the skin of the auricle and temporal region, but it does NOT mediate the cough reflex. While it does innervate part of the EAC, it lacks the reflex arc connection to the vagal motor nuclei that would trigger cough. This is a common distractor because students confuse sensory innervation of the ear with the specific reflex pathway. B. Great auricular nerve — The great auricular nerve (from the cervical plexus, C2–C3) provides sensory innervation to the auricle and lower part of the EAC, but like the auriculotemporal nerve, it does not have the necessary reflex connections to trigger cough. Students often select this because they recall it innervates the ear, but they miss that the cough reflex specifically requires vagal efferent pathways, which the great auricular nerve cannot provide. C. Facial nerve — The facial nerve (CN VII) provides motor innervation to muscles of facial expression and parasympathetic fibers to lacrimal and salivary glands, but it does NOT provide sensory innervation to the EAC or mediate the cough reflex. While the facial nerve does have a small sensory component (taste from anterior 2/3 tongue), it is not involved in the auricular cough reflex arc. This option may trap students who confuse cranial nerve anatomy with reflex pathways.
High-Yield Facts
- Arnold's nerve (auricular branch of vagus) innervates the posteroinferior EAC and triggers cough on mechanical stimulation—the only cranial nerve branch with this reflex property.
- Auricular cough reflex is mediated by vagal afferent → medullary nucleus tractus solitarius → vagal motor efferent → laryngeal/pharyngeal muscles.
- Cerumen impaction and otitis externa commonly elicit this reflex in Indian clinical practice; patients may cough during otoscopic examination.
- Auriculotemporal (CN V3) and great auricular (C2–C3) innervate the ear but lack efferent vagal connections—they do NOT cause cough.
- Post-auricular surgery (mastoidectomy, canal wall down) may alter this reflex due to nerve injury; clinically important for counseling patients.
Mnemonics
COUGH REFLEX = VAGus (Arnold's) Cough on scratching EAC → Vagus (Arnold's nerve) → Afferent to medulla → Goes back via vagal motor → Unexpected cough in clinic → Sensory posteroinferior EAC. Use this when you see 'cough + ear canal' in a question. EAC Innervation Rule: 'VAG + TGB' VAG = Vagus (posteroinferior EAC, cough reflex) | T = Trigeminal/auriculotemporal (anterior/superior) | G = Great auricular (inferior) | B = Branches of facial (small part). Only VAG triggers cough.
NBE Trap
NBE pairs "ear innervation" with "trigeminal and cervical plexus" to lure students into selecting auriculotemporal or great auricular nerve—both DO innervate the ear, but neither mediates the cough reflex. The trap is confusing sensory supply with reflex arc completion.
Clinical Pearl
In Indian outpatient ENT clinics, patients with cerumen impaction or acute otitis externa often cough unexpectedly during otoscopic examination—this is Arnold's reflex in action. Forewarning the patient ("You may feel a cough when I examine your ear") improves compliance and reduces anxiety, especially in pediatric cases seen in government health centers.
_Reference: Bailey & Love Ch. 42 (Otology); Robbins Ch. 28 (Cranial Nerves); Harrison Ch. 379 (Cough)_