Correct Answer: C. Cricothyroid
The cricothyroid muscle is the sole tensor (lengthener) of the vocal cords. It is the only intrinsic laryngeal muscle innervated by the external branch of the superior laryngeal nerve (a branch of the vagus nerve), while all other intrinsic muscles are innervated by the recurrent laryngeal nerve. The cricothyroid muscle has two parts: the pars recta (straight part) and pars obliqua (oblique part). When it contracts, it tilts the thyroid cartilage forward and downward on the cricoid cartilage, increasing the distance between the thyroid and arytenoid cartilages. This elongates and tenses the vocal cords, raising pitch and enabling phonation at higher frequencies. This mechanism is critical for voice modulation and is tested frequently in NEET PG anatomy. The cricothyroid is unique in its action and innervation pattern, making it the definitive answer for any question on vocal cord tensors.
Why the other options are wrong
A. Lateral cricoarytenoid — The lateral cricoarytenoid is an abductor of the vocal cords (opens the airway), not a tensor. It rotates the arytenoid cartilages laterally, pulling the vocal cords apart. It is innervated by the recurrent laryngeal nerve. This is a common trap—students confuse abduction with tensioning. B. Thyroarytenoids — The thyroarytenoid muscles are relaxers (shorteners) of the vocal cords, not tensors. They run from the thyroid cartilage to the arytenoid cartilages and, when contracted, shorten and relax the cords, lowering pitch. They are innervated by the recurrent laryngeal nerve. NBE often pairs this with cricothyroid to test understanding of opposing actions. D. Posterior cricoarytenoids — The posterior cricoarytenoid is an abductor of the vocal cords, rotating the arytenoid cartilages medially to open the airway. It is the only muscle that abducts the cords and is innervated by the recurrent laryngeal nerve. It has no role in tensioning the vocal cords.
High-Yield Facts
- Cricothyroid is the only tensor of vocal cords; all other intrinsic laryngeal muscles are relaxers or abductors/adductors.
- External branch of superior laryngeal nerve (vagal branch) innervates cricothyroid; all other intrinsic muscles are innervated by recurrent laryngeal nerve.
- Cricothyroid contraction tilts the thyroid cartilage forward, elongating vocal cords and raising pitch.
- Thyroarytenoid muscles shorten and relax cords (lower pitch); lateral cricoarytenoid abducts cords; posterior cricoarytenoid abducts cords.
- Unilateral recurrent laryngeal nerve palsy spares cricothyroid function (external SLN intact), allowing some pitch control despite vocal cord paralysis.
Mnemonics
Intrinsic Laryngeal Muscle Actions (LAPP) Lateral cricoarytenoid = ABduct; Arytenoid (Interarytenoid) = ADduct; Posterior cricoarytenoid = ABduct; Pitch control = Cricothyroid (Tensor). Use this to remember that cricothyroid stands alone as the tensor. Innervation Rule: 'External = Cricothyroid' External branch of superior laryngeal nerve → Cricothyroid (the only exception to recurrent laryngeal nerve rule). All other intrinsic muscles = recurrent laryngeal nerve. Memorize this one pairing.
NBE Trap
NBE often pairs cricothyroid with thyroarytenoid in the same question to test whether students understand that these muscles have opposite actions—one tenses (raises pitch), the other relaxes (lowers pitch). Students who confuse "intrinsic laryngeal muscle" with "vocal cord controller" may pick thyroarytenoid by mistake.
Clinical Pearl
In Indian clinical practice, recurrent laryngeal nerve injury (from thyroid surgery, lung cancer, or aortic aneurysm) causes vocal cord paralysis in the paramedian position—but the cricothyroid remains functional via the external superior laryngeal nerve, allowing patients to maintain some hoarseness rather than complete aphonia. This preserved tensioning is why voice is breathy rather than absent.
_Reference: Robbins & Cotran Pathologic Basis of Disease (anatomy section on larynx); Gray's Anatomy (Chapter on Larynx and Phonation); Harrison's Principles of Internal Medicine Ch. 29 (voice disorders)_