Thyroid & Larynx – Anatomy · FCPS Part 1

💡Thyroid surgery — nerve risks

Surgical site	Nerve at risk	Consequence of injury
Superior pole of thyroid	External branch of superior laryngeal nerve (EBSLN)	Weakness of cricothyroid; difficulty producing high-pitched sounds
Inferior pole / tracheoesophageal groove	Recurrent laryngeal nerve (RLN)	Vocal fold movement impairment, hoarseness; bilateral injury can threaten the airway
Tracheostomy near the thyroid / tracheoesophageal groove	RLN may be injured depending on the level and technique	Vocal fold paralysis

Exam distinction: EBSLN injury impairs tension of the vocal folds; RLN injury impairs most intrinsic laryngeal muscles.

💡Vocal fold movements

Structure / function	Nerve supply	Common FCPS question
Sensory above vocal cords	Internal laryngeal nerve	Branch of superior laryngeal nerve
Sensory below vocal cords	Recurrent laryngeal nerve	Also called inferior laryngeal nerve near its termination
Cricothyroid muscle	External laryngeal nerve	Tenses and lengthens vocal cords
Posterior cricoarytenoid	Recurrent laryngeal nerve	Only muscle that abducts vocal cords
Lateral cricoarytenoid	Recurrent laryngeal nerve	Adducts vocal cords
Transverse and oblique arytenoids	Recurrent laryngeal nerve	Adduct arytenoid cartilages
Thyroarytenoid	Recurrent laryngeal nerve	Relaxes and shortens vocal cords
Vocalis	Recurrent laryngeal nerve	Fine adjustment of vocal cord tension

Clinical pearl: Bilateral RLN injury can cause severe airway obstruction because both vocal folds may remain near the midline. Cricothyroid function is generally preserved because it is supplied by the EBSLN.

💡Laryngeal cartilages

Category	Cartilages	FCPS point
Unpaired (3)	Thyroid, cricoid, epiglottis	Three unpaired cartilages
Paired (3 pairs)	Arytenoid, corniculate, cuneiform	Six individual paired cartilages
Most clinically important	Cricoid	Only complete cartilaginous ring of the airway
Largest cartilage	Thyroid	Forms the laryngeal prominence
Elastic cartilage	Epiglottis, corniculate, cuneiform	Helps maintain flexibility
Mainly hyaline cartilage	Thyroid, cricoid, arytenoid	Arytenoid vocal processes anchor vocal ligaments

💡Important anatomical levels

Landmark	Vertebral level / location	FCPS pearl
Larynx begins	C3–C4 region	Approximate upper extent
Upper border of thyroid cartilage	C4	Anatomical landmark
Cricoid cartilage	C6	Larynx continues as trachea
Pharynx continues as esophagus	C6	Esophagus begins behind the cricoid
Tracheal bifurcation (carina)	T4–T5	Near the sternal angle
Right main bronchus	Wider, shorter, more vertical	Aspirated foreign bodies more often enter here