Overview of Laryngeal Muscles
The larynx contains intrinsic muscles that control vocal fold movement and voice production, plus extrinsic muscles that position the larynx in the neck for phonation, swallowing, and airway protection.
Intrinsic Laryngeal Muscles
The intrinsic laryngeal muscles are highly specialized striated muscles that control vocal fold position, tension, and the glottic aperture for phonation, respiration, and sphincteric protection. 1
Muscle Groups and Functions
The intrinsic muscles can be functionally categorized into three groups:
Adductors (close the vocal folds): The thyroarytenoid, lateral cricoarytenoid, and interarytenoid muscles bring the vocal folds together for phonation and airway protection 1, 2
Abductor (opens the vocal folds): The posterior cricoarytenoid is the sole abductor, opening the glottic airway for respiration 2
Tensor (adjusts vocal fold tension): The cricothyroid muscle lengthens and tenses the vocal folds by tilting the thyroid cartilage forward, crucial for pitch control 2, 3
Neural Innervation Pattern
The recurrent laryngeal nerve (RLN) innervates all intrinsic laryngeal muscles except the cricothyroid, which receives motor supply from the external branch of the superior laryngeal nerve. 1, 3
Key anatomical considerations:
- Motor neurons originate in the nucleus ambiguus in the medulla and project ipsilaterally 1
- The RLN carries motor fibers with a 4:1 adductor to abductor ratio, reflecting the predominance of adductor function 1
- The RLN also provides sensory innervation to the vocal folds and subglottic region 1
- Nonrecurrent RLN occurs in <1% of cases (right-sided only) and poses increased surgical injury risk 1
Unique Muscle Characteristics
The intrinsic laryngeal muscles differ significantly from limb skeletal muscles:
- Higher percentage of type 1 (slow-twitch) muscle fibers compared to limb muscles 3
- Specialized neuromuscular junction morphology with distinct motor end-plate distribution patterns 3
- Optimized for rapid, precise movements required for voice and airway protection 4, 3
Extrinsic Laryngeal Muscles
The extrinsic muscles position the larynx vertically in the neck and are divided into suprahyoid and infrahyoid groups 4, 2:
Laryngeal Opening Muscles
- Geniohyoid, mylohyoid, sternothyroid, and middle constrictor elevate and open the laryngeal airway 2
- The posterior cricoarytenoid works synergistically with the cricothyroid to accentuate airway opening 2
Laryngeal Closing Muscles
- Thyrohyoid, cricothyroid, sternohyoid, and inferior constrictor depress and close the laryngeal airway 2
- These muscles are critical during swallowing to protect against aspiration 2, 5
Functional Integration
The larynx functions as a three-level system 4:
- Subglottic tract: Power source providing airflow 4
- Vocal folds: Oscillator creating sound through myoelastic-aerodynamic principles 4, 5
- Supraglottic tract: Resonator modifying sound quality 4
Clinical Implications
Injury to laryngeal nerves during thyroid surgery can cause devastating morbidity including voice loss, dysphagia with aspiration pneumonia, and airway compromise requiring tracheostomy. 1
Important clinical considerations:
- Unilateral vocal fold immobility causes breathy dysphonia and aspiration risk, particularly to liquids 1
- Bilateral vocal fold immobility can cause life-threatening airway obstruction 1
- Abnormalities in muscle innervation occur in multiple sclerosis, amyotrophic lateral sclerosis, vocal cord paralysis, and spasmodic dysphonia 2
- The larynx also functions as a sphincter during swallowing, with three-level closure and superior-anterior movement to prevent aspiration 5
Common Pitfalls
Surgeons must recognize that the RLN can have anatomical variations, particularly the nonrecurrent RLN associated with retro-esophageal right subclavian artery, which increases injury risk if not identified preoperatively 1. The complex 4:1 adductor-to-abductor fiber ratio means that partial nerve injuries often preferentially affect abductor function, leading to paradoxical vocal fold positioning 1.