What type of muscle are vocal cords?

Vocal Cords Are Skeletal Muscle

The vocal folds (vocal cords) consist of epithelium and lamina propria overlying the vocalis (thyroarytenoid) muscle, which is a skeletal muscle. 1

Anatomical Structure of Vocal Cords

The vocal folds are twin infoldings of mucous membrane that:

• Cover the upper surface of each vocalis (or thyroarytenoid) muscle
• Extend from the midline anterior attachment to the thyroid cartilage
• Project posteriorly to the vocal process of the arytenoid cartilage
• Vibrate to modulate airflow from the lungs during phonation 1

The structure consists of three main components:

1. Epithelium (outer layer)
2. Lamina propria (middle layer)
3. Vocalis muscle (innermost layer) - skeletal muscle tissue

Functional Characteristics Supporting Skeletal Muscle Classification

Several characteristics confirm the skeletal muscle nature of vocal fold musculature:

• Voluntary Control: The vocalis muscle can be voluntarily controlled during speech and singing, a hallmark of skeletal muscle 1

• Electromyographic Properties: Laryngeal electromyography (LEMG) studies show motor unit action potentials (MUAPs) characteristic of skeletal muscle, including:

  • Concentric electrode recordings showing mean MUAP durations of 4.5 ms
  • Mean amplitudes of 350 microvolts in the thyroarytenoid muscle 1

• Motor Unit Recruitment: The thyroarytenoid muscle follows the Henneman size principle (smallest to largest motor unit recruitment) typical of skeletal muscles 1

Intrinsic Laryngeal Muscles

The vocal folds are part of a complex of intrinsic laryngeal muscles that include:

1. Thyroarytenoid muscle (vocalis muscle) - primary muscle of the vocal fold
2. Cricothyroid muscle - tensor of the vocal fold
3. Posterior cricoarytenoid muscle - sole abductor of the vocal folds 2
4. Lateral cricoarytenoid muscle - adductor of the vocal folds

All these muscles are skeletal muscles innervated by branches of the vagus nerve (recurrent laryngeal nerve for all except cricothyroid, which is innervated by the external branch of the superior laryngeal nerve) 2.

Clinical Relevance of Vocal Fold Muscle Type

Understanding that vocal folds are skeletal muscle has important clinical implications:

• Voice Disorders: Conditions like spasmodic dysphonia involve abnormal laryngeal muscle spasms 3

• Electromyographic Assessment: LEMG can be used to assess vocal fold function and diagnose vocal fold paresis or paralysis 1, 2

• Surgical Considerations: During thyroid surgery, care must be taken to preserve the recurrent laryngeal nerve to maintain vocal fold function 1

Common Misconceptions

A common misconception is confusing the vocal folds with smooth muscle due to their location in the airway. However, their voluntary control, innervation pattern, and histological characteristics clearly identify them as skeletal muscle.

Key Functional Aspects

• During phonation, the thyroarytenoid muscle shows increased activity compared to respiration 4
• The cricothyroid muscle serves as an additional tensor of the vocal cord, particularly important in high-pitched voice production 5
• The posterior cricoarytenoid is the only abductor muscle of the vocal folds, critical for airway patency 2

Understanding the skeletal muscle nature of vocal folds is essential for proper diagnosis and management of voice disorders, as well as for surgical planning in procedures that may affect laryngeal function.