Intrinsic Muscles of Larynx

 Anatomy of the Vocal Folds

🔹 Key Structures of the Vocal Folds (from medial to lateral)

1. Vocal Ligament (Most Medial)

   • Attached to the vocal process of the arytenoid cartilage

   • Provides tensile support and elastic recoil essential for vibration

2. Thyroarytenoid Muscle Group

   • Divided into two parts:

     • Thyrovocalis (Vocalis) Muscle (immediately lateral to the vocal ligament)

       • Fine-tunes vocal fold tension and pitch

     • Thyromuscularis Muscle (lateral to the thyrovocalis)

       • Controls vocal fold position and adduction

3. Superficial Mucosa

   • Covers all these internal structures

   • Provides smooth gliding surface for vibration

   • Contains stratified squamous epithelium to withstand mechanical stress

 

🧠 Functional Summary

• The vocal ligament and vocalis muscle together form the vibratory core of the true vocal fold.

• The thyromuscularis acts more on adduction and relaxation.

• The mucosal covering is essential for efficient vibration and protection.

Coronal Section of the True Vocal Folds: Anatomical & Histological Overview

Structures Visible in Coronal View (Posterior to Anterior)

• Surface Layer:

  • Epithelium / Mucosa

  • Stratified squamous epithelium covers the vocal folds, providing protection against mechanical stress during vibration.

• Vocal Ligament:

  • Fibroelastic structure composed of collagen and elastin fibers

  • Provides tensile strength and elasticity for phonation.

• Thyrovocalis Muscle (Medial portion of thyroarytenoid muscle):

  • Adjacent and lateral to the vocal ligament

  • Controls fine tension adjustments of the vocal folds for pitch modulation.

• Thyromuscularis Muscle (Lateral portion of thyroarytenoid muscle):

  • Lateral to thyrovocalis

  • Functions in vocal fold adduction and relaxation.

• Lateral Arytenoid Muscle:

  • Located posterior-laterally

  • Important for adduction of the vocal folds.

• Cartilages:

  • Inferiorly: Cricoid cartilage, covered by the conus elasticus (fibroelastic membrane)

  • Superiorly: Thyroid cartilage

 

🧫 Histological Importance

• The conus elasticus forms the inferior lining and contributes to the vocal ligament.

• Stratified squamous epithelium allows the vocal folds to resist trauma from vibration and airflow.

• Underlying muscle and ligament provide structure and dynamic movement essential for phonation.

Coronal Section: False Vocal Fold vs True Vocal Fold

🔹 False Vocal Fold (Vestibular Fold) — Superior

• Epithelium:

  • Covered by respiratory epithelium — pseudostratified columnar epithelium

  • This type contains cilia and mucus-secreting glands

• Underlying Structures:

  • Contains glands to keep the mucosa moist and protected

• Function:

  • Primarily protective, helps close the airway during swallowing

• Anatomical Landmark:

  • Ventricle — the space (gap) between false and true vocal folds

 

🔸 True Vocal Fold (Vocal Cord) — Inferior

• Central Core:

  • Vocal ligament — composed of elastic and collagen fibers, providing structural support and elasticity

• Deep to Ligament:

  • Thyrovocalis muscle — controls tension of the vocal fold for phonation

• Epithelium Covering:

  • Stratified squamous epithelium (denoted as SE)

  • Provides durability against mechanical stress from vibration and airflow during phonation

• Function:

  • Primary structure for sound production (phonation)

 

🧠 Summary

Structure	Epithelium Type	Function
False Vocal Fold	Pseudostratified columnar	Protective, mucus secretion
Ventricle (gap)	—	Space between folds
True Vocal Fold	Stratified squamous	Sound production

Vocal Nodules: Pathology and Clinical Features

🔍 Histology & Pathophysiology

• Vocal nodules are a reactive lesion of the underlying connective tissue (stromal layer) of the true vocal folds.

• They result from excessive or inappropriate voice use (voice abuse or misuse).

• Histological changes include:

  • Edema (fluid accumulation in the stroma)

  • Focal haemorrhage (small bleeding spots)

  • Fibrosis (scarring and thickening of connective tissue)

• Nodules do not involve malignant transformation or epithelial dysplasia—there is no cancer risk.

 

👁 Macroscopic Appearance (Laryngoscopy)

• Small, pink-white swellings located at the free edge of the vocal folds.

• Can be:

  • Unilateral or bilateral

  • Bilateral nodules may create an hourglass shape of the glottis during phonation.

 

🗣 Clinical Presentation

• Common symptoms include:

  • Hoarseness

  • Voice breaks or instability

  • Vocal fatigue

 

🧠 Summary

Feature	Description
Cause	Excessive/inappropriate voice use
Histology	Reactive stromal changes (edema, haemorrhage, fibrosis)
Macroscopic Appearance	Small pink-white nodules at free vocal fold edge
Malignancy Risk	None
Symptoms	Hoarseness, voice breaks, vocal fatigue

Positions and Movements of the Vocal Folds

🔹 Key Positions

• Resting Position:

  • Slight abduction (vocal folds slightly apart)

  • Allows for quiet breathing and voiceless speech sounds.

• Forced Abduction:

  • Vocal folds are widely separated

  • Occurs during heavy respiration or deep breathing (e.g., during exercise).

• Adduction:

  • Vocal folds are brought together

  • Necessary for phonation (voiced sounds) and whispering.

  • In whispering, the vocal folds are adducted but the cartilaginous portions remain slightly apart, allowing air to pass.

• Intermediate Positions:

  • Between full abduction and adduction for producing various speech sounds requiring precise control.

 

🔄 Functional Summary

Position	Description	Function
Slight Abduction	Vocal folds slightly apart	Quiet breathing, voiceless sounds
Forced Abduction	Vocal folds widely separated	Heavy respiration
Adduction	Vocal folds closed	Phonation, voiced sounds, whispering
Intermediate	Partial opening/closing	Fine speech sound modulation

Intrinsic Muscles of the Larynx: Overview and Naming

🔹 Muscle Naming Principles

• Named based on:

  • Cartilages they attach to (e.g., cricoid, arytenoid)

  • Position relative to the arytenoid cartilages (posterior, lateral, or between)

 

🔸 Intrinsic Muscles Attached to Cricoid & Arytenoid Cartilages

Muscle Name	Location/Position	Function Summary
Posterior Cricoarytenoid	Posterior, between cricoid & arytenoid	Only abductor of vocal folds; opens glottis for breathing
Lateral Cricoarytenoid	Lateral, between cricoid & arytenoid	Adductor; closes vocal folds for phonation
Interarytenoid	Between arytenoids (transverse and oblique fibers)	Adducts arytenoid cartilages, closing posterior glottis

 

 

🧠 Functional Highlights

• Posterior cricoarytenoid is key for opening the vocal folds (abduction).

• Lateral cricoarytenoid and interarytenoid muscles work to close the vocal folds (adduction), essential for voice production.

Posterior Cricoarytenoid Muscles

🦴 Anatomical Location

• Located on the posterior surface of the cricoid cartilage (hence the name).

• Origin: Posterior cricoid lamina

• Insertion: Muscular process of the arytenoid cartilage

 

🔄 Function

• Abducts the vocal folds by rotating the arytenoid cartilages laterally.

• This action opens the glottis, allowing air passage for breathing.

• The only intrinsic laryngeal muscle responsible for vocal fold abduction (opening).

 

👨‍🏫 Clinical/Workshop Tip

• Movement can be visualised by rotating arytenoids away from the midline on larynx models, showing how vocal folds separate.

Lateral Cricoarytenoid Muscles

🦴 Anatomical Location

• Positioned on the lateral surface of the cricoid cartilage, mainly on the arch.

• Origin: Lateral cricoid cartilage (arch)

• Insertion: Muscular process of the arytenoid cartilage

 

🔄 Function

• Adducts the vocal folds by swinging the arytenoid cartilages anteriorly and inferiorly.

• This movement closes the glottis, particularly the interligamentous (vocal fold) portion.

• Responsible for bringing the true vocal folds together for phonation.

 

👁 Visualisation Tip

• You can see their location and action clearly in anatomical apps like the Visible Body app, highlighting their role in vocal fold closure.

Interarytenoid Muscles

🦴 Anatomical Location

• Located between the arytenoid cartilages.

• Composed of two parts:

  • Transverse interarytenoid (runs horizontally)

  • Oblique interarytenoids (run diagonally in an X shape)

 

🔄 Function

• Adduct the arytenoid cartilages, effectively closing the posterior (intercartilaginous) part of the glottis.

• This action complements the lateral cricoarytenoid muscles to fully close the vocal folds.

 How Pitch Is Changed in the Larynx

🔑 Key Principles of Pitch Variation

• Pitch depends on the length, tension, and thickness of the vocal folds — similar to strings on a musical instrument:

  • Longer vocal folds → lower frequency → lower pitch

  • Tense vocal folds → higher frequency → higher pitch

  • Thinner vocal folds → higher frequency → higher pitch

 

🦾 Muscles Controlling Length and Tension

Muscle	Role in Pitch Control
Cricothyroid	Increases tension and lengthens vocal folds → raises pitch
Thyrovocalis	Adjusts tension (fine control of vocal fold tension)
Thyromuscularis	Can relax vocal folds, reducing tension → lowers pitch

 Cricothyroid Muscle

🦾 Action

• Pulls the thyroid cartilage anteriorly and downward (rocks it forward on the cricoid cartilage).

• This movement stretches and thins the vocal folds, increasing their tension.

• Result: Raises pitch by increasing the frequency of vocal fold vibration.

 

🦴 Attachments

• Origin: Anterolateral aspect of the cricoid cartilage

• Insertion: Inferior border and inferior horn of the thyroid cartilage

 

⚡ Innervation

• Innervated by the external branch of the superior laryngeal nerve (branch of the vagus nerve).

• This is unique compared to other intrinsic laryngeal muscles, which are mostly innervated by the recurrent laryngeal nerve.

 Thyromuscularis Muscle

🦾 Action

• Pulls the arytenoid cartilages anteriorly toward the thyroid cartilage.

• This shortens the vocal folds, relaxes the vocal ligament, and decreases pitch.

 

🦴 Attachments

• Origin: Internal surface of the thyroid cartilage

• Insertion: Muscular process of the arytenoid cartilage

 

⚡ Innervation

• Innervated by the inferior laryngeal nerve (terminal branch of the recurrent laryngeal nerve, itself a branch of the vagus nerve).

• This innervation is common to most intrinsic muscles of the larynx, except the cricothyroid.

Thyrovocalis

🦾 Action

• Located medial to the thyromuscularis within the vocal fold.

• Has complex actions:

  • Maintains tension in the anterior part of the vocal fold (helps control fine tension for phonation).

  • Can relax the posterior part of the vocal fold, allowing subtle modulation of vocal fold stiffness.

 

🦴 Attachments

• Origin: Vocal processes of the arytenoid cartilage

• Insertion: Vocal ligament

 

⚡ Innervation

• Innervated by the inferior laryngeal nerve (terminal part of the recurrent laryngeal nerve, branch of the vagus nerve), similar to the thyromuscularis

 Intrinsic Muscles of the Larynx

Muscle	Action	Innervation
Posterior cricoarytenoid	Abducts (opens) vocal folds (glottis)	Inferior laryngeal nerve (branch of recurrent laryngeal nerve, CN X)
Lateral cricoarytenoid	Adducts (closes) vocal folds (interligamentous part)	Inferior laryngeal nerve (branch of recurrent laryngeal nerve, CN X)
Transverse and oblique arytenoids	Adducts (closes) vocal folds (intercartilaginous part)	Inferior laryngeal nerve (branch of recurrent laryngeal nerve, CN X)

 

 

🔹 Key Points

• Posterior cricoarytenoid is the only abductor of the vocal folds.

• Lateral cricoarytenoid and transverse/oblique arytenoids are adductors, with different focus areas of the vocal folds.

• All three muscles are innervated by the inferior laryngeal nerve, which is the terminal branch of the recurrent laryngeal nerve, itself a branch of the vagus nerve (cranial nerve X).

Pathway of the Vagus Nerve & Laryngeal Branches

Vagus Nerve (CN X) Pathway in the Neck:

• Main trunk of the vagus nerve runs within the neurovascular bundle alongside the common carotid artery and internal jugular vein.

 

Branches relevant to the larynx:

1. Superior Laryngeal Nerve (SLN):

   • Internal branch:

     • Provides sensory innervation to the larynx above the vocal folds (supraglottic region).

   • External branch:

     • Provides motor innervation to the cricothyroid muscle, which controls tension and pitch.

2. Recurrent Laryngeal Nerve (RLN):

   • Branches off the vagus nerve inferiorly in the neck (right loops under right subclavian artery; left loops under the aortic arch).

   • Runs back upwards (recurrent) to enter the larynx from below.

   • Its terminal part is called the inferior laryngeal nerve.

   • Provides motor innervation to all intrinsic muscles of the larynx except the cricothyroid.

   • Also provides sensory innervation below the vocal folds (infraglottic region).

 

🔹 Summary

Nerve Branch	Function	Target Structures
Internal branch of SLN	Sensory	Larynx above vocal folds
External branch of SLN	Motor	Cricothyroid muscle
Recurrent laryngeal nerve	Motor & sensory (via inferior laryngeal nerve)	All intrinsic muscles except cricothyroid; larynx below vocal folds

Course of the Recurrent Laryngeal Nerve (RLN) & Clinical Importance

Pathway Differences:

• Left RLN:

  • Loops under the arch of the aorta.

  • Runs upward in the tracheoesophageal groove.

  • Because of its course around the aortic arch, it is at risk of injury during cardiac surgeries, especially cardiac bypass or aortic arch procedures.

• Right RLN:

  • Loops under the right subclavian artery.

  • Travels upward similarly in the neck.

 

Common Course and Surgical Risk:

• Both RLNs pass posterior to the thyroid gland.

• They are at risk of injury during:

  • Thyroid surgery

  • Parathyroid surgery

 

Clinical Consequences of RLN Injury:

• Paralysis of the true vocal fold on the affected side.

• This can cause:

  • Hoarseness or breathy voice.

  • Possible aspiration risk due to incomplete glottic closure.

  • Breathing difficulties if bilateral injury occurs (rare but serious).

 

🔹 Summary

Side	Nerve Course	Surgical Risk Area	Potential Injury Consequence
Left	Loops under aortic arch	Cardiac surgery; thyroid/parathyroid surgery	Ipsilateral vocal fold paralysis
Right	Loops under right subclavian artery	Thyroid/parathyroid surgery	Ipsilateral vocal fold paralysis

 Blood Supply of the Larynx

Main Arterial Sources:

• Superior laryngeal artery

  • Branches from the superior thyroid artery (which itself branches from the external carotid artery).

  • Supplies the upper part of the larynx including the supraglottic region.

• Inferior laryngeal artery

  • Branches from the inferior thyroid artery.

  • The inferior thyroid artery runs with the inferior laryngeal nerve.

  • Supplies the infraglottic region and lower parts of the larynx.

 

Additional Notes:

• These arteries are closely associated with thyroid vasculature.

• Rich blood supply ensures good oxygenation for the vocal folds and laryngeal muscles critical for voice production and airway protection