Describe laryngeal innervation and support your answer with neat labeled diagrams. (10) June 2013

Motor Supply of Larynx:

• ALL intrinsic muscles of larynx which move vocal folds are supplied by Recurrent Laryngeal nerve (RLN):
  • Posterior CricoArytenoid (Abductor)
  • Lateral CricoArytenoid (Adductor)
  • InterArytenoid (Adductor)
  • ThyroArytenoid (Adductor)
  • Vocalis(Tensor)
• EXCEPT:
  • CricoThyroid (Main Tensor + Adductor)
    • External branch of Superior Laryngeal Nerve (SLN)

Sensory Supply of Larynx:

• Laryngeal mucosa Above Vocal folds:
  • Internal branch of Superior Laryngeal Nerve(SLN).
• Laryngeal mucosa Below Vocal folds:
  • Recurrent Laryngeal Nerve(RLN).

Laryngeal innervation –Superior Laryngeal Nerves (SLN):

• Originate from vagus nerve as it exits base of the skull.
• Runs transversally behind the carotid artery.
• Couses with Superior thyroid artery until approximately 1 cm before the artery enters the capsule of Superior pole of the thyroid.
• Two primary branches:
  • External branch:
    • Primarily motor:
      • 1. Inferior constrictor muscle.
      • 2. Cricothyroid muscle.
    • Travels with Superior thyroid artery unti approximately 1 cm before the artery enters superior thyroid pole.
    • Divides into branches that enter Lateral inferior pharyngeal constrictor muscle and Cricothyroid muscle.
  • Internal branch:
    • Sensory to Supraglottis and Glottis.
    • Afferent branch of Cough reflex arc.
    • Enters the larynx through Thyrohyoid membrane superior to the external branch.

Laryngeal innervation – Recurrent Laryngeal Nerve (RLN):

• Provides both sensory and motor function to Larynx.
• Sensory to Subglottis and Trachea.
• Innervates all muscles to the larynx Except Cricothyroid muscle.
• Courses of Left RLN:
  • Originates from Left Vagus nerve below Arch of Aorta in the Thorax.
  • Loops posterior to Arch of Aorta.
• Common Course:
  • Associated with Inferior Thyroid artery at junction of lower and middle thirds of thyroid gland.
    • Posterior to Inferior Thyroid Artery (40%).
    • Between branches of the artery (35%)
    • Anterior to Inferior Thyroid Artery (20%)
  • Ascends to the side of Trachea at Tracheoesophageal groove.
  • Terminal portion of RLN accompanies Laryngeal branch of Inferior Thyroid Artery to enter the Larynx behind Crico Thyroid Joint.
  • In 90% of cases, RLN divides into two to three branches just before entering the Larynx deep to inferior constrictor muscle.

Galen Anastomosis (Ansa of Galen):

• Connection between RLN and internal branch of SLN
• Provide purely sensory and autonomic innervation.
• May also contain motor fibres.
• Present as a single nerve, single trunk, several branches, or a plexus.

Read Laryngeal electromyography here

Blood Supply

1. Superior Thyroid Artery:

• 1st branch of External Carotid Artery.
• Arises just below Greater cornu of Hyoid bone.
• Descends along Inferior constrictor muscle to reach upper pole of Thyroid gland.
• Cephalic to upper pole, External branch of Superior Laryngeal Nerve runs with Superior Thyroid Artery before turning medially to supply Cricothyroid muscle.
• Provides blood supply to Supraglottis and Glottis.
• Gives rise to:
• 1. Superior Laryngeal Artery:
  • Prices Thyrohyoid membrane to supply the larynx.
  • Accompanied with Internal branch of Superior Laryngeal nerve.
• 2. Cricothyroid Artery:
  • Runs cephalic to upper pole and runs toward the midline on the cricothyroid ligament.
  • Lacerated during emergent cricothyroidotomy.

2. Inferior Thyroid Artery:

• Branch of Thyrocervical trunk of Subclavin Artery.
• Ascends vertically along Medial border of Anterior Scalene Muscle to enter Tracheoesophageal groove.
• Penetrates posterior aspect of lower lobes of Thyroid.
• Provides the blood supply to Glottis, Subgottis and Cervical Trachea.
• Gives rise to:
• 1. Inferior Laryngeal Artery:
  • Reaches posterior border of larynx atlevel of CricoThyroid joint.
  • Lies immediately deep to Inferior constrictor muscle.
  • Travels beside Recurrent laryngeal nerve

Venous Drainage

• 1. Superior Thyroid Vein:
  • Ascends along Superior Thyroid Artery.
  • Drains into Internal Jugular Vein.
• 2. Middle Thyroid Vein:
  • Follows a direct course laterally to drain into Internal Jugular Vein.
• 3. Inferior Thyroid Vein:
  • Drains into Left Brachiocephalic Vein.
  • Occasionly, both inferior veins form a common trunk called Thyroid ima vein, which empties into the left brachiocephalic vein.

Additional Resourse

https://sborl.es/wp-content/uploads/2018/01/Larynx-anatomy.pdf

https://teachmeanatomy.info/neck/viscera/larynx/muscles/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4527097/