The Larynx

The larynx is a functional sphincter at the beginning of the respiratory tree. It protects against foreign bodies and is used for phonation. It is lined with ciliated columnar epithelium.

larynx

Innervation

larynxvagus

Sensation
Internal brand of superior laryngeal nerve: Above the cords.
Recurrent laryngeal nerve: Below the cords.

Power
Recurrent laryngeal nerve: All muscles except the cricothyroid muscle which is innervated by the external brand of the superior laryngeal nerve.

Of note is the different sensory innervations affected during tracheal intubation and the haemodynamic effects these can have. The valeculla has sensory supply from the glossopharyngeal nerve, whereas beneath the epiglottis has sensory innervation from the vagus nerve. Using a standard Macintosh blade seated in the valeculla produces a sympathetic response both due to nociception and due to the glossopharyngeal nerve’s connection to the nucleus tractus solitarius and subsequent effects on heart rate and blood pressure (see Control of circulation). However when a Miller blade is used this stimulates the sensory afferents of the vagus nerve which can in turn produce vagal stimulation and bradycardia. This is particularly evident in children who do not have a high sympathetic resting tone.

Blood supply

Arterial supply from the laryngeal branches of superior and inferior thyroid arteries. Venous drainage from laryngeal brances of superior and inferior thyroid veins.

Laryngeal muscles

laryngealmuscles

There are three extrinsic and six intrinsic muscles.

Extrinsic

  1. Sternothyroid – Arises from manubrium, inserts into thyroid cartilage lamina. Functions as a depressor of the larynx.
  2. Thyrohyoid – Connects thyroid lamina to greater horn of hyoid. Functions as an elevator of the larynx.
  3. Inferior constrictor – Constricts laryngeal inlet. Propofol relaxes these muscles very effectively and so aids placement of a laryngeal mask airway.

Intrinsic

These are all paired muscles, except transverse arytenoid which is a midline structure.

  1. Cricothyroid – Anterior horn of cricoid to inferior horn of thyroid cartilage. Contraction tilts cricoid upwards, moving arytenoids posteriorly and therefore tensing the vocal cords.
  2. Posterior cricoarytenoid – Posterior cricoid to muscular surface of arytenoid. Contraction externally rotates arytenoids causing abduction of the cords.
  3. Lateral cricoarytenoid – Outer cricoid to muscular surface of arytenoid. Contraction adducts vocal cords.
  4. Transverse arytenoid – Posterior surface of both arytenoids. Contraction narrows distance between the two arytenoids, constricting glottis.
  5. Aryepiglottic – Causes a minor constriction of laryngeal inlet.
  6. Thyroarytenoid – Thyroid lamina to anterior arytenoid. Contraction pulls arytenoid anteriorly relaxing the cords.

Summary

Abductors Adductors Tenses cords Relaxes cords
Posterior cricoarytenoids Lateral cricoarytenoids Cricothyroids Thyroarytenoids
Transverse arytenoid
Thyroarytenoids

Recurrent laryngeal nerve injury

This is a problem because all intrinsic muscles except the cricothyroid muscles are supplied by these nerves. Therefore the only muscle with any tone after a RLN injury is a muscle that moves the arytenoids posteriorly and tenses the cords. A bilateral RLN injury can therefore cause upper airway obstruction.

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