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.

Transverse section through the neck at C6

Landmarks in relation to the cervical vertebrae

  • At C1, base of the nose and the hard palate
  • At C2, the teeth of a closed mouth
  • At C3, the mandible and hyoid bone
  • At C4, the common carotid artery bifurcates
  • From C4-5, the thyroid cartilage
  • From C6-7, the cricoid cartilage

Transverse section through C6

transversesectionC6

The thyroid covers the 2nd to 4th tracheal rings. When performing a surgical tracheostomy the isthmus of the thyroid is general displaced downwards by blunt dissection, through if this is not possible it may need to be divided. When performing a percutaneous dilatational tracheostomy the space between the 2nd and 3rd tracheal ring is usually chosen. Tracheostomy higher than this increases the risk of tracheal stenosis. The use of ultrasound to select puncture site to avoid aberrant midline vessels can make the procedure safer, but the use of bronchoscopic guidance is essential for a percutaneous approach.

Coronary anatomy

Sinoatrial node: At the junction of the SVC and right atrium on the posterolateral surface.

Atrioventricular node: Lies in atrial septum above coronary sinus.

Left coronary artery arises from the posterior aortic sinus, and the right coronary artery arises from the anterior aortic sinus. The sinuses of valsalva (also known as the aortic sinuses) are shaped to encourage equal bilateral flow.

Coronary arteries

  • Right Coronary Artery (RCA):  Supplies the  RA, RV, and interatrial septum.  It usually supplies both the SA and AV nodes.
  • Posterior Descending Artery (PD):  Supplies the inferior portion of the LV and the posterior septum.  The PD arises from the RCA in 70% of cases and the CFX in the remaining 20%.
  • Left Main Coronary Artery (LCA):  Gives rise to the LAD and CFX.
  • Left Anterior Descending artery (LAD):   Supplies the LV, RV, and interventricular septum.  Arises from LCA.  May also be called the anterior interventricular artery.
  • Circumflex artery (CFX):  Supplies the LA and LV.  Arises from the LCA and anastamoses with the RCA.

Schematic of coronary arteries

coronaryarteries

The right coronary artery supplies to SA node in 60% of people, and it supplies the AV node in 90% of people.

Dominance refers to which side supplies the posterior interventricular artery (also called the posterior descending artery). 70% of people are right side dominant, 20% co-dominant and 10% left side dominant.

coronaryvesselsschematic

Coronary perfusion pressure

CPP = aortic pressure – intraventricular pressure

Left ventricle:

Systole: [SBP-LVESP] = 120-120 = 0mmHg

Diastole: [DBP-LVEDP] = 70-10 = 60mmHg

In the right ventricle flow occurs throughout the cardiac cycle.