Pharyngeal Anatomy: Innervation, Reflexes, and Clinical Considerations

Overview¹

• The pharynx is a midline muscular structure in the neck that extends from the inferior surface of the base of the skull to the level of the cricoid cartilage at C6. It is involved in vital functions related to the gastrointestinal and respiratory systems, including food swallowing, voice production, and air production.
• The pharynx comprises 3 regions (Figure 1).
  • Nasopharynx: Located behind the posterior nasal choanae, it extends from the base of the skull to the soft palate.
  • Oropharynx: Located posterior to the opening of the oral cavity, it extends from the soft palate to the epiglottis.
  • Laryngopharynx: Extends from the epiglottis to the esophagus.

Pharyngeal Muscles²

• The pharyngeal constrictor muscles include the superior pharyngeal constrictor, middle pharyngeal constrictor, and inferior pharyngeal constrictor muscles.
• The function of the constrictor muscles is to propel the food bolus downward during swallowing.
• The cricopharyngeus muscle, part of the inferior constrictor, prevents reflux of esophageal contents into the pharynx.

• The longitudinal muscles include the stylopharyngeus, palatopharyngeus, and salpingopharyngeus.
• These muscles function to elevate the pharynx and larynx during swallowing.

Pharyngeal Innervation^2,3

• The pharynx is innervated by the pharyngeal plexus which consists of the vagus nerve (CN X), glossopharyngeal nerve (CN IX), and superior cervical ganglion (sympathetic fibers).
• The pharyngeal plexus provides motor innervation to most pharyngeal muscles and sensory innervation to the pharyngeal mucosa.
• All pharyngeal muscles, except the stylopharyngeus, receive motor innervation from the vagus nerve.
• The stylopharyngeus receives motor innervation through the glossopharyngeal nerve.
• The nasopharynx receives sensory innervation from the maxillary division of the trigeminal nerve (CN V2), the oropharynx from the glossopharyngeal nerve (CN IX), and the laryngopharynx from the vagus nerve (CN X).

Gag Reflex⁴

• The gag reflex is a protective pharyngeal reflex that results in the contraction of the pharyngeal muscles and elevation of the soft palate in response to tactile stimulation of the posterior pharyngeal wall, soft palate, or base of the tongue.
• The gag reflex prevents foreign bodies from entering the airway or oropharynx.
• The afferent limb of the gag reflex is mediated by the glossopharyngeal nerve, and the efferent limb is mediated by the vagus nerve, which innervates the majority of the pharyngeal constrictor muscles via the pharyngeal plexus.
• Central integration of the gag reflex occurs in the medullary reticular formation, with involvement of the nucleus of the solitary tract for sensory input and the nucleus ambiguus for motor output to the pharyngeal muscles.
• An absent gag reflex may indicate dysfunction of the glossopharyngeal or vagus nerves of their central connections, but it is not a reliable predictor of dysphagia or aspiration risk in isolation. Many healthy individuals lack a gag reflex.
• In patients with known bulbar dysfunction or neuromuscular disease, absence of the gag reflex may signal impaired airway protection and increased aspiration risk.
• An exaggerated gag reflex is often seen in upper motor neuron lesions.

Clinical Correlates^1,5

• Glossopharyngeal nerve damage
  • The glossopharyngeal nerve provides sensation to the oropharynx and posterior tongue.
  • It is particularly vulnerable to injury during tonsillectomy, leading to loss of taste and sensation in the posterior 1/3 of the tongue, and/or absence of the afferent limb of the gag reflex.
• Vagus nerve damage
  • The vagus nerve provides motor innervation to most pharyngeal muscles.
  • It can be injured during pharyngeal surgeries, leading to hoarseness, dysphagia, aspiration, and/or loss of the efferent limb of the gag reflex.