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Anatomy

Cranial Nerve Lesions

Question 22 of 180

Which of the following nerves is the afferent pathway of the gag reflex:

Answer:

The glossopharyngeal nerve is the afferent pathway of the gag reflex. The vagus nerve is the efferent pathway of the gag reflex.

Cranial Nerve IX: Glossopharyngeal Nerve

The glossopharyngeal nerve (CN IX) mediates taste, salivation and swallowing (together with CN X).

Table: Overview of the Glossopharyngeal Nerve

Cranial Nerve Glossopharyngeal Nerve (CN IX)
Key anatomy Originates from medulla, exits skull via jugular foramen
Sensory function Posterior one-third of tongue, tonsils, oropharynx, soft palate, middle ear, taste from posterior one-third of tongue, visceral afferents from carotid body and sinus, afferent pathway of gag reflex
Motor function Stylopharyngeus muscle (facilitates swallowing), parasympathetic fibres to parotid gland
Assessment Gag reflex, swallowing
Clinical effects of injury Loss of gag reflex, dysphagia, loss of carotid sinus reflex, loss of taste from posterior one-third of tongue
Causes of injury Occipital condyle fractures, lateral medullary syndrome, jugular foramen syndrome, carotid artery dissection

Anatomical Course

The glossopharyngeal nerve originates from the medulla and travels lateral in the posterior cranial fossa before emerging from the cranial cavity via the jugular foramen.

Glossopharyngeal Nerve. (Image by Henry Vandyke Carter [Public domain], via Wikimedia Commons)

Function

The glossopharyngeal nerve carries:

  • General visceral afferent fibres from the carotid body and sinus
  • General sensory afferent fibres from the posterior one-third of the tongue, palatine tonsils, oropharynx, and mucosa of the middle ear, pharyngotympanic tube and mastoid ear cells
  • Special afferent fibres for taste from the posterior one-third of the tongue
  • Parasympathetic secretomotor fibres to the parotid salivary gland
  • Motor fibres to the stylopharyngeus muscle (elevates larynx and pharynx facilitating swallowing)

Assessment

The glossopharyngeal nerve can be assessed together with the vagus nerve (CN X) by:

  • Asking the patient to cough
  • Asking the patient to open the mouth wide and say 'ah' while visualising the palate and posterior pharyngeal wall (the soft palate should move upwards centrally)
  • Testing the gag reflex

Likely Causes of Disease or Injury

Causes of damage to CN IX include:

  • Occipital condyle fractures
  • Lateral medullary syndrome
  • Ischaemia secondary to vertebral artery damage
  • Jugular foramen syndrome (palsy of CN IX, X, XI and XII caused by tumours, meningitis and infection from the middle ear spreading into the posterior fossa or trauma)
  • Carotid artery dissection

Isolated glossopharyngeal nerve palsy is rare. It is usually damaged with CN X and XI, close to the jugular foramen.

Common Clinical Effects

CN IX palsy will result in:

  • Loss of gag reflex (afferent pathway)
  • Loss of carotid sinus reflex
  • Loss of taste from posterior one-third of tongue
  • Dysphagia

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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