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CN10 – Vagus nerve

  • Complex mixed nerve consisting of:
    • Preganglionic parasympathetic fibres to the ganglia in the thorax and upper abdomen until about 2/3 of transverse colon
    • Motor fibres to muscles of the pharynx and larynx – for speech and swallowing
    • Visceral sensory fibres from the lungs and bronchi and the upper GIT – thoracic and abdo. Viscera
    • Taste fibres from the palate
    • A few somatosensory fibres from the larynx, external ear and trachea
  • Emerges from the brainstem as rootlets along the lateral side of the olive, caudal to CN9
    • Collect and enter the jugular foramen before entering the neck, posterolateral to the pharynx
    • Passes in the carotid sheath posterior to the internal and common carotid arteries
    • Branches to the pharynx and larynx (superior and recurrent laryngeal nerves)
      • Thus enters the thorax etc
  • Controls the cough reflex
  • Small area of the tongue (about 5c coin size) in front of the epiglottis is sensed by vagus
  • Motor: Muscles of pharynx & larynx; Recurrent laryngeal nerves
  • Special sensory: taste (few taste buds on epiglottis)
  • Sensory: pain, temperature and touch from lower pharynx, larynx (involved in cough reflex) and oesophagus, from external acoustic meatus & tympanic membrane
  • Parasympathetic: viscera of thorax and abdomen; glands associated with GIT
    • Most important function is parasympathetic and its innervation of larynx
    • Cut recurrent laryngeal nerve on one side = hoarseness. Both sides = lose voice.
  • Pathways:
    • leaves medulla oblongata between olive and inferior cerebellar peduncle
    • leaves skull through jugular foramen
    • joined by cranial accessory
    • Gives off laryngeal nerves
    • Passes down into thorax and abdomen
  • Note the vagal ganglia (two swellings), that behave similarly to the dorsal root ganglia in spinal nerves
  • Note that below the spleen/liver branches, the Lt and Rt vagus nerves blend together and form the anterior and posterior vagal trunks in the abdomen

CN11 – Accessory nerve

  • Supplies the sternomastoid (rotates and flexes head) and trapezius muscles (shrugs shoulders etc)
  • Two roots: cranial part of accessory nerve (that belongs to the vagus nerve and actually does vagus functions) and a bigger part (spinal part of accessory nerve - which we consider the accessory nerve proper).
  • Arises as rootlets from the ventrolateral surface of the medulla and C1-5
    • Cervical fibres enter the cranial cavity via foramen magnum before joining other fibres to form nerve (runs alongside the spinal cord as it enters the foramen magnum, then leaves the skull via the jugular foramen to head to effectors)
    • Nerve leaves by the jugular foramen (with the vagus and glossopharyngeal)
      • These 3 can be affected simultaneously by meningiomas at the base of the skull
    • Thus it innervates the internal surface of the sternocleidomastoid (it runs underneath sternocleidomastoid) from where it travels across the posterior triangle onto the lateral neck supplying the trapezius


CN12 – Hypoglossal nerve

  • Supplies the tongue muscles (motor)
  • Arises as a vertical line of several rootlets on the ventral surface of the medulla between pyramids and olives
    • Fibres unit and exit the cranial cavity via the hypoglossal canal
    • Nerve travels downwards behind the internal carotid before turning anteriorly
      • Crosses the lateral side of the external carotid reaching the sides of the tongue (this is the ONLY nerve that crosses on the lateral surface of the carotid arteries)
  • Lesions cause problems with swallowing and speech
    • Test: ask them to poke out tongue/jaw, good side will be forward, bad side doesn’t move

Corticobulbar tract

  • While the body skeletal muscle is controlled by the corticospinal tract, the corticobulbar tract is specifically for cranial nerves (only those with motor function)
  • Skeletal muscles involved with cranial nerves are connected to the cortex via the corticobulbar (corticonuclear) tract
    • Originates in the sensorimotor cortex and travels through the forebrain and brainstem with the corticospinal tract, goes through the crus cerebri
      • Limbs and trunk voluntary activity is controlled by the contralateral side of the cortex
      • In the head, most voluntary activity is controlled by both ipsilateral and contralateral tracts
      • Therefore when you see a lesion in one side cranial nerve motor, you get bilateral lesion
        • This is true except for facial (7) and hypoglossal (12) nerves, which don't result in bilateral lesions (the nerves supply the muscles only on one side)
    • Corticospinal and corticobulbar together make up the pyramids
  • Pathology
    • A lesion in one corticobulbar tract may not cause paralysis since fibres from the opposite tract may compensate
    • Exception: muscles of facial expression in the lower half of the face – supplied exclusively by the contralateral corticobulbar tract
      • Facial nerves may recover, have Schwann cells, corticobulbar may not, no Schwann

Cases

  • Case 1
  • Case 2
    • Oculomotor nerve lesion
    • Can't move her eye vertically and medially due to lack of superior/inferior rectus
    • Strabismus, double vision (unopposed lateral rectus)
    • Eye deviates inferiorly and laterally because superior and inferior oblique are unopposed
    • Superior oblique = always down and out
  • Case 3
    • Abducent nerve lesion
    • Diplopia with medial deviation of the eye
  • Case 4
    • Hypoglossal nerve lesion
    • Tongue deviates to the side of the lesion (because of the right and left genioglossus muscles for extruding the tongue)