Cranial Nerves

• The cranial nerves are the peripheral nerves of the head and neck
• The olfactory nerve (CN I) and the optic nerve (CN II) arise from the cerebrum, the rest of the cranial nerves arise from the brainstem
• Cranial nerves III, IV, and VI have a long path and therefore are the most susceptible to injury
• The cranial nerves exit the skill via the foramina in the skull base
• The olfactory nerve passes through the cribriform plate - fracture can lead to anosmia
• Glaucoma:

  • open angle - outflow of aqueous humour through the trabecular meshwork is reduced. Causes gradual reduction of peripheral vision
  • Closed angle - iris is forced against the trabecular meshwork, preventing any drainage of aqueous humour - ophthalmic emergency
• Cranial nerve II - optic nerve:

  • Purely sensory
  • Visual acuity
  • Visual field
  • Pupillary reaction
  • Fundoscopy
• Temporal vision from both eyes cross over at the optic chiasm
• A lesion at the optic chiasm causes bitemporal hemianopsia - loss of temporal vision in both eyes

  Typical presentation of an enlarging pituitary tumour
• Relative afferent pupillary defect = defect in the direct response due to optic nerve damage or severe retinal disease
• Horner's syndrome is caused by interruption of the sympathetic supply to the eye and face - causes ptosis, miosis and anhidrosis

  Can also get ptosis from CN III palsy - usually worse

  Superior tarsal muscle - sympathetic innervation

  Levator palpebrae superiosis - CN III
• Cranial nerve IV controls the superior oblique - looking down and laterally

  CN IV palsy causes eye to move up and medially - double vision reading and going down stairs
• Cranial nerve VI controls the lateral rectus

  Palsy of cranial nerve VI causes failure of lateral movement and nystagmus
• Cranial nerve III controls the muscles of eye movement except the lateral rectus and superior oblique

  A cranial nerve III palsy causes a down and out palsy - complete ptosis and dilated pupil which is not responsive to light or accommodation
• Cranial nerve V - trigeminal nerve:

  • Sensory - skin, mucous membranes and sinuses of the face
  • Motor - muscles of mastication - temporalis and masseter muscles
• CN V (trigeminal) has 3 sensory branches:

  • V1 = ophthalmic nerve
  • V2 = maxillary nerve
  • V3 = mandibular nerve
• The corneal reflex can illicit the sensory functions of CN V and motor function of CN VII

  • Sensory - sensation of cotton on eye ball
  • Motor - scrunching eye shut - CN VII
• There are 5 major branches of the facial nerve:

  • Temporal
  • Zygomatic
  • Buccal
  • Mandibular
  • Cervical
• Functions of the facial nerve (CN VII):

  • Motor - muscles of facial expression
  • Sensory - taste sensation to the anterior 2/3 of the tongue via chordae tympani
• The facial nerve gives parasympathetic supply to many glands of the head and neck

  • Submandibular and sublingual salivary glands
  • Lacrimal glands
• The facial nerve (CN VII) gives off its branches in the parotid gland - parotid tumour and their management can cause facial nerve palsies
• Facial nerve (CN VII) lesions:

  • LMN - ipsilateral paralysis - no forehead sparing
  • UMN - contralateral paralysis - forehead sparing
• CN VIII - vestibulocochlear nerve:

  • Sensory only
  • Vestibular component - innervates the vestibular system of the inner ear which is responsible for detecting balance
  • Cochlear component - forms the spiral ganglia which serve the sense of hearing
• Cranial nerve IX - glossopharyngeal nerve:

  • Sensory - oropharynx, tonsils, posterior 1/3 of tongue (taste)
  • Parasympathetic - innervation to the parotid gland
  • Motor - pharynx
• The gag reflex is a 2 nerve reflex:

  • Sensory - glossopharyngeal nerve
  • Motor - vagus nerve
• Cranial nerve X - vagus nerve:

  • sensory - laryngopharynx and larynx. Visceral sensation to the heart and abdominal viscera
  • Parasympathetic - innervates the smooth muscle of the trachea, bronchi and GI tract - regulates respiratory rate and heart rate
  • Motor - innervation to most of the muscles of the pharynx, soft palate and larynx - important vocal cord nerve
• The vagus nerve travels in carotid sheath with carotid artery and internal jugular vein
• Cranial nerve XI - accessory nerve:

  • Motor function of turning head
  • Travels across posterior triangle of the neck
  • Innervates the sternocleidomastoid and trapezius muscles
• Cranial nerve XII - hypoglossal nerve:

  • Purely motor
  • Innervates muscles of the tongue
  • lesion - tongue deviates towards side of the lesion
• CN III and CN IV arise from the midbrain
• CN V to CN VIII arise from the pons
• CN IX to CN XII arise from the medulla
• Cranial nerve III paresis causes:

  • Raised ICP
  • Posterior communication artery aneurysm
  • Cavernous sinus infection or trauma
  • Diabetes
  • MS
  • Myasthenia gravis
  • Temporal arteritis
• Cranial nerve VI paresis causes:

  • Space-occupying lesion (pressing on the brainstem)
  • Diabetic neuropathy
  • Thrombophlebitis of cavernous sinus
• Cranial nerve VII paresis causes:

  • Bells palsy
  • Intracranial lesion
  • Parotid gland pathology
  • Myasthenia gravis
• Cranial nerve IX paresis causes:

  • Stroke
  • MS
  • Parkinson's disease
  • Neurosyphilis
  • Traumatic brain injury