Cranial Nerves

To remember all the cranial nerves in order remember this useful mnemonic:

  1. Oh  -  Olfactory
  2. Oh  -  Optic
  3. Oh  -  Oculomotor
  4. To  -  Trochlear
  5. Touch  -  Trigeminal
  6. And  -  Abducens
  7. Feel  -  Facial
  8. Virgin  -  Vestibulocochlear
  9. Girls  -  Glossopharyngeal
  10. Vagina  -  Vagus
  11. And  -  Accessory
  12. Hymen  -  Hypoglossal

Another useful mnemonic, is one which identifies the role the nerve, be it sensory, motor or both:

  1. Some  -  Sensory  Olfactory
  2. Say  -  Sensory  Optic
  3. Marry  -  Motor  Oculomotor
  4. Money  -  Motor  Trochlear
  5. But  -  Both  Trigeminal
  6. My  -  Motor  Abducens
  7. Brother  -  Both  Facial
  8. Says  – Sensory  Vestibulocochlear
  9. Big  -  Both  Glossopharyngeal
  10. Boobs  -  Both  Vagus
  11. Matter  -  Motor  Accessory
  12. More  -  Motor  Hypoglossal

Foramina of the Nerves:

  1. Cribiform Plate (Olfactory)
  2. Optic canal (Optic)
  3. Superior Orbital Fissure (Oculomotor)
  4. Superior Orbital Fissure (Trochlear)
  5. Superior Orbital Fissure (Trigeminal – Ophthalmic)/Foramen Rotundum (Trigeminal – Maxillary)/Foramen Ovale (Trigeminal – Mandibular)
  6. Superior Orbital Fissure (Abducens)
  7. Internal Acoustic Meatus (Facial)
  8. Internal Acoustic Meatus (Vestibulocochlear)
  9. Jugular Foramen (Glossopharyngeal)
  10. Jugular Foramen (Vagus)
  11. Jugular Foramen (accessory)
  12. Hypoglossal Canal (Hypoglossal)

 

 

Now we understand the order of the cranial nerves and there relative neurone function and accompanying foramen we can look into their function:

  • Olfactory nerve:
    • This nerve uses sensory receptors in the roof and walls of the nasal cavity to detect smell
  • Optic nerve:
    • This transmits impulses from light sensitive sensory receptors within the retina of the eye
      • Remember that the optic nerve lies on the nasal aspect of the eye when examining on fundoscopy
  • Occulomotor nerve:
    • This nerve has motor functions going to the extrinsic eye muscles except for the lateral rectus and superior oblique
    • It also has motor neurones via the parasympathetic NS to the ciliary and papillary sphincter muscles
      • This goes via the ciliary ganglion in the orbit
  • Trochlear
    • Motor function going to the superior oblique muscles In the eye
  • Trigeminal
    • This cranial nerve has sensory function via three divisions in the face:
      • Ophthalmic
      • Maxillary
      • Mandibular
    • It also has motor function for:
      • the muscles of mastication as well as:
        • Tensor tympani muscle
        • Diagastric muscle
        • Tensor veli palatine muscle
        • Mylohyoid muscle
  • Abducens
    • This muscle supplies motor neurones to the lateral rectus muscle of the eye
  • Facial
    • This has sensory neurones that detect:
      • Taste receptors from the anterior of the tongue
      • The external ear
    • The main function is its motor neurones which involve:
      • Parasympathetic fibres to glands of the nasal/oral cavity, lacrimal glands, submandibular salivary glands
      • To facial expression muscles, stapedius (Found in middle ear), posterior diagastric muscles, stylohyoid
  • Vestibulocochlear
    • Sensory functions sensitive to hearing and head balance/movement
  • Glossopharyngeal
    • The glossopharyngeal nerve has both sensory and motor function. The sensory function of this nerve involves:
      • Taste receptors from the posterior 1/3rd of the tongue
      • External ear and external auditory canal
      • From mucous membranes of posterior mouth, pharynx, auditory tube + middle ear
      • Finally, pressure and chemical receptors in carotid body and common carotid artery
    • The motor function of the glossopharyngeal nerve involves:
      • Superior constrictor of the pharynx and stylopharyngeus
      • Parasympathetic action to the parotid gland
  • Vagus
    • This has sensory receptors at the base of the tongue and epiglottis
    • It also has sensory receptors from external ear and external auditory canal
    • There are deeper sensory functions involving the pharynx, larynx, thoracic and abdominal viscera
    • Motor function of the vagus nerve involves muscles of the palate, pharynx and larynx as well as visceral parasympathetic fibres to the muscles of the thoracic and abdominal viscera
  • Accessory
    • The accessory nerve has a cranial root and a spinal root
      • The cranial root is involved with sensory neurones to the laryngeal muscles following the path of the vagus
      • The spinal root (C1-C5) is involved in innervations of the trapezium and sternocleidomastoid muscle.
  • Hypoglossal
    • This has motor function supplying the extrinsic and intrinsic muscles of the tongue

 

Now we understand the motor and sensory functions of the cranial nerves, we can apply them clinically, to begin to understand common clinical problems.

  • Olfactory
    • Loss of smell
      • This can be either:
        • Anosmia – complete loss of smell
        • Hyposmia– partial loss of smell
          • Hyposmia can be caused temporarily due to an URTI, whereas anosmia, which is permanent could be due to:
            • Frontal meningioma (Tumour of the meninges)
            • Trauma to the ethmoid bone (This is the bone that separates the nasal cavity from the brain)
  • Optic
    • Monocular field loss (Loss of vision in one eye)
      • Occlusion of a retinal artery
      • Demyelination
      • Nerve compression
    • Bitemporal Hemianopia (Tunnel vision)
      • This is compression on the optic chiasm
        • The most common cause is a pituitary adenoma
    • Homonymous hemianopia (Loss of vision in the same visual field of both eyes)
      • This type of blindness is most commonly due to
        • Tumour
        • Vascular accident
    • Homonymous hemianopia with sparing of the macular
      • This is the same as the previous condition except a small central circle of vision is spared, resembling the macular
      • An example of this would be a posterior stroke
  • Occulomotor/Trochlear/Abducens(All have motor functions involving the eye)
  • The occulomotor nerve is involved in constriction of the pupil
  • Causes of pupil dilatation include:
    • Myotonic pupil(Holmes-Adie pupil)
      • This is mainly due to the blocking of the parasympathetic drive
      • Can be due to denervation of ciliary ganglion, which is usually unilateral
    • 3rdNerve palsy
      • This presents as a eye which is moved downwards and outwards
      • The eye has a dilated pupil and slight ptosis
      • In diabetic nerve lesions, the pupil reflex will be spared
    • Drugs (Antimuscarinics)
  • Causes of pupil constriction include:
    • Drugs (Such as morphine/Heroin)
    • Argyll Robertson pupil
      • This is usually the result of tertiary syphilis (accommodates but doesn’t react – Whore’s Pupil)
      • Pupils are constricted at rest, by characteristic of the following:
        • The pupils accommodate to objects near and far
          • However, they do not react to light
    • Horner’s syndrome
      • This is the result of a loss in sympathetic drive to the eye, which could happen in one of three sympathetic nerves.
  • Horner’s syndrome will present clinically as:
    • Ptosis (Lid-lag)
    • Miosis (Pupil constriction)
    • Hemifacial anhidrosis (Loss of sweating – on affected side)
    • Apparent endophthalmos

 

  • Trochlear and abducens nerve palsy’s are slightly rarer
    • Trochlear – 4thnerve palsy
      • This palsy is extremely hard to diagnose
      • The patient may present with a painful neck, due to twisting of the affected eye resulting In tilting of the head away from the lesion
    • Abducens – 6thnerve palsy
      • This will stop the effect of the lateral rectus muscle and thus the affected eye will move inwards
      • This will result in diplopia (Double vision)
  • Trigeminal
    • It is very rare to have an isolated fifth nerve palsy
    • However, many patients over 50 may experience trigeminal neuralgia
      • This is the result of compression of one of the trigeminal branches by blood vessels
      • This could present as pain over several minutes from an impulse such as brushing teeth
        • Treatment usually involves carbamazepine, which is an anti-convulsant that dampens nervous conduction
  • Facial
    • Nerve palsy’s that involve the 7thnerve depend on whether the lesion is an upper neuron or a lower neuron
      • UMNL
        • An example of this type of lesion is a stroke
        • With this type of lesion however, the frontalis and orbicularis muscles are spared
        • LMNL
          • An example of this type of lesion is Bell’s palsy – (Remember that a LMNL of the facial nerve is only termed Bell’s palsy if it is idiopathic)
          • With this type of lesion there is no frontalis or orbicularis sparing
            • Other examples of LMNL’s include parotid tumours and Ramsay-hunt syndrome, which is a herpes zoster infection of the facial nerve.
  • Vestibulocochlear
    • Damage to the vestibulocochlear nerve will either be sensorineural or conductive
    • To identify which one it is Weber’s and Rinne’s tests can  be conducted, which are exampled in year 2 case study 2
      • Causes of conductive loss include:
        • Wax
        • Glue ear (Otitis media with effusion)
        • Perforation
        • Chronic Suppurative otitis media (+/- Cholesteatoma = Squamous cells within the middle ear)
        • Otosclerosis (Abnormal growth of the middle ear)
      • Causes of sensorineural loss include:
        • Age (presbycusis)
        • Acoustic neuroma
        • Infection
        • Meniere’s disease
        • Drugs
          • Gentamicin
  • Bulbar Nerves (Glossopharyngeal/Vagus/ Accessory/ Hypoglossal)
    • Isolated lesions of these nerves are rare
    • These lesions are usually associated with dysphagia (Difficulty swallowing) and dysarthria (difficulty speaking)
    • Bulbar lesions can be split into UMN and LMN
      • UMN
        • This is usually the result of strokes and Tumours
        • These are also known as pseudobulbar:
          • Small spastic tongue
          • No fasciculations (Twitching)
          • Emotional Lability
          • Jaw Jerk increased
      • LMN
        • This are the result of tumour, or general compression of nerves
        • These are known as bulbar:
          • Wasting of the tongue
          • Fasciculations
          • All muscles of the tongue work in random
          • Jaw jerk normal