Overview

Cranial nerves are pairs of nerves that emerge directly from the brain and brainstem. They are considered to be part of the peripheral nervous system, although technically speaking, the nerves originating from the brain are part of the CNS.

The cranial nerves include:

  • Cranial nerves emerging from the cerebrum:
    • CN I – olfactory nerves
    • CN II – optic nerves
  • Cranial nerves emerging from the brainstem:
    • Cranial nerves emerging from the midbrain:
      • CN III – oculomotor nerves
      • CN IV – trochlear nerves
    • Cranial nerves emerging from the pons:
      • CN V – trigeminal nerves
      • CN VI – abducens nerves
      • CN VII – facial nerves
      • CN VIII – vestibulocochlear nerves
  • Cranial nerves emerging from the medulla:
    • CN IX – glossopharyngeal nerves
    • CN X – vagus nerves
    • CN XI – accessory nerves
    • CN XII – hypoglossal nerves

Bilateral and unilateral innervation

Upper motor neurones (UMNs) from the cerebral cortex synapse with cranial nerve nuclei in the brainstem. Most corticobulbar tracts decussate above each cranial nerve nucleus and most of the cranial nerve nuclei receive bilateral UMN innervation except for:

  • CN VII (the facial nerve)
  • CN XII (the hypoglossal nerve)

Their implications are discussed below.

CN I – Olfactory Nerve

Function

  • Sensory function:
    • Sense of smell

Causes of dysfunction

  • Trauma
  • Congenital e.g. Kallmann’s syndrome
  • Neurodegenerative disorders e.g. Alzheimer’s and Parkinson’s

Signs and symptoms of lesions

  • Reduced taste and smell

CN II – Optic Nerve

Function

  • Sensory function:
    • Sense of vision

Causes of dysfunction

Signs and symptoms of lesions

  • Visual field defects dependent on the cause for example:
    • Bitemporal hemianopia due to pituitary adenoma/craniopharyngioma
    • Homonymous hemianopia: due to a lesion behind the optic nerve e.g. optic radiations or occipital cortex
  • Relative afferent pupillary defect
  • Papilloedema
  • See Visual Field Defects and Pupil Abnormalities for more information. 

CN III – Oculomotor Nerve

Function

  • Motor function:
    • Movement of the extraocular muscles (medial rectus, superior rectus, inferior rectus, inferior oblique, levator palpebrae superioris)
    • Controlling the eyelid muscles (levator palpebrae superioris)
  • Parasympathetic function:

Causes of dysfunction

Signs and symptoms of lesions

  • Eye is deviated “down and outwards”
  • If the pupil is fixed and dilated this is called a surgical third nerve palsy:
  • Surgical third nerve palsies are where external structures are compressing on the nerve or trauma. The most common cause is a posterior communicating aneurysm which presents with a headache and dilated pupil that is “downwards and outwards”
  • Medical third nerve palsies are where there is no pupil involvement, they tend to be due to microvascular disease. 
  • Ptosis

CN IV – Trochlear Nerve

Function

  • Motor function:
    • Innervates superior oblique eye muscle

The trochlear nerve is unique as it decussates within the brainstem before emerging from it.

An injury to the trochlear nucleus causes contralateral palsies and an injury to the trochlear nerve (after it has emerged from the brainstem) causes an ipsilateral palsy. This is the opposite of other cranial nerves where damage to their nuclei causes ipsilateral palsies.

Causes of dysfunction

Signs and symptoms of lesions

  • Vertical diplopia due to weakness in downward and inward eye movement
  • Patient may tilt their head to compensate for this
  • Eye is deviated “up and outwards”

CN V – Trigeminal Nerve

Divisions of the trigeminal nerve

The trigeminal nerve is divided into three divisions:

  • Ophthalmic division (CN V1):
    • Supplies sensation to the skin of the forehead, eye, and medial nose
    • Responsible for the corneal reflex
  • Maxillary division (CN V2):
    • Supplies sensation to the skin of the cheek, upper lip, and some of the skin from the eye and mouth to the mid-temporal region
  • Mandibular division (CN V3):
    • Supplies sensation to the lower lip and chin and anterior 2/3 of the tongue
    • Supplies motor innervation to the masseter, temporalis, and pterygoid muscles

Causes of dysfunction

Signs and symptoms of lesions

  • Pain due to trigeminal neuralgia
  • Loss of corneal reflex
  • Loss of sensation in the face depending on which branch is affected
  • Paralysis of the muscles supplied and deviation of the jaw to the affected side

CN VI – Abducens Nerve

Function

  • Motor function:
    • Innervates the lateral rectus eye muscle

Causes of dysfunction

  • Multiple sclerosis
  • Pontine cerebrovascular accident

Signs and symptoms of lesions

  • Horizontal diplopia due to ineffective eye abduction

CN VII – Facial Nerve

Unilateral and bilateral innervation

The facial nerve nucleus is divided horizontally in half:

  • The superior half represents the superior half of the face and is bilaterally innervated by UMNs from the cerebral cortex
  • The inferior half represents the inferior half of the face is contralaterally innervated by UMNs from the cerebral cortex

Function

  • Motor function:
    • Supplies the muscles of the face
    • Supplies the nerve to the stapedius bone in the middle ear and stabilises it
  • Sensory function:
    • Supplies taste to the anterior 2/3 of the tongue
  • Parasympathetic function:
    • Responsible for lacrimation
    • Responsible for salivation

Causes of dysfunction

Signs and symptoms of lesions

  • Paralysis and flaccid upper and lower face (depending on if LMN/UMN)
  • Loss of taste
  • Impaired salivation
  • Impaired lacrimation
  • Loss of corneal reflex
  • Hyperacusis due to stapedius bone dysfunction

CN VIII – Vestibulocochlear Nerve

Function

  • Sensory:
    • Sense of hearing and balance

Causes of dysfunction

  • Meniere’s disease
  • Herpes zoster
  • Acoustic neuroma
  • Brainstem cerebrovascular accident
  • Drugs: aspirin, aminoglycosides, and furosemide

Signs and symptoms of lesions

CN IX – Glossopharyngeal Nerve

Function

  • Sensory:
    • Supplies taste to the posterior 1/3 of the tongue
  • Motor:
    • Swallowing
  • Parasympathetic:
    • Salivation

Causes of dysfunction

  • Dysfunction is very rare
  • Trauma
  • Brainstem lesions
  • Guillain-Barre syndrome 
  • Cerebellopontine/neck tumours

Signs and symptoms of lesions

  • Absent gag reflex
  • Unilateral lesions may not cause any deficit

CN X – Vagus Nerve

Function

  • Sensory:
    • Supplies the pharynx, larynx, and viscera
  • Motor:
    • Supplies the palate, pharynx, and larynx
    • Swallowing
  • Parasympathetic:
    • Provides innervation to the heart, lungs, and abdominal organs

Causes of dysfunction

  • Trauma
  • Guillain-Barre syndrome
  • Lesions to the brainstem
  • Tumours in the cerebellopontine angle or jugular foramen

Signs and symptoms of lesions

  • Hoarse and nasal speech
  • “Bovine cough” – due to the inability to close the glottis
  • Loss of gag reflex
  • Deviation of the uvula away from the side the lesion is on

CN IX – Spinal Accessory Nerve

Function

  • Motor function:
    • Provides motor innervation to the trapezius and sternocleidomastoid muscles

Causes of dysfunction

  • Trauma
  • Guillain-Barre syndrome
  • Lesions to the brainstem
  • Tumours in the cerebellopontine angle or jugular foramen

Signs and symptoms of lesions

  • Weakness turning the head to the opposite side
  • Wasting of these muscles
  • Difficulties shrugging shoulders

CN XII – Hypoglossal Nerve

Function

  • Motor function:
    • Supplies motor fibres to the tongue

Causes of dysfunction

Signs and symptoms of lesions

  • Tongue deviates to the side of the lesion
  • Tongue weakness

Author

  • Ishraq Choudhury

    FY1 doctor working in North West England.

    MB ChB with Honours (2024, University of Manchester).
    MSc Clinical Immunology with Merit (2023, University of Manchester).<br Also an A-Level Biology, Chemistry, Physics, and Maths tutor.
    Interests in Medical Education, Neurology, and Rheumatology.
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