Cranial nerve summary
|I - Olfactory||Smell||Exits at the cribiform plate|
|II - Optic||Vision||Exits via optic canal|
|III - Oculomotor||Moves the eye (4 of 6 extraocular muscles)||Exits via superior orbital fissure|
|IV - Trochlear||Moves the eye down & out (innervates superior oblique)||Exits via superior orbital fissure|
|V - Trigeminal||Sensation to the face. Motor innervation to the muscles of mastication.||V1 exits via the superior orbital fissure. V2 exits via foramen rotunda. V3 exits via foramen ovale.|
|VI - Abducens||Moves the eye laterally. (Innervates lateral rectus muscle)||Exits the skull via the superior orbital fissure.|
|VII - Facial||Muscles of facial expression. Taste to the anterior 2/3 of the tongue. Tear & salivary ducts. Innervates stapedius muscle.||Exits the skull via the internal acoustic meatus and the skull itself via the stylomastoid foramen.|
|VIII - Vestibulocochlear||Balance (vestibular division). Hearing (cochlear division).||Exits the skull via the internal acoustic meatus.|
|IX - Glossopharyngeal||Taste for posterior 1/3 of tongue. Sensation to pharynx. Innervates stylopharyngeus.||Exits via the jugular foramen.|
|X - Vagus||Parasympathetic innervation to all viscera above splenic flexure. Innervates laryngeal muscles and controls cough reflex.||Exits via the jugular foramen.|
|XI - Accessory||Spinal portion innervates SCM & Trapezius. Cranial portion runs with Vagus nerve.||Exits via the jugular foramen.|
|XII - Hypoglossal||Innervates all tongue muscles apart from palatoglossus (innervated by accessory nerve).||Exits via hypoglossal canal.|
5 Simple Rules
All eye related nerves (apart from the optic nerve) exit the skull via the superior orbital fissure.
Nerves 9, 10 and 11 leave the skull through the large J shaped Jugular foramen.
SO4, LR6 (See CN4 and 6 for details)
The names of the nerves tell you a lot about what they do.
Don’t bother too much with acronyms, they’re easy to forget and you need fluid rather than rigid knowledge of these nerves.
CN I – Olfactory nerve
Gives us our sense of smell. Exits the cranial cavity through the cribriform plate. The structure you often see on diagrams is not the nerve, but rather the tract.
Smell is affected in early-stage Parkinson’s, Kallman’s syndrome and in fracture of the anterior cranial fossa which may transect the nerve tracts.
CN II – Optic nerve
Gives us our sense of sight. Exits the cranial cavity through the optic canal.
Transection leads to blindness.
Glaucoma (high intraocular pressure) leads to tunnel vision.
Diabetics suffer progressive visual defects due to damage to the microvasculature of the eye.
CNIII – Oculomotor nerve
Plays a significant role in the movement of the eye.
Innervates 4 of the 6 extraocular muscles (Inferior oblique / Medial rectus / Superior rectus / Inferior rectus) as well as constrictor pupillae and levator palpebrae superioris (elevates the upper eyelid).
Exits the skull through the superior orbital fissure.
Palsy leads to a “down and out” appearance of the eye (CN4 AND 6 still exert their actions) in addition to a dilated pupil with some ptosis (upper eyelid drooping).
Constrictor pupillae is innervated by the parasympathetic component of the nerve. The parasympathetic nerve fibres run on the outside of the nerve. Hence one of the first signs of raised intracranial pressure is a ‘blown pupil’.
CNIV – Trochlear nerve
From the Greek word for ‘pulley’.
Long thin nerve and the only one to originate from the back of the brainstem.
SO4, LR6 is an acronym for remembering the muscles innervated by two of the cranial nerves:
- SO4 – The 4th nerve (Trochlear) innervates the Superior Oblique muscle.
- LR6 – The 6th cranial nerve, known as the Abducens nerve, innervates the Lateral Rectus.
Palsy of the Trochlear nerve leads to an upward facing eye.
CNV – Trigeminal nerve
Tri means three and Gemini means twins.
It, therefore, an apt name for this nerve that has three divisions:
- V1 – Ophthalmic
- V2 – Maxillary
- V3 – Mandibular
These nerves supply sensation to the face.
The mandibular division also innervates the chewing muscles:
- Medial and lateral pterygoid
Palsy of any of these divisions leads to a loss of facial sensation as well as an inability to chew.
CNVI – Abducens nerve
Abducens is from the word ‘abduct’ which means move away from the midline. It’s therefore logical that the nerve innervates the lateral rectus that moves the eye laterally.
Palsy results in the eye being unable to be abducted past the midline.
CNVII – Facial nerve
Innervates the muscles of facial expression. Also has a few special branches:
Nerve to stapedius – This is a muscle attached to the stapes footplate that impacts on the oval window of the inner ear. It dampens the sound sent through the malleus, incus and stapes.
Chorda Tympani – Provides taste to the anterior two thirds of the tongue. It is named this way as it actually passes through the middle ear on its way to the mouth, and makes a ‘chord’ across the ‘tympanic membrane’ (ear drum), hence the ‘chorda tympani.’
Lacrimal (tear ducts) and salivary glands – Causes tears to be formed as well as saliva. The branch to the facial muscles passes through the parotid gland on its way to the face, but it does not innervate it. It is innervated by the glossopharyngeal nerve.
Corneal reflex – If you feel something on your sclera (white part of your eye), this stimulates the V1 division of the trigeminal nerve, which causes the facial nerve to contract the orbicularis oculi muscle (shut your eyes).
Pathology of the Facial Nerve (VII)
Bell’s palsy – Hemi facial paralysis, taste affected, sounds sound louder than normal, dry eyes, inability to close eyes fully.
CNVIII – Vestibulocochlear nerve
- Vestibular – Balance
- Cochlear – Perception of sound (hearing)
Damage to this nerve can lead to balancing difficulties, tinnitus, and nystagmus.
A vestibular schwannoma is a tumour of the Schwann cells of this nerve. Hearing is affected first, as the cerebellum can compensate for any deficits in balance for a long time.
Cochlear division issues can lead to sensorineural deafness.
CNIX – Glossopharyngeal nerve
‘Glosso’ means tongue, pharyngeal clearly relates to the pharynx. Therefore it is clear the nerve provides taste to the posterior one third of the tongue, as well as the sensation to the pharynx (hence it is the afferent/sensory limb of the gag reflex).
Innervates one muscle –> Stylopharyngeus
Palsy leads to lack of pharyngeal sensation and affected taste, as well as a uvula deviated away from the affected side.
CNX – Vagus nerve
Vagus means ‘wanderer’ in Latin. This is apt as the nerve wanders all over the body as far as the splenic flexure to provide parasympathetic innervation (rest and digest functions i.e. slows the heart down, causes intestines to contract etc).
Palsy leads to a bovine (weakened) cough, severe sympathetic overdrive depending on the level of transection.
CNXI – Accessory nerve
Has a Spinal and a Cranial component.
The cranial portion quickly joins with the vagus nerve.
The spinal portion innervates sternocleidomastoid and trapezius.
Palsy means the patient can’t shrug shoulders or turn the head as powerfully (SCM weakness).
CNXII – Hypoglossal nerve
Innervates all the muscles that move the tongue, apart from palatoglossus (innervated by the accessory nerve).
Palsy leads to deviation of the tongue toward the affected side.