Cranial nerve exits from the skull
Cranial nerves leave the skull through foramina, openings in a bone for nerves and vessels, so damage at one opening gives a predictable pattern of nerve loss.
Remember
Nerves that share an opening can be damaged together: IX, X and XI share the jugular foramen (a gap in the base of the skull behind the ear), while VII and VIII share the internal acoustic meatus (a bony canal that carries nerves towards the inner ear).
Memory aid
the three nerves numbered nine, ten and eleven travel as a trio through the jugular foramen
Use it to link the numbers nine, ten and eleven with the jugular foramen.
The jugular foramen carries the glossopharyngeal nerve (IX, which gives sensation and taste from the back of the tongue), the vagus nerve (X) and the accessory nerve (XI) together. The vagus supplies the pharynx and larynx, so damage to the vagus can cause difficulty swallowing and a hoarse voice. The accessory nerve supplies sternocleidomastoid (the neck muscle that turns the head to the opposite side), so a weak sternocleidomastoid reduces head turning to the opposite side.
The internal acoustic meatus is shared by the facial nerve (VII), which moves the face, and the vestibulocochlear nerve (VIII), which carries hearing and balance. Surgery to remove a vestibular schwannoma (a benign tumour growing on the nerve for hearing and balance) inside this narrow canal can put both nerves at risk. After the meatus, the facial nerve leaves the skull through another opening, the stylomastoid foramen.
The trigeminal nerve, which gives sensation to the face and supplies the chewing muscles, has three divisions, and each leaves the skull by its own opening. V1 (ophthalmic) uses the superior orbital fissure (a narrow slit at the back of the eye socket). V2 (maxillary) uses the foramen rotundum (a round opening behind and below that fissure). V3 (mandibular) uses the foramen ovale, which lies behind and to the side of the foramen rotundum.
The same fissure also carries the oculomotor nerve (III), the trochlear nerve (IV) and the abducens nerve (VI), which supply the muscles of the eye, so damage at the fissure can affect several of them together.
Key facts
- Jugular foramen
- transmits cranial nerves IX, X and XI, plus the internal jugular vein.
- Internal acoustic meatus
- carries the facial (VII) and vestibulocochlear (VIII) nerves, so damage or an operation there can cause hearing loss and facial weakness together.
- Trigeminal exits
- V1 leaves through the superior orbital fissure, V2 through the foramen rotundum and V3 through the foramen ovale.
- Optic canal and hypoglossal canal
- the optic canal carries the optic nerve (II), and the hypoglossal canal carries the hypoglossal nerve (XII) to the tongue.
- Corneal reflex
- touching the cornea (the clear dome at the front of the eye) brings a blink, with V1 as the sensory limb (the half carrying the signal towards the brainstem) and the facial nerve as the motor limb.
Exam traps
- The foramen magnum, the large opening where the brainstem becomes the spinal cord, is a tempting near miss for weak head turning because part of the accessory nerve climbs through it, but it does not carry the glossopharyngeal or vagus nerves, so it does not explain a reduced gag reflex or a hoarse voice.
- Hearing loss with facial weakness is not explained by the facial and vagus nerves at the jugular foramen: the facial nerve does not use the jugular foramen, and the pairing leaves the hearing loss unexplained.
- The abducens nerve also passes through the superior orbital fissure, but it supplies only the lateral rectus (the muscle that pulls the eye outwards), so an injury would stop the eye turning outwards and cause double vision, not numbness.
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