Anatomy & Function

The olfactory nerve is unique in that it is not truly a “nerve” in the classical sense — it is actually a bundle of delicate unmyelinated nerve filaments (the fila olfactoria) arising from bipolar sensory neurons in the nasal mucosa. These tiny filaments pass superiorly through the cribriform plate of the ethmoid bone — a sieve-like bony structure — to synapse directly on the olfactory bulb, which sits on the floor of the anterior cranial fossa.

Common Pathology: Anterior Cranial Fossa Fracture

A significant blow to the head (e.g., road traffic accident, fall from height) can fracture the anterior cranial fossa, shearing the delicate olfactory filaments where they pierce the cribriform plate. This results in two classic signs:

• Anosmia — permanent loss of smell. This is often permanent because central nerve fibres cannot regenerate.
• CSF Rhinorrhoea — leakage of cerebrospinal fluid into the nose. The fluid looks like clear water and will test positive for glucose (unlike mucus). This is a serious complication as it creates a direct path for bacteria to enter the intracranial space.

Anatomy & Function

The optic nerve is formed by the axons of retinal ganglion cells converging at the optic disc. It exits the orbit through the optic canal (alongside the ophthalmic artery) and the two optic nerves meet at the optic chiasm, sitting directly above the pituitary gland in the sella turcica.

At the chiasm: medial (nasal) fibres cross to the opposite side, while lateral (temporal) fibres remain ipsilateral. This is the anatomical basis for all visual field defects.

Visual Field Defects — The Key Rule

Anatomy & Innervation Rule

These three nerves control all movement of the eyeball. The single most important rule to memorise is:

Skull Exits — Superior Orbital Fissure

CN III, IV, and VI all pass through the Superior Orbital Fissure (SOF), along with branches of CN V. Use this mnemonic to remember everything passing through:

Cavernous Sinus — O TOM CAT

These nerves also run through or adjacent to the cavernous sinus. Use O TOM CAT:

Clinical Pathologies

Uncal Herniation — The “Blown Pupil” Emergency

An expanding intracranial haematoma (classically an extradural haematoma from rupture of the middle meningeal artery after temporal bone fracture) raises intracranial pressure, forcing the uncus of the temporal lobe over the edge of the tentorium cerebelli. This crushes CN III against the rigid tentorial edge.

• The parasympathetic fibres travel on the outside of CN III and are compressed first.
• This causes a fixed, dilated (“blown”) ipsilateral pupil — a neurosurgical emergency.
• The eye then adopts a characteristic “down and out” position as the unopposed lateral rectus (VI) and superior oblique (IV) take over.

CN IV Palsy — Staircase Difficulty

The trochlear nerve supplies the superior oblique, which intorts and depresses the eye. An isolated CN IV palsy causes vertical diplopia worst when looking down — classically described as difficulty reading or descending stairs. The patient will often compensate with a characteristic head tilt away from the lesion.

Anatomy & Function

The trigeminal nerve is the largest cranial nerve and the main sensory nerve of the face. It originates in the trigeminal (Gasserian) ganglion in Meckel’s cave and divides into three branches. Only V3 carries motor fibres (to the muscles of mastication).

Foramen Ovale Contents — MALE

Trigeminal Neuralgia

One of the most severe pain syndromes known to medicine. Characterised by brief, intense, “electric shock-like” or “lightning bolt” attacks of facial pain, usually lasting only seconds but repeated many times a day. Crucially, these are triggered by innocuous stimuli — shaving, a breeze, talking, or chewing.

• Affects V2 and V3 territories most commonly (the cheek, jaw, teeth).
• Usually unilateral. Always investigate bilateral neuralgia.
• Most commonly caused by vascular compression of the nerve root by the superior cerebellar artery.
• First-line treatment: Carbamazepine. Surgical: microvascular decompression (MVD).

Anatomy & Function

The facial nerve is wonderfully complex — it carries four different types of nerve fibres in one trunk: somatic motor, parasympathetic, special sensory (taste), and general sensory. It enters the internal acoustic meatus alongside CN VIII, then travels through the facial canal within the petrous temporal bone.

Intracranial Branches — “Great People Can’t Sing”

Extracranial Branches — “The Zebra Bit My Cheek”

After exiting via the stylomastoid foramen and passing through the parotid gland, the facial nerve divides into its five terminal branches supplying the muscles of facial expression:

Bell’s Palsy vs. Stroke — The Most Tested Distinction

Anatomy & Function

CN VIII has two distinct divisions — the cochlear nerve (hearing) and the vestibular nerve (balance and spatial orientation). Both enter the brainstem via the internal acoustic meatus alongside CN VII.

Acoustic Neuroma (Vestibular Schwannoma)

A benign, slow-growing tumour arising from the Schwann cells of the vestibular division of CN VIII, most commonly at the cerebellopontine angle. Despite the name, it arises from the vestibular nerve, not the cochlear nerve — but hearing loss is often the first symptom.

Anatomy & Function

CN IX exits via the jugular foramen and is named for its territory — the “gloss” (tongue) and “pharynx” (throat). It provides both taste and general sensation to the posterior third of the tongue, and it is the afferent (sensory) limb of the gag reflex.

Referred Otalgia — The Tonsil-Ear Connection

Because CN IX supplies both the tonsillar bed and the middle ear, disease in one area is often felt in the other. This is why patients with:

• Tonsillitis often get ear pain
• Post-tonsillectomy patients can have significant ear ache
• Throat malignancies (tongue base, tonsil) commonly present with ipsilateral ear pain

Always examine the throat and tonsil in any patient with unexplained ear pain.

Anatomy & Function

The vagus is the great wanderer of the nervous system (vagus = Latin for “wandering”). It has the longest course of any cranial nerve, providing parasympathetic supply from the neck all the way to the transverse colon. It exits the skull via the jugular foramen and descends in the carotid sheath.

Thyroid Surgery — The Two Nerves at Risk

During thyroidectomy, two branches of the vagus are vulnerable, each with a different consequence. Remember: “High Road = Pitch, Low Road = Power”.

Anatomy & Function

The spinal accessory nerve arises from the upper cervical spinal cord (C1–C5), ascends through the foramen magnum, exits via the jugular foramen, and then crosses the posterior triangle of the neck. It supplies the Sternocleidomastoid (SCM) and Trapezius muscles.

Scapular Winging — Medial vs. Lateral

This distinction is very commonly examined. Both cause the scapula to “wing” away from the chest wall, but the direction and cause are different:

Anatomy & Function

The hypoglossal nerve (“hypo” = under, “glossal” = tongue) exits via the hypoglossal canal and supplies all intrinsic and extrinsic muscles of the tongue — with one crucial exception:

Tongue Deviation — Which Way Does It Point?

The key muscle is the genioglossus — it protrudes the tongue forward. When one side is paralysed:

• The healthy side’s genioglossus pushes unopposed.
• The tongue therefore deviates TOWARDS the side of the lesion.
• Mnemonic: “Lick your wounds” — the tongue licks the side that hurts (the damaged side).

Common Cause: Carotid Endarterectomy

The hypoglossal nerve crosses anteriorly over the carotid bifurcation before looping down and back to the tongue. During carotid endarterectomy, it must be carefully identified and protected. Injury causes ipsilateral tongue weakness and deviation toward the lesion side.