Neurology | Cranial Nerves: Overview

The olfactory nerve originates in the olfactory mucosa at the roof of the nasal cavity. It supplies the nasal cavity for the sense of smell, carrying special visceral afferent (SVA) fibers. It is a sensory nerve that passes through the olfactory foramina of the cribriform plate of the ethmoid bone to the olfactory bulb.

The optic nerve originates in the retina, specifically from the ganglion cells, and supplies the photoreceptors (rods, cones, and ganglion cells with melanopsin). It carries special sensory afferent fibers for vision. This sensory nerve travels through the optic canal.

Originating in the midbrain, the oculomotor nerve supplies several extraocular muscles (superior rectus, levator palpebrae superioris, inferior rectus, inferior oblique, medial rectus) and the pupil and ciliary muscle. It carries general somatic efferent (GSE) and general visceral efferent (GVE) fibers, making it a motor nerve. It passes through the superior orbital fissure within the common tendinous ring.

The trochlear nerve originates in the midbrain, at the level of the inferior colliculus. It supplies the superior oblique muscle, controlling eye movement (depression and lateral rotation of the eyeball). It consists of general somatic efferent (GSE) fibers and is a motor nerve, passing through the superior orbital fissure outside the common tendinous ring.

This complex nerve has origins in the midbrain (mesencephalic nucleus), pons (principal pontine nucleus, trigeminal motor nucleus), and medulla/spinal cord (spinal nucleus). It supplies extensive areas of the face (skin, scalp, eyelids, conjunctiva, external ear, jaw, chin, nose, nasal and oral cavities, tongue), as well as the muscles of mastication, tensor tympani, and tensor veli palatini. It carries special visceral efferent (SVE) and general somatic afferent (GSA) fibers, classifying it as both sensory and motor. Its three branches (ophthalmic V1, maxillary V2, mandibular V3) pass through the superior orbital fissure, foramen rotundum, and foramen ovale, respectively.

The abducens nerve originates in the inferior pons. It supplies only the lateral rectus muscle, responsible for lateral rotation (abduction) of the eye. It contains general somatic efferent (GSE) fibers and is primarily a motor nerve, traveling through the superior orbital fissure inside the common tendinous ring. The mnemonic 'LR6 SO4 ATR3' is introduced to remember the innervation of extraocular muscles (Lateral Rectus by CN VI, Superior Oblique by CN IV, and All The Rest by CN III).

Originating in the pons, the facial nerve supplies the muscles of facial expression, stapedius, digastric, and stylohyoid muscles. It also innervates lacrimal, nasal, and palatine glands, and picks up taste from the anterior two-thirds of the tongue and sensation from the ear/tympanic membrane. It contains special visceral efferent (SVE), general visceral efferent (GVE), special visceral afferent (SVA), and general somatic afferent (GSA) fibers, making it a mixed nerve. It passes through the internal acoustic meatus and the stylomastoid foramen.

This nerve originates at the pons-medulla junction. It supplies the inner ear, specifically the vestibule (for static equilibrium) and semicircular canals (for dynamic equilibrium), and the cochlea (for sound). It primarily carries special sensory afferent (SSA) fibers for balance and hearing, but also has minor efferent fibers to outer hair cells, making it arguably 'both' but predominantly sensory. It passes through the internal acoustic meatus.

Originating in the medulla, the glossopharyngeal nerve supplies the stylopharyngeus muscle, pharynx, soft palate, tonsils, posterior one-third of the tongue, ear, and parotid gland. It also receives information from carotid baroreceptors and chemoreceptors. It contains special visceral efferent (SVE), general visceral afferent (GVA), general somatic afferent (GSA), and general visceral efferent (GVE) fibers, functioning as a mixed nerve. It exits the skull through the jugular foramen.

The vagus nerve originates in the medulla and is a 'wanderer', supplying a vast array of structures including the pharynx, larynx, heart, lungs, and most abdominal viscera (up to the splenic flexure of the colon). It picks up taste from the epiglottis and sensation from the ear, and receives information from aortic arch baroreceptors and chemoreceptors. It contains general visceral efferent (GVE), special visceral afferent (SVA), general somatic afferent (GSA), special visceral efferent (SVE), and general visceral afferent (GVA) fibers, making it a mixed nerve. It is the main parasympathetic nerve and also passes through the jugular foramen.

The accessory nerve has cranial (medulla) and spinal (cervical spinal cord C1-C5) origins. It primarily supplies the sternocleidomastoid and trapezius muscles, affecting neck flexion and head rotation, and scapular elevation and retraction. It consists mainly of general somatic efferent (GSE) fibers, but the cranial part has special visceral efferent (SVE) components that join the vagus nerve to supply pharyngeal/soft palate muscles. It is mainly a motor nerve. The cranial part exits via the jugular foramen, while the spinal part ascends through the foramen magnum and then joins the cranial part to exit through the jugular foramen.

Originating in the medulla, the hypoglossal nerve supplies all the extrinsic and intrinsic muscles of the tongue. It contains general somatic efferent (GSE) fibers and is solely a motor nerve, responsible for changing the tongue's shape, protraction, retraction, elevation, and depression. It exits the skull through the hypoglossal canal.