Overview of cranial nerves
The cranial nerves are a set of 12 paired nerves that arise directly from the brain. The first two nerves (olfactory and optic) arise from the cerebrum, whereas the remaining ten emerge from the brain stem.
The names of the cranial nerves relate to their function and they are also numerically identified in roman numerals (I-XII).
Origin of the Cranial Nerves
There are twelve cranial nerves in total. The olfactory nerve (CN I) and optic nerve (CN II) originate from the cerebrum.
Cranial nerves III – XII arise from the brain stem. They can arise from a specific part of the brain stem (mid brain, pons or medulla), or from a junction between two parts:
Midbrain – the trochlear nerve (IV) comes from the posterior side of the mid brain. It has the longest intracranial length of all the cranial nerves.
- Midbrain-pontine junction – oculomotor (III).
- Pons – trigeminal (V).
- Pontine-medulla junction – abducens, facial, vestibulocochlear (VI-VIII).
- Medulla Oblongata – posterior to the olive: glossopharyngeal, vagus, accessory (IX-XI). Anterior to the olive: hypoglossal (XII).
The cranial nerves are numbered by their location on the brain stem (superior to inferior, then medial to lateral) and the order of their exit from the cranium (anterior to posterior).
Simplistically, each cranial nerve can be described as being sensory, motor or both. They can more specifically transmit seven types of information; three are unique to cranial nerves (SSS, SVS and SVM). See table 1 for a summary of the cranial nerves, their modalities and functions.
Sensory (afferent) Modalities:
- General somatic sensory (GSS) – general sensation from skin.
- General visceral sensory (GVS) – general sensation from viscera.
- Special somatic sensory (SSS) – senses derived from ectoderm (sight, sound, balance).
- Special visceral sensory (SVS) – senses derived from endoderm (taste).
Motor (efferent) Modalities:
- General somatic motor (GSM) – skeletal muscles.
- General visceral motor (GVM) – smooth muscles of gut and autonomic motor.
- Special visceral motor (SVM) – muscles derived from pharyngeal arches.
|1 (CNI)||Olfactory||Cribriform plate||Sensory(SVS)||Smell|
|2 (CNII)||Optic||Optic canal||Sensory(SSS)||Vision|
|3 (CNIII)||Oculomotor||Superior orbital fissure||Motor(GSM & GVM)||GSM: 4 extrinsic eye muscles and levator palpebrae superioris.GVM: pupillary sphincter|
|4 (CNIV)||Trochlear||Superior orbital fissure||Motor(GSM)||Superior oblique|
|Ophthalmic||Superior orbital fissure||GSS||Scalp, forehead and nose.|
|Maxillary||F. rotundum||GSS||Cheeks, lower eye lid, nasal mucosa, upper lip, upper teeth and palate.|
|Mandibular||F. ovale||GSS SVM||GSS: anterior 2/3 tongue, skin over mandible and lower teeth.SVM: muscles of mastication.|
|6 (CNVI)||Abducens||Superior orbital fissure||Motor(GSM)||Lateral rectus|
|7 (CNVII)||Facial||Internal acoustic meatus > stylomastoid f.||Both:GSS
|GSS: sensation to part of ext. ear.SVS: taste from ant. 2/3 tongue, hard and soft palate.
GSM: muscles of facial expression.
GVM: lacrimal, submandibular, sublingual glands and mucous glands of mouth and nose.
|8 (CNVIII)||Vestibulocochlear||Internal acoustic meatus||Sensory(SSS)||Hearing and balance|
|9 (CNIX)||Glossopharyngeal||Jugular f.||Both:GSS
|GSS: post. 1/3 tongue, ext. ear, and middle ear cavity.GVS: carotid body and sinus.
SVS: taste from post. 1/3 tongue.
GVM: parotic gland. SVM: stylopharyngeus
|GSS: ext. ear, larynx and pharynx.GVS: larynx, pharynx and, thoracic & abdominal viscera.
SVS: taste from epiglottis region of tongue
GVM: smooth muscles of pharynx, larynx and most of the GIT.
SVM: most muscles of pharynx and larynx.
|11 (CNXI)||Spinal accessory||Jugular f.||Motor(GSM & SVM)||GSM: trapezius and sternocleidomastoid.SVM: a few fibres run with CNX to viscera.|
|12 (CNXII)||Hypoglossal||Hypoglossal canal||Motor(GSM)||Intrinsic and extrinsic tongue muscles (except the palatoglossus).|