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 brainstem. The names of the cranial nerves relate to their function and they are numerically identified in roman numerals (I-XII).
In this article, we shall summarise the anatomy of the cranial nerves – their origin, course, and functions.
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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 brainstem (Figure 1).
They can arise from a specific part of the brainstem (midbrain, pons or medulla), or from a junction between two parts:
- Midbrain – the trochlear nerve (IV) comes from the posterior side of the midbrain. 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 brainstem (superior to inferior, then medial to lateral) and the order of their exit from the cranium (anterior to posterior) (Figures 1 & 2).
Figure 1
The location of the cranial nerves on the cerebrum and brainstem.
Figure 2
Superior view of the skull base showing the foramina and which cranial nerves pass through them.
Tip: Cranial nerves with the number 2 in them (e.g. 2-optic and 12-hypoglossal) exit through a canal of the same name. They are the only cranial nerves to pass through canals.
Modalities
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).
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 (e.g. sight, sound, balance).
- Special visceral sensory (SVS) – senses derived from endoderm (e.g. taste, smell).
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.
Summary Table – Cranial Nerves
| Number | Name | Exit | Modality | Function |
| 1
(CN I) |
Olfactory | Cribriform plate | Sensory
(SVS) |
Smell |
| 2
(CN II) |
Optic | Optic canal | Sensory
(SSS) |
Vision |
| 3
(CN III) |
Oculomotor | Superior orbital fissure | Motor
(GSM & GVM) |
GSM: 4 extrinsic eye muscles and levator palpebrae superioris.
GVM: pupillary sphincter |
| 4
(CN IV) |
Trochlear | Superior orbital fissure | Motor
(GSM) |
Superior oblique |
| 5
(CN V) |
Trigeminal: | |||
| 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
(CN VI) |
Abducens | Superior orbital fissure | Motor
(GSM) |
Lateral rectus |
| 7
(CN VII) |
Facial | Internal acoustic meatus > stylomastoid f. | Both:
GSS SVS SVM GVM |
GSS: sensation to part of ext. ear.
SVS: taste from ant. 2/3 tongue, hard and soft palate. SVM: muscles of facial expression. GVM: lacrimal, submandibular, sublingual glands and mucous glands of mouth and nose. |
| 8
(CN VIII) |
Vestibulocochlear | Internal acoustic meatus | Sensory
(SSS) |
Hearing and balance |
| 9
(CN IX) |
Glossopharyngeal | Jugular f. | Both:
GSS GVS SVS GVM SVM |
GSS: post. 1/3 tongue, ext. ear, and middle ear cavity.
GVS: carotid body and sinus. SVS: taste from post. 1/3 tongue. GVM: parotid gland. SVM: stylopharyngeus |
| 10
(CN X) |
Vagus | Jugular f. | Both:
GSS GVS SVS GVM SVM |
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
(CN XI) |
Spinal accessory | Jugular f. | Motor
(GSM & SVM) |
GSM: trapezius and sternocleidomastoid.
SVM: a few fibres run with CNX to viscera. |
| 12
(CN XII) |
Hypoglossal | Hypoglossal canal | Motor
(GSM) |
Intrinsic and extrinsic tongue muscles (except the palatoglossus). |
Prosection Images
Prosection 1
The base of the cerebrum, demonstrating the origin of the cranial nerves.