Cranial nerves
Nervi craniales
Definition
There are 12 pairs of cranial nerves, which, along with the spinal nerves, form the peripheral nervous system. These nerves typically originate from the base of the brain, with each pair emerging in a specific sequence. Cranial nerve I begins at the front of the brain, while cranial nerve XII appears along the brainstem.
After emerging from the brain, cranial nerves pass through specific foramina at the base of the skull. These pathways allow the nerves to exit the cranial cavity and innervate various structures in the head, neck, and some viscera in the thoracic and abdominal regions.
Cranial nerves consist of bundles of neuronal fibers (i.e. neurons), which can be sensory or motor. Motor or efferent neurons originate in the brain from motor nerve nuclei within the gray matter, known as their nuclei of origin. In contrast, sensory or afferent neurons extend to sensory nuclei within the gray matter. These sensory afferent neurons arise from ganglia located outside the brain, such as the ganglia located in the trunk of the cranial nerves (analogous to the dorsal root ganglia in the sensory roots of spinal nerves). The central processes of these sensory neurons enter the brain and synapse at the sensory nuclei, termed the nuclei of termination.
The motor nerve fibers' nuclei of origin and the sensory nerve fibers' nuclei of termination are linked to the cerebral cortex. Motor fibers connect through upper motor neurons, decussating from the cortex, while sensory fibers progress via second-order neurons through the thalamus, projecting to the cortex via third-order neurons.
Below is a table listing the 12 pairs of cranial nerves:
Cranial nerve | Nuclei | Site of origin from brain | Passage through cranial cavity | Sensory, motor, or mixed | Function |
Olfactory nerve (I) |
| Olfactory mucosa | Passes through cribriform plate of the ethmoid bone; synapses in the olfactory bulb and tract | Sensory | Sense of smell |
Optic nerve (II) |
| Retina | Pass through optic canal; forms optic chiasma; continues as optic tracts | Sensory | Vision |
Oculomotor nerve (III) | Front of the midbrain in the interpeduncular fossa, at superior colliculus level | Motor | Innervates extraocular muscles (superior, inferior, medial rectus, inferior oblique), levator palpebrae superioris (to raise the upper eyelid), ciliary muscle (for accommodation), and constrictor pupillae (for pupil constriction) | ||
Trochlear nerve (IV) | *From the back of the midbrain at the inferior colliculus level. | Superior orbital fissure | Motor | Innervates the superior oblique muscle (moves the eyeball downward and outward) | |
Trigeminal nerve (V) | Mesencephalic n., Main sensory n., Spinal trigeminal n., Trigeminal motor n. | Trigeminal (sensory and motor) roots emerge from the ventral surface of the pons | Ophthalmic branch: passes through superior orbital fissure to enter orbital cavity. Maxillary branch: passes through foramen rotundum to enter pterygopalatine fossa, then courses through the floor of the orbital cavity. Mandibular branch: passes through foramen ovale to enter the mandibular fossa region | Ophthalmic and maxillary are sensory, mandibular branch is mixed | Ophthalmic: General sensation from cornea, skin of forehead, scalp, eyelids, nose; mucous membrane of paranasal air sinuses and nasal cavity. Maxillary: General sensations from skin of face over maxilla; upper teeth; mucosa of nose, the maxillary sinus, and palate. Mandibular: General sensations from skin of cheek, mandible and side of head, lower teeth and temporomandibular joint; mucosa of mouth and anterior tongue. Provides motor innervation to mastication muscles (temporalis, masseter, pterygoids); tensor tympani, tensor veli palatini, mylohyoid, anterior digastric belly. |
Abducent nerve (VI) | Junction of pons and medulla, close to the midline | Superior orbital fissure; reaches orbital cavity | Motor | Innervates lateral rectus muscle, causing abduction of the eyeball | |
Facial nerve (VII) | Facial motor n., superior salivatory n., n. of tractus solitarius | Junction of pons and medulla (cerebellopontine angle) | Internal acoustic meatus (with vestibulocochlear nerve and labyrinthine artery) | Mixed | Muscles of facial expression, taste sensation from anterior 2/3 of tongue, salivation from submandibular and sublingual glands |
Vestibulocochlear nerve (VIII) | Four vestibular nuclei, Two cochlear nuclei | Junction of pons and medulla (cerebellopontine angle) | Internal acoustic meatus (with facial nerve and labyrinthine artery) | Sensory | Innervates the inner ear, mediates hearing and balance regulation |
Inferior salivatory n., n. of tractus solitarius, nucleus ambiguous | Emerge from the medulla oblongata behind the olive | Mixed | Innervates stylopharyngeus muscle, taste and general sensations from posterior 1/3 of tongue, salivation from parotid gland | ||
Vagus nerve (X) | Dorsal motor n. of vagus, n. of tractus solitarius, nucleus ambiguous | Emerge from the medulla oblongata behind the olive | Jugular foramen | Mixed | Motor to larynx and pharynx muscles, parasympathetic to thoracic/abdominal viscera, taste from oropharynx |
Accessory nerve (XI) | n. of tractus solitarius, Spinal cord | Emerge from the medulla oblongata behind the olive | Jugular foramen | Motor | Innervates sternocleidomastoid and trapezius muscles |
Hypoglossal nerve (XII) | Emerge from the medulla oblongata in front of the olive | Motor | Innervates tongue musculature |
Note: (n = nucleus, * = Trochlear nerve is unique because it's the only nerve originating from the back of the brainstem, whereas all others emerge from the front).
References
Snell, R.S. (2010). ‘Chapter 11: The cranial nerve nuclei and their central connections and distribution’, in Clinical Neuroanatomy. (7th ed.) Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, pp.331-370.
Sonne J, Lopez-Ojeda W. Neuroanatomy, Cranial Nerve. [Updated 2022 Dec 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470353/
