Cranial Nerves: Origins, Functions, and Innervation

Real and Apparent Origins

Cranial nerves have a real origin, which is the nucleus of gray substance inside the brain, except for cranial nerves I, II, and VIII, whose real origin is outside the brain. The apparent origin is the attachment surface of the brainstem.

Specific Cranial Nerves

Olfactory Nerve (CN I)

Origin: Roof of the nasal cavity.

Optic Nerve (CN II)

Origin: Ganglionic cells of the retina.

Oculomotor Nerve (CN III)

Origin: Sulcus on the medial side of the cerebral peduncle.

• Superior division: Supplies the superior rectus and levator palpebrae superioris.
• Inferior division: Supplies the medial rectus, inferior rectus, and inferior oblique muscle.
• Function: Parasympathetic innervation, constricts the pupil via the ciliary ganglion.

Trochlear Nerve (CN IV)

Origin: Inferior colliculus.

Unique features:

• Nerve exits from the dorsal side of the brain.
• Only nerve whose fibers decussate.
• Long intracranial course.

Trigeminal Nerve (CN V)

The trigeminal nerve is located in the middle cranial fossa. The sensory root forms the trigeminal ganglion. This ganglion is located in the trigeminal depression on the anterior surface of the petrous part of the temporal bone. The anterior border of the trigeminal ganglion has three divisions: V1, V2, and V3.

Abducens Nerve (CN VI)

Origin: Lower border of the pons, above the pyramids of the medulla.

Function: Supplies the lateral rectus muscle.

Facial Nerve (CN VII)

The facial nerve is also known as the intermediate nerve. Its parasympathetic fibers have terminal branches inside the parotid gland. It has special afferent fibers responsible for taste from the anterior 2/3 of the tongue via the chorda tympani. It innervates the muscles of the face and scalp, as well as the stapedius, posterior belly of the digastric, and stylohyoid muscles.

Vestibulocochlear Nerve (CN VIII)

• Vestibular part: Arises in the vestibular ganglion (Scarpa's ganglion) and is the nerve of equilibrium.
• Cochlear part: Nerve of hearing, originates from the ganglion of Corti.

Glossopharyngeal Nerve (CN IX)

Motor fibers supply the constrictor pharyngeal muscles. It provides sensation from the posterior 1/3 of the tongue. Parasympathetic fibers innervate the parotid gland via the lesser petrosal nerve, synapsing in the otic ganglion. Fibers from the otic ganglion travel with the auriculotemporal nerve to innervate the parotid gland. It communicates with the vagus nerve (CN X) on the lateral wall of the pharynx, forming the pharyngeal plexus.

Vagus Nerve (CN X)

The vagus nerve travels between the internal jugular vein and the internal carotid artery to the chest and abdomen.

• Pharyngeal branch: Innervates the constrictor muscles of the pharynx except for the stylopharyngeus muscle, which is innervated by CN IX. It also innervates the palatoglossus and soft palate, except for the tensor veli palatini muscle.
• Laryngeal nerve: Motor to all intrinsic muscles of the larynx except the cricothyroid.
• Sensory: Provides taste sensation from the epiglottic region.

The nasopharynx is innervated by V2, the oropharynx by CN IX via the pharyngeal plexus, and the laryngopharynx by CN X via the internal branch of the superior laryngeal artery.

Accessory Nerve (CN XI)

Function: Innervates the trapezius and sternocleidomastoid muscles.

Hypoglossal Nerve (CN XII)

Function: Supplies the tongue, specifically the genioglossus, styloglossus, and hyoglossus muscles. The palatoglossus muscle is supplied by CN X. The descending ramus joins the cervical plexus to form a loop called the ansa cervicalis or ansa hypoglossi.