What are the cranial nerves?

The Cranial Nerves

There are 12 pairs of cranial nerves that provide specialized sensory and motor innervation to the head and neck region, designated by Roman numerals I through XII and organized from most rostral to most caudal in descending order. 1

Complete List of the 12 Cranial Nerves

The cranial nerves emerge in orderly fashion from the rostral portion of the embryologically developing neural tube, which matures to form the brain and brainstem. 1 The complete list includes:

Sensory and Special Sense Nerves

• CN I (Olfactory nerve): Responsible for the sense of smell 2
• CN II (Optic nerve): Dedicated to vision, transmitting visual information from the eyes to the brain 2
• CN VIII (Vestibulocochlear nerve): Controls hearing and balance 2

Motor Nerves for Eye Movement

• CN III (Oculomotor nerve): Governs eye movements 2
• CN IV (Trochlear nerve): Controls eye movements 2
• CN VI (Abducens nerve): Ensures ability to track objects and focus 2

Mixed Sensory and Motor Nerves

• CN V (Trigeminal nerve): Controls facial sensations in three divisions (ophthalmic, maxillary, mandibular) and jaw movements 3, 2
• CN VII (Facial nerve): Facilitates facial expressions and taste perception to the anterior two-thirds of the tongue 3, 2
• CN IX (Glossopharyngeal nerve): Provides sensory innervation to the posterior third of tongue and motor function to pharyngeal muscles 3, 2
• CN X (Vagus nerve): Has the longest course of any cranial nerve, influencing numerous internal organs including heart, lungs, and digestive system, plus motor innervation to pharynx and larynx 3, 2
• CN XI (Accessory nerve): Supplies sternocleidomastoid and upper trapezius muscles for neck muscle control 3, 2
• CN XII (Hypoglossal nerve): Provides somatic motor innervation to intrinsic and extrinsic tongue muscles (except palatoglossus), essential for speaking, swallowing, and breathing 3, 2

Important Anatomic Distinctions

CN I and CN II are actually tracts formed from the telencephalon and diencephalon respectively, and are not considered true nerves. 1 This is a critical anatomic distinction, though they are still classified as cranial nerves clinically. 1

Functional Classification

Cranial nerve functions are divided into three sensory and three motor categories: 1

Sensory components:

• Visceral sensory (supplies sensory input from internal organs) 1
• General sensory (supplies tactile, pain, temperature sensations) 1
• Special sensory (smell, vision, taste, hearing, balance) 1

Motor components:

• Somatic motor (innervates muscles from body somites) 1
• Branchial motor (innervates muscles from branchial arches) 1
• Visceral motor (innervates viscera, glands, smooth muscle) 1

Critical Clinical Considerations

Individual nerve fibers, particularly autonomic nerves, may travel with several different cranial nerves from their nuclei of origin to their ultimate destinations, meaning loss of a specific function may indicate involvement of potentially more than one cranial nerve. 1, 3 Due to the close proximity of many cranial nerve nuclei and exiting sites, mass lesions frequently involve multiple cranial nerves simultaneously. 1, 3