Sensory cranial nerves

List of cranial nerves

The 12 pairs of cranial nerves are anatomically numbered from I to XII based on the location of their origin in the brainstem (rostral to caudal).

• CN I: olfactory nerve
• CN II: optic nerve
• CN III: oculomotor nerve
• CN IV: trochlear nerve
• CN V: trigeminal nerve
• CN VI: abducens nerve
• CN VII: facial nerve
• CN VIII: vestibulocochlear nerve
• CN IX: glossopharyngeal nerve
• CN X: vagus nerve
• CN XI: accessory nerve
• CN XII: hypoglossal nerve

Each cranial nerve has functional components, such as afferent fibers (carry information towards the brainstem) or efferent fibers (carry information away from the brainstem) or both. Sensory information is conducted by cranial nerves using afferent fibers, and include general somatic afferents (GSA), general visceral afferents (GVA), special somatic afferents (SSA) or special visceral afferents (SVA).

Sensory cranial nerves

The primarily sensory cranial nerves include the olfactory (CN I), optic (CN II) and vestibulocochlear nerves (CN VIII) as they preferentially carry sensory information. Each of these cranial nerves carries a different sensation. The olfactory nerve picks up the sensation of smell, while the optic nerve is responsible for vision. The vestibulocochlear nerves have two components, the cochlear nerve and the vestibular nerve, sensing hearing and equilibrium respectively.

Other special senses such as taste and general somatic sensation from the face are carried by mixed cranial nerves such as the facial (CN VII), trigeminal (CN V), glossopharyngeal (CN IX) and vagus (CN X) nerves.

Olfactory nerve (CN I)

The olfactory nerve (CN I) has special afferent (SSA/SVA) fibers carrying the sensation of smell. It is formed by the axons of olfactory sensory neurons, which are bipolar neurons located in the roof of the nasal cavity, within a modified region of nasal mucosa known as the olfactory epithelium. The axons of these neurons form olfactory fiber bundles that pass through foramina in the cribriform plate of the ethmoid bone. These bundles are collectively known as the olfactory nerve. The olfactory nerve synapses with second order relay neurons (mitral and tufted cells) in the olfactory bulbs located on the inferior aspect of the frontal lobe.

In the central nervous system, the smell information travels in the olfactory tracts and olfactory striae. Using this olfactory pathway, information about odor reaches the primary olfactory cortex of the temporal lobe.

Note: There are varied opinions regarding the types of functional fibers in the olfactory nerve. Some neuroanatomy authors categorize the olfactory nerve (CN I) as special somatic afferent (SSA), while some use special visceral afferent (SVA).

The olfactory nerve is just the beginning. It’s the first of 12 cranial nerves, learn them all with our cranial nerves labelling diagrams and quizzes!

Optic nerve (CN II)

The optic nerve (CN II) has special somatic afferent (SSA) fibers that pick up visual information from the eye. The retina of the eye has retinal ganglion cells, whose axons collectively form the optic nerve. This nerve leaves the eye at the optic disc and, after exiting the orbital cavity at the orbital apex, it travels through the optic canal to reach the cranial cavity.

Within the cranial cavity, the optic nerve terminates with some fibers crossing over at the optic chiasm. From here, the visual pathway continues with the optic tract, lateral geniculate nucleus of the thalamus and optic radiation to reach the primary visual cortex in the occipital lobe.

Take a deeper look at the eye and the optic nerve in these study units!

Vestibulocochlear nerve (CN VIII)

The vestibulocochlear nerve (CN VIII) consists of the cochlear and vestibular nerves. Both these nerves have special somatic afferent (SSA) fibers, responsible for the sensations of hearing and equilibrium respectively.

The vestibulocochlear nerve originates from the rhomboid fossa of the brainstem. The cochlear nerve emerges from the cochlear nuclei, while the vestibular nerve originates from the vestibular nuclei. The two nerves emerge from the brainstem at the junction between the pons and the medulla oblongata (pontomedullary junction). They travel through the posterior cranial fossa and internal auditory meatus to reach the cochlea and vestibular apparatus of the internal ear.

The cochlear nerve is responsible for hearing. Sound is converted into electrical signals by cochlear hair cells located in the spiral organ (Organ of Corti) in the cochlea. The peripheral processes of afferent neurons carry impulses to the spiral ganglion, from where central processes form the cochlear nerve carrying sound information in the form of electrical impulses to the brainstem.

The vestibular nerve plays an important role in maintaining equilibrium. Changing head position is sensed by vestibular hair cells located in the utricle, saccule and semicircular ducts. Here electrical impulses travel along peripheral processes of afferent neurons to reach the vestibular ganglion, from where the central processes form the vestibular nerve carrying information about position and spatial orientation in the form of electrical impulses to the brainstem.

Take a break from reading and test yourself with this quiz on the vestibulocochlear nerve!