Which of the following cranial nerves is purely sensory in function?

Explanation

## Cranial Nerve Classification by Function **Key Point:** The optic nerve (CN II) is the only purely sensory cranial nerve. It carries visual information from the retina to the visual cortex and has no motor component. ### Functional Categories of Cranial Nerves | Nerve | Number | Function Type | Primary Role | |-------|--------|---------------|-------------| | Olfactory | CN I | Sensory | Smell | | Optic | CN II | Sensory | Vision | | Oculomotor | CN III | Motor | Eye movement, pupil constriction | | Trochlear | CN IV | Motor | Superior oblique eye movement | | Trigeminal | CN V | Mixed | Facial sensation, mastication | | Abducens | CN VI | Motor | Lateral rectus eye movement | | Facial | CN VII | Mixed | Facial expression, taste (anterior 2/3 tongue) | | Vestibulocochlear | CN VIII | Sensory | Hearing, balance | | Glossopharyngeal | CN IX | Mixed | Taste (posterior 1/3 tongue), swallowing | | Vagus | CN X | Mixed | Visceral motor and sensory | | Accessory | CN XI | Motor | Neck/shoulder movement | | Hypoglossal | CN XII | Motor | Tongue movement | **High-Yield:** Only CN I (olfactory) and CN II (optic) are purely sensory. All others are either purely motor or mixed (sensory + motor). **Mnemonic:** **"Oh, Oh, to touch and feel very good velvet, ah!"** — This classic mnemonic lists all 12 cranial nerves in order. Notice CN I and CN II are the only "sensory-only" entries. ### Why CN II Stands Alone The optic nerve is a true sensory nerve that: - Originates from the retinal ganglion cells (not the eye muscles) - Carries only afferent (sensory) information - Has no parasympathetic or sympathetic motor fibers - Is actually an extension of the central nervous system (diencephalon), not a peripheral nerve [cite:Standring Anatomy 42e Ch 28]