Pathophysiology
Clinical meaning
The twelve cranial nerves emerge from the brainstem (except CN I and II from the cerebrum) and serve sensory, motor, or mixed functions. CN I (olfactory) passes through the cribriform plate; fractures here cause anosmia and CSF rhinorrhea. CN II (optic) transmits visual information via the optic chiasm where nasal fibers decussate; lesions produce characteristic visual field defects (bitemporal hemianopia from chiasmal compression). CN III (oculomotor) carries parasympathetic fibers on its surface, making pupil dilation the earliest sign of compression (uncal herniation). The corneal reflex tests CN V (afferent) and CN VII (efferent). CN VII upper vs lower motor neuron distinction is critical: central lesions spare the forehead (bilateral cortical innervation), peripheral lesions (Bell palsy) affect the entire half of face.