The corneal reflex is a protective reflex that causes both eyes to blink when the cornea of either eye is touched. In this video, I discuss the neural pathways underlying the corneal reflex as well as what types of damage might cause a deficit in the reflex. TRANSCRIPT: The corneal reflex is a protective reflex that occurs when the cornea of either eye is touched. When this happens, the corneal reflex causes the eye being touched to blink---but it also causes the other eye to blink at the same time. This ensures that both eyes are protected from any foreign objects or irritants in the environment. The corneal reflex involves sensory fibers in the ophthalmic division of the trigeminal nerve that are activated when the cornea is touched. These fibers synapse on neurons in the spinal trigeminal nucleus, and those spinal trigeminal neurons then project to the facial motor nuclei on both sides of the brainstem and synapse on neurons of the facial nerve. The facial nerve fibers leave the brainstem and travel to the orbicularis oculi muscle, which closes the eyelid. Thus, the reflex arc uses the sensory input being received in one eye to generate a motor signal that causes both eyes to blink simultaneously. The corneal reflex is used clinically to test the integrity of the trigeminal and facial nerves and related structures, typically by touching the cornea with a piece of cotton. The healthy response to touching one cornea would be that both eyes would blink, but damage to the trigeminal or facial nerves or other related structures may cause a deficit in the reflex. For example, damage to the right trigeminal nerve could cause a corneal touch to the right eye to elicit no response in either eye, since the sensation might not be detected. But damage to the facial nerve on the right side could cause a corneal touch on the right side to elicit a blink in only the left eye, since the touch would be detected but the right facial nerve may be incapable of carrying out the reflex response. REFERENCES: Peterson DC, Hamel RN. Corneal Reflex. [Updated 2019 Jun 22]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK534247/ Vanderah TW, Gould DJ. Nolte's The Human Brain. 7th ed. Philadelphia, PA: Elsevier; 2016. Special thanks to Srikar Gade for help with fact-checking the script for this video.