Corneal Nerve Anatomy

Abstract

The trigeminal ganglion's ophthalmic region contains a small number of primary sensory neurons whose peripheral axons provide sensory innervation to the eye. Most sensory axons enter the eyeball through the long and short ciliary nerves, reaching all ocular tissues except the lens and the retina. The cornea has rich sensory innervation, organized into four levels from penetrating stromal nerve trunks to intraepithelial nerve terminals. The cornea is also provided with a modest autonomic innervation in some species (Marfurt et al., 2019). Although all nerves in the cornea exhibit a similar appearance, consisting of unmyelinated fibers that end as free nerve endings, they differ in neuropeptide and other neurotransmitter content. Certain nerve fibers contain one or several neurotransmitters, many with relevant trophic functions in corneal tissue. Cornea nerves also differ in the expression of ion channels involved in stimuli transduction and action potential generation. Consequently, corneal sensory nerve fibers are functionally diverse, including low mechanoreceptor, mechano- and polymodal nociceptor, and cold thermoreceptor nerve fibers. Corneal nerves, especially nerve terminals in the corneal epithelium, undergo continuous remodeling in adults. Damage to ocular sensory nerves from accidental or surgical injury or due to pathological processes leads to corneal nerve loss and reduced sensitivity to stimulation. Subsequent nerve regeneration is gradual and incomplete, and it often fails to fully restore the corneal innervation to its original density, architecture, and function, leading to long-lasting changes in neural excitability that can result in ocular dysesthesia and neuropathic pain in the eye.