The lacrimation reflex

• stimulated by irritation of the conjunctiva and cornea, which elicits a large increase in tear volume.

• afferent limb involves branches of the ophthalmic nerve, with an additional contribution from the infraorbital nerve if the conjunctiva of the lower eyelid is involved

• Impulses enter the brain and spread by interneurones to activate parasympathetic neurones in the superior salivatory nucleus (associated with the facial nerve) and sympathetic neurones in the upper thoracic spinal cord.

The efferent pathway

• the greater petrosal nerve[preganglionic parasympathetic secretomotor fibres]

• the deep petrosal nerve[postganglionic sympathetic fibres]

• the parasympathetic fibres relay in the pterygopalatine ganglion

• sympathetic fibres pass through the ganglion without synapsing.

Lacrimation reflex

The aqueous component (consisting primarily of proteins, ions and water) is produced by the main and accessory lacrimal glands. Tarsal glands in the

eyelids produce the lipid layer of the tear film and the mucous component is derived from conjunctival goblet cells.