Blinking Mechanism and Tear Film Dynamics

Blinking and Tear Film

• We blink due to the evaporation of tear film, which leads to hypertonicity (high salt concentration) and draws water from epithelial cells, causing a slight sting that triggers a blink to restore isotonicity.

The Drainage System

• The drainage system is driven by pressure gradients, with three stages:
  • Before blinking, the system is open, and canaliculi are filled with approximately 1.5 μl fluid, with no pressure gradient.
  • During the closing phase, puncta meet and close, compressing canaliculi and sac, increasing pressure, and fluid moves into the duct.
  • During reopening, the sac and canaliculi re-expand, creating a negative pressure in the sac, and fluid flows into puncta and canaliculi.

Assessing Tear Film Condition

• The Schirmer test is used to assess tear production, with paper tabs inserted into the lower lids and removed after 5 minutes; less than 10 mm wetting is considered abnormal.
• Topical proparacaine hydrochloride anaesthesia reduces mean tear flow rate by 40% and prevents tearing due to irritation from the paper.

Tear Breakup Time (BUT)

• Tear Breakup Time (BUT) measures the time the eye can be kept open before the tear film ruptures spontaneously.
• The test is performed by treating the eye with fluorescein and scanning the tear film with cobalt blue light; the endpoint is recorded when a dry spot appears.

Problems with BUT

• Uneven mixing of fluorescein, unknown fluorescein concentration, and lag time between appearance of a dry spot and its discovery by the observer are potential problems with the test.

Nervous Involvement in Tears

• Tears can be secreted as a reflex response to various stimuli, including thermal stimuli, irritation of the cornea or conjunctiva, peppery foods, bright light, coughing, yawning, vomiting, and emotional upset.
• Tears produced above the resting level have more aqueous and less protein content.
• Lacrimal cells are under the influence of the hypothalamus, which is involved in controlling emotions and sexual activity.

Autonomic Nervous System

• The sympathetic nervous system (SNS) activates the "fight or flight" response, involved in producing adrenaline and controlling the cardiovascular system.
• The parasympathetic nervous system (PNS) conserves energy, slowing heart rate, increasing intestinal and gland activity, and relaxing sphincter muscles in the gut.

Lacrimal Gland Innervation

• The lacrimal gland is innervated by parasympathetic fibers, which stimulate secretion, and sympathetic fibers, which mainly control blood supply to the lacrimal gland.