Blinking Mechanism and Tear Film Dynamics

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Questions and Answers

What is the primary function of the parasympathetic nervous system in relation to the lacrimal gland?

Secretion of tears (correct)

Regulation of emotional crying

Control of blood supply to the lacrimal gland

Production of adrenaline

Which neurotransmitter is involved in stimulated secretion by the lacrimal gland?

Acetylcholine (correct)

Norepinephrine

Dopamine

Serotonin

What is the primary cause of the blink reflex in response to tear film evaporation?

Dryness of the corneal surface

Hypertonicity of the tear film (correct)

Increased pressure gradient in the drainage system

Hypotonicity of the epithelial cells

What is the role of the trigeminal ganglion in relation to tear production?

Sensation in the face Signup and view all the answers

During the closing phase of a blink, what happens to the canaliculi and sac?

They compress and fluid moves into the duct Signup and view all the answers

What is the difference between tears produced during emotional crying and tears produced to simply moisten the eye?

Tears from emotional crying carry toxins not normally found in the tear film Signup and view all the answers

What is the role of the hypothalamus in relation to tear production?

Regulation of emotional crying Signup and view all the answers

What is the purpose of the Schirmer test in assessing tear production?

To evaluate the effect of topical anesthesia on tear flow rate Signup and view all the answers

What is a common problem with the Tear Breakup Time (BUT) test?

All of the above Signup and view all the answers

Which nerve is responsible for relaying sensory information between the cerebellum and cerebrum?

Pons Signup and view all the answers

What is the effect of the parasympathetic nervous system on the heart rate?

Slows heart rate Signup and view all the answers

What is the advantage of using Rose Bengal Stain in assessing the tear film?

It stains dry epithelium while cells are still in situ Signup and view all the answers

What is the effect of age on tear production?

Tear production does not drop significantly with age Signup and view all the answers

What is the role of G proteins in stimulated secretion by the lacrimal gland?

Find and activate enzyme systems involved in production of secretory products Signup and view all the answers

What is the mechanism by which the drainage system is driven?

Pressure gradients Signup and view all the answers

What is the purpose of the paper tabs in assessing the tear film?

To assess the condition of the tear film Signup and view all the answers

Study Notes

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.

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Description

This quiz covers the reasons behind blinking, including the evaporation of tear film, hypertonicity, and the drainage system. It explains how the pressure gradient hypothesis drives the drainage system.