Function And Physiology Of The Tear Film Flashcards Preview

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Disorder of the tear film caused by tear deficiency or excessive tear evaporation that causes damage to the interpalpebral ocular surface nad is associated with symptoms of ocular discomfort

Dry eye syndrome


Function of tear film

-provides a smooth refractive surface for clear vision
-maintins the health of corneal and conjunctival epithelial
-acts as the first line of defense against microbial infections


Tear film structure from anterior to posterior

Lipid layer
Aqueous layer
Mucous layer


Mucous layer and aqueous layer of the tear film

They are sort of mixed together and have a gradient of mucins


Mucus layer composition

Intermost layer composed of mucin, immunoglobulins, urea, salts, glucose, leukocytes, cellular debris, and enzymes


High molecule weight glycoproteins that are heavily glycosylated



Two types of mucins

Membrane associated mucins
Secretory mucins.


A dense barrier to pathogen penetrate, at the epithelial cell-tear film interface

Membrane associated mucin


Act as a cleaning crew, moving through the tear fluid and collecting debris that can be removed via the nsaolacrimal duct during blinking

Secretory mucins


Function of mucus layer

-Maintain water on the surface fo the eye
-The mucins also interact with tear lipids layer to lower surface tension thereby stabilizing the tear film
-provides lubircation for eyelid movement
-traps exploited surface cell, foreign particles and bacteria


Sources of mucus for the mucus layer

Goblet cells and squamous cells of cornea and conjunctiva
Goblet cells major one

-neural control
-sensory nerves
-PNS-increases secretion
-SNS help regulate the goblet cell proliferation, not really to do with mucin increasing


Aqueous layer composition

-composed of water, electrolytes, protein, peptides growth factors, vitamins, and hormones
-middle layer
-electrolytes include sodium, chloride, potassium, calcium


What does sodium and chloride do in the aqueous layer

Main contributors to osmolarity of tear film (osmolarity increases with dry eye)


What does potassium and calcium do to aqueous layer of the tear film

Important for maintaining corneal epithelium health


Osmaolrty and dry eye

Osmolarity increases with dry eye
-Na and CL


Proteins of the aqueous layer

Albumin, immunoglobulins (IgA) histamine, lactoferin, lysozyme, interleukins, antimicrboials


Peptide growth factors in the aqueous layer

EGF, TGF-B, HGF (hepatocyte grwoth facotr), and vitamin A


Aqueous layer function

Electrolytes act as a buffer to maintain a constant pH and contribute to maintaining epithelial integrity of the ocular surface


The buffer system of the aqueous layer of the tear film allows the eye to tolerate ophthalmic solution varying from ______ to _____ pH



When does pH decrease in the tear film

During sleep


When does pH increase in tear film

Dry eye due to osmolarity increase


Function of aqueous layer

Offers strong defense system to protect against invading microorganism through contained proteins
-peptide growth factor and vitamin A act via auto rinse and paracrine mechanisms to regulate epithelial proliferation, motility and differentiation


Reflex and emotional tearing

Main lacrtimal gland


Maintenance of tearing

Accessory lacrtimal gland


Neural control of lacrimal glands

PNS, SNS, sensory nerves to main lacrimal gland

PNS only: accessory lacrimal glands


Lipid layer composition

Outermost layer
-meibomian oil
-composed of two phases
—polar surfactant phase
—nonmoral phase (overlaid polar phase)


Primarily composed of phospholipids and glycilpids. Acts as a surfactant between the hydrophilic aqueous mucin layers and the thick, no polar lipid layer

Polar surfactant phase of the lipid layer
Inner most layer of lipid layer


Primarily composed of wax, cholesterol esters, and trigs. Provides the air-tear film interface and is responsible for retarding evaporation

Nonpolar phase of the lipid layer (outermost layer of the lipid layer)


Lipid layer function

-major barrier to evaporation from the ocular surface
-provides stability to the tear film through interactions with the aqueous mucin phase
-provides a smooth optical surface fro the cornea
-acts as a barrier against foreign particle


Resources of lipid layer

Meibomian glands: with assistance by two other sebaceous glands, zeis and Moll

Neural control: PNS