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Fundamentals of Ophtho > Chapter 7: Tear Film > Flashcards

Chapter 7: Tear Film Flashcards

1
Q

What is the primary function of the tear film?

A
  • provide a smooth optical surface @ air-cornea interface
  • mediun for debris removal
  • protect ocular surface
  • contains antimicrobial agents
  • lubricates cornea-eyelid interface
  • prevents dessication of ocular surface
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2
Q

Where does the marginal tear strip cover? The precorneal tear film? The conjunctival tear film?

A

Marginal tear film= tear meniscus
Pre-Corneal tear film covers exposed bulbar conjunctiva and cornea, and is trilaminar
conjunctival film covers b/c eyelids and bulbar conjunctiva

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3
Q

What are the three layers of the pre-corneal tear film?

A

Outer lipid layer
Middle Aqueous layer
Inner mucin layer

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4
Q

What are the functions of the outer lipid layer?

A
  • slow tear evaporation
  • contribute to optical properties at air-cornea interface
  • Increase surface tension via maintenance of a hydrophobic barrier, preventing tear overflow
  • prevent damage to eyelid margin skin by tears
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5
Q

How thick is the outer lipid layer of precorneal tear film?

A

about 100 molecules thick

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6
Q

How many glands produce the lipid layer? Where are they located?

A

Glands are located in the tarsal plates of the upper and lower lid.
Upper lid= 30-40 orifices
Lower Lid= 20-30 orifices

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7
Q

Where do the meibomian glands of the tarsal plates open to?

A

they open onto the skin of the eyelid margin, b/w the tarsal grey line and mucocutaneous junction

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8
Q

What is the innervation of the meibomian glands?

A

Parasympathetics (main source; cholinesterase, vip)
Sympathetics
Sensory
Neuropeptide Y

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9
Q

Why are tear lipids not susceptible to lipid peroxidation?

A

They are not susceptible to lipid peroxidation because they contain extremely low amounts of polyunsaturated fatty acids

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10
Q

What are the functions of the aqueous layer of the pre corneal tear film?

A
  • supply 02 to avascular corneal epithelium
  • maintain constant electrolyte concentration
  • Antibacterial/Antiviral defenses
  • smooth minute irregularities
  • wash away debris
  • modulate corneal and conjunctival epithelial cell fx
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11
Q

What cells secrete the aqueous layer? What does the aqueous layer consist of?

A

The main and accessory lacrimal glands (Krause and wolfing)

Consists of electrolytes, H2O, proteins, and a variety of solutes

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12
Q

How much do the glands of Krause contribute to the aqueous layer? Where are they located?

A

They contribute 2/3 of the product of the accessory lacrimal glands. They are located in the lateral portion of the upper fornix (some also present in the lower fornix as well)

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13
Q

How much do the glands of Wolffring contribute to the aqueous later? Where are they located?

A

1/3 of total accessory lacrimal gland production. They are located on the proximal margin of each tarsus.

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14
Q

What is the innervation of the main lacrimal gland?

A

Parasympathetics (Main innervation; ACh and VIP)
Sympathetics (NE and NPY)
Sensory (Substance P and CGRP)

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15
Q

What are the electrolyte concentrations of the aqueous layer compared to serum?

A

Na+- equivalent to serum
K+- 5-7x greater than serum value
HCO3- regulates tear pH

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16
Q

What other solutes are located in the aqueous layer of the pre corneal tear film?

A

Urea, glucose, lactate, citrate, ascorbate, and amino acids

17
Q

What proteins are found in the aqueous layer of the pre corneal tear film?

A

IgA, sIgA, IgM, IgD, IgE
IgA and sIgA are formed by plasma cells of interstitial tissues of the main/accessory lacrimal glands and substantia propria of conjunctiva

18
Q

What antimicrobial agents are found in the aqueous layer of the pre corneal tear film?

A

Lysozymes, lactoferrin, Group II phospholipase A2, lipocalins, degensins, IFNs, cytokines, and GFs

19
Q

What affect does vernal conjunctivitis have on the aqueous layer of the pre corneal tear film and why?

A

It increases tear and serum IgE, increases IgE-producing plasma cells in the giant papillae of the superior tarsal conjunctiva, and increases histamine.

20
Q

What does the mucin layer of the pre corneal tear film contain?

A

Mucins, proteins, electrolytes, and H2O

21
Q

What is the surface of the mucin layer of the pre corneal tear film?

A
  • converts the corneal surface from hydrophobic to hydrophilic
  • intersects with lipid layer to decrease surface tension (stabilizes tear film)
  • trap exfoliated surface cells/foreign particles/bacteria
  • lubricates eyelids as they pass over the globe
22
Q

What cells secrete the mucin layer of the pre corneal tear film?

A

The conjunctival goblet cells and stratified squamous epithelium, minimally by lacrimal glands of Henle and Manz.

23
Q

What is the daily production of the mucin layer of the pre corneal tear film?

A

2-3 microliters per day

24
Q

How do parasympathetic and sympathetic nerves act of the tear glands?

A

via G-protein linked receptors, and activating one of two pathways:

  • Ca2+/Protein dependent Kinase pathway
  • cAMP pathway
25
Q

Describe the Ca2+/Protein dependent kinase pathway of lacrimal gland activation

A

activation of muscarinic (via ACh) or alpha1 adrenergic (via NE–all except main lacrimal gland) receptors–> G Protein activation–> phospholipase C activation -> IP3 and Diacylglycerol–> Ca2+ release –> protein/h2O/electrolyte secretion

26
Q

Describe the cAMP pathway of lacrimal gland activation

A

Activation by NE/VIP-> stimulation of VIP/beta adrenergic receptors-> activates G Proteins-> activates adenylyl cyclase-> cAMP production-> activates cAMP-dependent kinase-> stimulates secretion

27
Q

What other types of stimulation can activate the tear glands?

A

Steroid and Peptide hormones can also activate tear gland secretion
Peptide- stimulate secretion from main lacrimal gland via cAMP pathway
Steroid-stimulate secretion of sIgA from main lacrimal gland and lipid secretion from meibomian glands via diffusing into nucleus, binding steroid receptors, and activating gene transcription and protein synthesis

28
Q

How does eyelid movement affect tear secretion?

A
  • As eyelids close, superior and inferior fornices are compressed by force of preseptal muscles and the eyelids moving toward each other.
  • force from upper lid clears surface of debris and expresses secretions of meibomian glands
  • lower lid moves horizontally toward the nose during blinking, which pushes tear fluid and debris toward the puncta.
  • When eyelids open, the tear film is re-distributed.
29
Q

What are diagnostic tests for tear dysfunction

A
  • tear breakup time
  • fluorescein staining
  • lisomine green staining
  • rose bengal staining
  • osmolarity test
  • schirmer test

Fundamentals of Ophtho (14 decks)

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