L7 Tear production Flashcards
What are the 3 layers of tears?
Mucous > aqueous > lipid
What is the function of the mucous layer?
- Coats the cornea
- provides a hydrophilic layer
- allows for even distribution of tear film
What is the function of the aqueous layer?
Promotes spreading of tear film
Promotes the control of infectious agents
promotes osmotic regulations
What’s the function of the lipid layer?
Coats the aqueous layer
provides hydrophilic barrier that retards evaporation
prevents tear spillage on cheek.
Which tissues are involved in the lipid layer?
Meibomian glands, Glands of Zeiss and Moll
Which tissues are involved in aqueous layer?
Lacrimal gland, Accessory lacrimal gland (Krause and Wolfring glands)
Which tissues are involved in mucous layer?
Conjunctival goblet cells and corneal epithelium
What are the lipid layer’s contents?
Sebum - wax, monoesters, sterol, esters, hydrocrabons, TGL, FFA’s, phospholipids
What are the aqueous layer’s contents?
Water, electrolytes, proteins (lipocalin, lactoferrin, lysozyme, lacritin, SIgnA, TGF, interleukins).
What does the mucous layer content consist of?
Mucin
Blinking assists in:
Distribution of tears, expels excessive tears
Lid closure assists in:
Wipe clean corneal surface
Assists in drainage of tear lake
Distributes fresh secretions from:
- Glands of Krause and Wolfring (aqeuous), and meibomian glands (oily layer)
Which gland secretes their substance via holocrine secretion?
Meibomian glands release Sebum via holocrine into the ducts. Blinking releases stored sebum
What is holocrine secretion?
In oil glands –> whole cell dies and ruptures, releasing their products.
Lacrimal gland protein secretion occurs via two pathways:
Constitutive and regulated pathway
What is the constitutive pathway of lacrimal gland protein secretion?
Proteins are in secretory granules that fuse with the membrane and are continually released into tears.
What is the regulated pathway of lacrimal gland protein secretion?
Proteins are stored in secretory vesicles and only fuse with apical membrane in response to an appropriate stimulus, i.e. neurotransmitters or peptide hormones.
Most proteins secreted by lacrimal gland use this pathway
What happens in the primary secretion of lacrimal glands?
Acinar cells mimin plasma cells by secreting Cl- into lumen, with it H2O via paracellular transport
Na/K ATPase sets up gradient for Cl and K to leave cell via their separate channels.
What happens during secondary secretion of lacrimal glands?
Lumen duct cells modify primary secretion by releasing KCl
Lacrimal secretion and therefore tear production is controlled by ___ release from nerves.
Neurotransmitter
During SNS response for tear production, ___ is released, and _____ is released during PNS. ___ is released for sensory.
Sympathetic –> Norepinephrine
Parasympathetic –> Ach and Vasointestinal peptide (VIP)
Sensory –> Substance P and calcitonin
Which peptide hormones are released during PNS activation of tear production?
a-melanocyte stimulating hormone (MSH)
Adrenocorticotrophic hormone
Which steroid hormones are released during PNS activation of tear production?
Androgens
Parasympathetic (Ach) elevates intracellular __ levels and ___ protein and electrolyte secretion
Ca+, stimulates
Receptor specific activation of Adenylate Cyclase ____ cAMP levels, ____ protein and electrolyte sectrion
Increases, stimulates
Lacrimal gland secretion is inhibited by…
Enkephalin’s that bind to receptors and prevent AC activation
The constitutive secretory pathway of lacrimal gland is regulated by…
Androgen steroid hormones
What is the role of conjunctiva and corneal epithelium?
It’s partly responsible for producing the moisture that covers the surface of the eye, facilitating eye movement. Goblet cells in conjunctiva are responsible for mucous later of the tear film.
What is conjunctiva?
Stratified columnar epithelium that lines the inner surfaces of the eylids and outer surface of eyeball
What are the two classifications conjunctiva?
Tarsal conjunctiva - lids
Bulbar conjunctiva - eyeball
In the conjunctiva cells, ___ and __ secretion is driven by Cl- transport out of cells
Electrolyte and water
Mucins, produced by goblet cells in conjunctiva, are released via ___ secretion.
Apocrine
What is the mucin function?
- Lubrication, protection, wetting the ocular surface
- Mucin binds to the glycocalyx of the corneal surface
Elimination of tears can occur by 3 routes:
- Passive evaporation
- Reabsorption by conjunctival nasolacrimal epithelium
- Active excretion by the drainage portions of the lacrimal system.
Structural components - pathway of tear drainage?
Puncta > Superior and inferior Lacriminal caniliculli > Nasolacrimal sac > Nasolacrimal duct > Valve of Hasner
The caniliculus tubes contain elastic tissue surrounded by ___
Orbicularis muscle
The nasolacrimal duct ends in the ___, allowing the sac to drain into back of nose, hence runny nose when crying
nose
What does the Valve of Hasner do?
Prevent movement in the other way
At the start of blink (eyelids fully open), system is ____
Filled with fluid
When lids are half closed, lid closure ____ canaliculi and sac
Squeezes
When lids have complete closure, canaliculi are _____ maximum amount until ____
squezed, all fluids are expelled.
When lids are opening again (half open), there is ____ of pressure on canaliculi, puncta is still _____
Release, occluded
At the end of blink, puncta are now ____, and fluid is drawn back in from ____
open, tear menisci
Tear duct blockage or failure of the capillary action of the drainage system causes….
Tears to be trapped into the lacrimal sac, where they become stagnant and infected.
This causes excessive watering, mucous discharge, eye irritation, and a painful swelling in the inner corner of your eye.
Infection can spread to other parts of the eye.
Meibomian glands and/or Lacrimal gland dysfunction causes…
Keratoconjunctivitis sicca or Dry eye
Dry eye can occur due to abnormality to any one of the three tear layers
- insufficient aqueous fluid production > lacrimal gland doesn’t produce enough tears to keep entire conjunctiva and cornea covered by a complete layer
- Abnormal tear composition > Meibomian gland dysfunction affects lipid layers and increases tear evaporation, leading to tear film instability
- Disorders of the eye > disrupt the blinking motion required to spread tears.
What are the symptoms of dry eye?
- dryness, buring and sandy-gritty irritation
- pain, redness, and pressure behind the eye
- increased sensitivity to bright light
What are reflex tears caused by?
Irritation to the eye causes reflex tears, however they don’t have the lubricating qualities necessary to prevent dry eye.
Dry eye incidence?
Increases in elderly - esp in post menopausal women
In contact lens wearers.
Dry eye syndrome is a ___ condition, which is managed by:
chronic, environmental control > dry, drafty environment and those with smoke and dust should be avoided. Using a humidifer, esp in the winter, can help by adding moisture to the dry indoor air.
Dry eye can be treated by:
Rehydration - artificial tears (lubricating eye drops that may alleviate the symptoms)
Dry eye treatment: medication?
Prescription medications:
1) Antibiotics to reduce eyelid inflammation
2) Eyedrops to control corneal inflammation
3) Tear simulating drugs - cholinergics - pilocarpine, cevimeline)
