Chapter 2: Ocular Physiology Flashcards
What are the 3 layers of the tear film, from deep to superficial
Mucus
Aqueous
Lipid
True of false, the mucus layer of the tear film is wiped off during blinking?
False, it is only thinned out
What is the mucus layer of the the tear film made of
Epithelial cell glycocalyx and tear mucins (glycoproteins) produced by goblet cells
Where are goblet cells located in the eye and how are they controlled?
Grouped in the tarsal conjunctival crypts (Henle’s crypts) and bulbar conjunctiva, nasal to the limbus (manz’s crypts).
They are stimulated by sensory nerves on the cornea and conjunctiva and are under sympathetic and parasympathetic control
Apart from goblet cells, what other cells produce mucins
Stratified squamous cells on the cornea and conjunctival epithelium
Why is mucus in the tear film so viscous
Composed of a central protein containing many threonine and serine residues attached to oligosaccharide chains. This gives it the ability to bind to hydrophobic cornea and hydrophilic aqueous layer.
What is the main mucin in the tear film?
MUC5UC
Which is the thickest part of the tear film?
Aqueous 7-10 micrometers
Where is the aqueous section of the tear film produced
Lacrimal and accessory lacrimal gland.
What is the aqueous layer of the tear film composed of and what properties does it have?
Mainly water (98%)
Salts: k and Cl
Proteins: IgA, lactoferrin, Gprotein, prealbumin, lysozyme, traces of plasma proteins.
Function: antibacterial, anti adhesive, lubrication
What happens to the aqueous layer composition when tear film production rate is increased?
Goes from being hypertonic due to there being plenty of time for salt and proteins to be released to isotonic.
The lacrimal glad is composed of which type of cell and what does it secrete?
Tuboacinar cells. They are exocrine in nature. Secretes, water, protein and salts
What is the basal rate of tear secretion
1.2microlitre per min
How is the lacrimal gland tear secretion controlled
Under autonomic control by the sympathetic and parasympathetic systems.
The parasympathetic system directly simulates the Acinar cells or the myoepithelium around them.
What is the effect of menopause on lacrimation
Decreases
What does testosterone cause in the lacrimal system?
Increased release of proteins like IgA
The accessory lacrimal gland and corneal epithelium are under the control of…
The sympathetic nervous system
Which is the thinnest layer of the tear film?
Lipid layer (0.2-1 micro litre).
Where is the tear firm lipid layer secreted and what is its composition
Secreted by the meibomian glands (Holocrine).
They have one polar surface facing the aqueous layer and one non polar surface facing the air
How is the lipid layer released from its ducts and what is its production controlled by?
Blinking released stored material from the ducts.
Controlled by androgen/ sex steroids as well as neurotransmitters.
The canaliculi are are lined with ….. ……. Cells and are surrounded by the …….. muscle.
1) stratified squamous epithelial
2) orbicularis.
True or false, in most cases, the superior canaliculus joins the inferior to form the common canaliculus?
True, in 10% of cases they don’t join.
What type of cells line the canaliculi?
Non ciliated columnar cells.
How is retrograde flow of tears prevented in the common canaliculus?
Angled anteriorly forming a valve
How much of the tear volume is lost to evaporation?
10-25%
True or false, lacrimal drainage relies only on gravity?
False,there is a dynamic pumping process causes by contraction of the surrounding muscles causing negative pressure during blinking.
What are the 5 main functions of the ciliary body?
- Accommodation
- Aqueous humour production
- Production of lens zonules.
- Production of GAG and collagen
- Non conventional outflow of aqueous humour
Which structure secretes aqueous humour?
Non pigmented ciliary epithelium of the anterior pars plicata
What percentage of aqueous humour is drained via the uveosceral route?
10%
What is the rate of production of aqueous humour, and by which processes is it released from the ciliary body (non-pigmented aspect of the pars plicata)
2 to 3 µL/s.
Diffusion
Active secretion
Ultrafiltration (pressure dependent excretion).
What percentage of aqueous humour is actively secreted? Which enzymes are responsible for this.
80 to 90%
Carbonic, anhydrase and sodium potassium pump
What is the difference in the composition of the aqueous compared to plasma?
In general, aqueous contains less protein.
Aqueous contains more, G AL: glucose, ascorbate, lactate
True false, the sclera is more hydrated than the cornea
False, the cornea is 80% hydrated compared to the sclera, which is 70% hydrated
Which molecules are responsible for the cornea is high, swelling pressure
Glycosaminoglycan (lumican).
How are corneal fibres arranged?
Quasirandom configuration
How is the swelling pressure of the cornea counteracted?
- H2O and CO2 are pulled from the stroma into the endothelium where it is converted into H+ and HCO3-
- Na/ HCO3- co transporter transports Na/ HCO3- into the aqueous.
- H+ is taken into the stroma
- Na H+ exchanger pulls H+ out of stroma into aqueous and Na into the stroma to replenish the loss
Where does the cornea derive most of its glucose from?
Stroma via the aqueous humour
How is the glycogen store in the cornea utilised?
Converted into glucose 6 phopshate.
Most of this is converted into pyruvate via the glycolytic pathway. This subsequently produces lactic acid.
During sleep, how is oxygen supplied to the cornea
Via the palpebral conjunctiva.
Where does the corneal deep stroma and endothelium derive its oxygen from?
Aqueous humour
Which part of the corneal is more metabolically active, the epithelium or endothelium
Endothelium.
Mainly uses anaerobic respiration.
Endothelium also uses citric acid cycle more than epithelium
How many layers are in the corneal epithelium
6
In corneal healing, which process is used to generate energy
Anaerobic glycolysis. There is an increased expression of GLUT1 to help with this.
Which layer of the cornea cannot regenerate
Bowman’s layer and endothelium.
As endothelium controls swelling pressure, if a lot of cells are damaged, cornea recompenses and becomes opaque.
Which aspects of the cornea obtain oxygen from the aqueous?
Stroma and endothelium.
