2. Special senses/ the eye and seeing Flashcards
Trachoma is an infectious disease caused by…
Chlamydia
Top 4 causes of visual impairment?
- URE (uncorrected refractive error)
- Cataract
- Glaucome
- Trachoma, AMD
Top 4 causes of blindness?
- Cataract
- Glaucoma
- AMD
- CO (cornea opacity)
What are the 5 layers of the cornea?
Is 6 when including the tear film
(Outer) + tear film Epithelium Bowman's membrane Stroma Descemet's membrane Endothelium (keep at keeping cornea clear)
What is the major light focussing element of the eye?
The cornea at 40D of “fixed power”
Layers of the tear film?
- LIPID LAYER
Oily lipid outer layer to reduce evaporation - WATER LAYER
Aqueous middle layer containing lysozymes and antibodies - MUCIN LAYER
Mucin layer containing glycoprotein, wets the cornea and allows good coverage
Function of tear film?
- To wash the eye of debris
- To provide the dissolved o2 to the cornea external epithelium
- Lubrication of the cornea
what are the different refractive elements of the eye?
• Cornea – largest element (40D) – Interfaces with air with low RI • Lens – lesser element (20D) – Interfaces with aqueous – similar RI – But can vary in power (accommodation) • Whole Eye Ball – About 60D
How to calculate power?
1/focal length(m)
or
(n2-n1)/ r
Note: radius of curvature = “r”
and “n” is the difference of refractive ability of the object. N2 = cornea, N1 is of the medium it is in.
Why is underwater vision poor?
Due to similar refractive ability of cornea and water. Need to be a difference in “power = (n2-n1)/ r” to produce a greater power.
Hence the light cannot be focused on the retina as corneal refraction has been dampened
When is accommodation needed?
To look at something UP CLOSE
What actions occur in accommodation?
To look at something upclose the eye must:
Constrict and turn in. The lens must narrow so ciliary muscles relaxes to allow the zonule fibres to slack off
What are the sources of the dual innervation of the iris?
PS fibres from CN III –> Cholinergic stimulation of pupillary CONSTRICTOR
Sympathetic fibres from the superior cervical ganglion –>Adrenergic stimulation of pupillary DILATOR
What is the near triad?
Miosis
Convergence
Accommodation
i.e. actions of the eye when focusing on something near
URE?
Uncorrected refractive error
Main type of URE?
Presbyopia (failure to accommodate when you are older) is the main type of URE
Myopia?
Short-sidedness
Hypermetropia? Associated with?
Long sightedness
Hypermetropia (long sightedness) is associated with squint and lazy eye (amblyopia) and acute closed angle glaucoma