Retina & Optic Nerve Flashcards
Detail the anatomy of the retina
It is a apart of the neural layer of the eye, sandwiched between the vitreous humour internally and choroid & sclera externally.
Composed of 10 layers whihc can bee simplified into two major layers:
Neurosensory
Retinal Pigment Epithelium
Describe the three sections of the retina neurosensory layer
Ganglion cells & nerve fibre layer converge on optic disc forming the optic nerve
Inner Nuclear Layer (INL):
Bipolar cells - Relay visual signals
Amacrine - Modulate neuronal activity
Muller cells - Structural skeleton
Outer Nuclear Layer (ONL)
Nuclei of rods & cones (thick band of dark dots)
Briefly explain the RPE and its pigmentation
Outermost layer, single layer of cells
Lies in front of tapetum
Despite its name, RPE is both pigmented and non-pigmented
- Pigmented in ventral fundus where tapetum is absent
Non-pigmented in dorsal fundus where tapetum is present – allows tapetum to be seen
Describe the physiology of the retina
Absorb light rays & convert light energy into electrical energy which travels as a nerve impulse up the optic nerve to the visual cortex
What is the function of the Retinal Pigment Epithelium in the physiology of the retina?
Essential for retinal integrity & function
Photoreceptors are embedded within RPE
Support system for neurosensory retina
Recycles 'used' photopigments
Phagocytosing & renewing photoreceptor outer segments
Stores vitamin A
Melanin pigments (in no tapetal areas only) absorb stray light &
scavenge free radicals
Form part of blood retinal barrier
Phagocytic role in retinal inflammation
What is the function of the neurosensory layer in regards to retinal physiology?
Contain visual pigment
Light energy is converted to a nerve impulse (phototransduction)
Photopigments found inside membrane sacks of rods & cones
consist of a protein (opsin) and a derivative of vitamin A (retinal)
1. Series of biochemical reactions
2. Result in hyperpolarisation of photoreceptor cell
3. Electrical impulse generated
4. Travels through neurosensory layers to ganglion cells, nerve
axons to optic nerve
Cones:
Colour vision
Visual acuity
Rods:
Night vision
Detail the anatomy of the optic nerve
Three regions
Intraocular portion
Can be directly observed in vivo
Retrobulbar portion (in orbit)
Intracranial portion
Describe the function of the optic nerve
Afferent pathway to visual cortex
Passes through lamina cribosa at posterior pole of globe
Sieve-like area in sclera
A weak point, sensitive to increased intraocular pressure as pressure on this weak point can damage the optic nerve causing blindness- (glaucoma)
May be myelinated (dog) or non-myelinated (cat)
Affects appearance of optic disc as seen with ophthalmoscopy
What is a fundus?
Portion of posterior segment of the eye that is viewed with the ophthalmoscope
Describe the anatomy of the fundus and how the different layers relate to their appearance on ophthalmoscopy
Sclera: Outermost fibrous white layer
Choroid & Tapetum: Posterior uveal tract. Radial large blood vessels.
Variable pigmented. Tapetum -Dorsal & triangular
RPE: Pigmented over non-tapetal area
Neural Retina: Transparent unless detached. Branching thin dark
vessels
Describe how the appearance of the fundus varies across species
Different species have different arrangement of blood vessels – will affect appearance of fundus
Holangiotic - retinal blood vessels supply whole retina e.g. dog, cat,
cow, sheep, goat
Paurangiotic - retinal blood vessels supply a small focal area of
retina e.g. Horse
In dogs the optic disc is myelinated - so appear pink whiteish
-Some breeds will have pronounced myelination
In cats the optic disc is non-myelinated - so appears grey
‘Stars of Winslow’ - ends of choroidal blood vessels in equine fundus
Pigment variation in animals will affect the appearance of the fundus
Coat colour, iris colour & fundus colour are linked
Describe the principles of the retina’s dual blood supply
High metabolic rate: dual blood supply
Inner retina: retinal blood vessels Outer retina: choroidal blood vessels
Explain the relationship between the retina and uveal tract (choroid) with respect to systemic disease
This can be expressed in both inherited disease (Generalised progressive retinal atrophy) and acquired disease (retinal detachment, retinal haemorrhage & posterior uveitis (Chorioretinitis))
What is generalised progressive retinal atrophy (Gpra)?
Important cause of inherited blindness in pedigree dogs
e.g. Toy and Miniature poodles, Labrador retrievers, English Cocker spaniel
Bilateral symmetrical degeneration of neurosensory retina
-Tapetal hyper-reflectivity due to thinned atrophic
neurosensory retina
Gradual loss of vision
Secondary cataract common
No treatment
Genetic tests available
Appearance in ophthalmoscopy:
-Vascular attenuation
-Pale optic disc
-Tapetal hyper-reflectivity
What is retinal detachment?
Retinal Detachment
Retina is strongly attached at only two points:
Around the optic disc
Ora ciliaris retinae (peripheral retina)
Detachment occurs between neurosensory retina and RPE Common causes: Systemic hypertension (high blood pressure) e.g. geriatric cats Congenital e.g. collie eye anomaly (inherited retinal disease) Trauma Inflammation Neoplasia
Complication of lens luxation or chronic cataract
