Visual System Anatomy Flashcards
What is the Limbus?
The border between the cornea and the sclera
What produces tears?
Lacrimal gland
Where do the afferent nerves travel from the eye?
Cornea
Cranial Nerve V1 - ophthalmic trigeminal
What neurotransmitter operates within the lacrimal system?
Acetylcholine * due to parasympathetic efferent nerves
Where do the tears drain?
through two puncta - which are openings on medial lid margin
Where do tears flow through?
Superior and inferior canalculi and gather in the tear sac
They exit the tear duct into the nasal cavity
Where is the lacrimal gland located?
Located within the orbit - latero-superior to the globe
It is innervated by the parasympathetic nerve
What is basal tear?
Tear even in the absence of irritation or stimulation
What is reflex tear?
Increased tear productions in response to ocular irritation
What is the tear film?
The healthy cornea is covered in the tear film which is a thin layer of FLUID
Made of three layers : - mucous area - thick watery layer - thin lipid layer it allows a smooth cornea to air surface
What are the functions of the tear film?
Oxygen supply to cornea as cornea has no blood vessels
It contains factors against harmful bacteria
- both functions of the aqueous tear film layer in the middle
What is the top lipid layer of the tear film responsible for?
Protecting tear film from rapid evaporation
Where does the lipid layer of the tear film come from?
The lipid layer is secreted by Meibomian glands which are situated along the eyelid margins
What is the layer adhered to the eye surface used for?
The bottom mucinous layer ensures that tear film sticks to eye surface
Mucin molecules act by binding water molecules to the hydrophobic corneal epithelial cell surface
What is the conjuctiva?
The thin transparent tissue that covers the outer surface of the eye
begins at outer edge of cornea, covers visible part of the eye AND lines the EYELIDS
Nourished by tiny blood vessels which are more prominent when inflammed
What is the eyeball made up of outside to inside?
Sclera : hard and opaque
Choroid : pigmented and vascular
Retina: neurosensory tissue lining inner part of the eye
What is the function of the sclera?
Protecting and maintaining shape of the eye
has a high water content
What is the function of the choroid?
Providing circulation * especially to the back of the eye
and shields out unwanted scattered light
What is the function of the retina?
conversion of light to neurological impulses to be transmitted to the brain via optic nerve
What is the cornea?
Transparent dome like window covering front of the eye
Low water content
Powerful refracting surface 2/3 of eyes focusing power
Made of 5 layers
What 5 layers is the cornea made of?
1 – Epithelium
2 – Bowman’s membrane
3 – Stroma – its regularity contributes towards transparency
4- Descemet’s membrane
5- Endothelium – pumps fluid out of corneal and prevents corneal oedema
What is the Uvea?
The vascular coat of the eyeball and lies between the sclera and retina
What three parts make up the uvea?
Iris
Ciliary body
Choroid
These are connected and disease to one affects the other portions
What does the Iris do?
Control light levels
Round opening in the centre of pupil
embedded with tiny muscles that dilate or contrict
Describe the structure of the lens?
Outer acellular capsule
Regular inner elongated cell fibres - transparancey
may loose transparency with age = cataract
What is the function of the lens?
- Transparency
- Regular structure
- Refractive Power
- 1/3 of the eye focusing power - higher refractive index than aqueous fluid and vitreous
- Accommodation
- Elasticity
What is the visible portion of the optic nerve called?
Optic disk
It connects to the back of the eye near the macula
What is the blind spot?
Where the optic nerve meets the retina there are no light sensitive cells = blind spot
What is the macula?
roughly located in the centre of the retina
Small and highly sensitive part responsible for detailed central vision
What is the fovea?
The very centre of the macula
What is the fovea test?
Since the fovea is the most sensitive part of the retina = because it has the highest concentration of cones but low concentration of rods
- hence why stars out of the corner of your eye are brighter than when you look directly as rods are on periphery of eye not fovea
Only your fovea has the concentration of cones to percieve in detail
What is the corresponding anatomic landmark for the physiological blind spot?
Optic disk
What is central vision?
Detail day vision
colour vision
fovea with highest concentration of cone photoreceptors
read and facial recognition
How to asses central vision?
visial acuity assessment
Loss of foveal vision : poor visual acuity
What is peripheral vision?
Shape, movement, night vision
Navigation vision
How to assess peripheral vision?
Assessed by visual field assessment
Extensive loss of visual field - unable to navigate in environment, px may need white stick even with perfect visual acuity
What is retinal structure?
Made of outer layer: retinal pigment epithelium which sits infront of choroid
Middle layer : made of bipolar 2nd order neuron cells
Inner layer : neuroretina made of retinal ganglion 3rd order neurones
What is the function of the retinal pigment epithelium?
Made of photoreceptors = rods
Used for detection of light
What is the function of the Middle layer of retina?
Local signal processing to improve contrast sensitivity, regulate sensitivity
What is the function of the inner layer of retina?
transmission of signal from eye to the brain
What two photoreceptors are there?
Rods and Cons
Rods : 100X more sensitive to light and night vision
Cons : colour and day vision
What are the structural differences between cones and rods?
Rods: have longer outer segment with photo sensitive pigment
Have a slower response to light
Cones: Shorter outer segment.
6 million cones vs 120 million rods
What happens to the outer segment distal discs in photoreceptors?
Outer segment : made of stacks of discs, distal discs which are deactivated pigments are shedded and phagocytosed by retinal epithelial cells
How are photoreceptors distrivuted in the eye?
In periphery = higher rods in central higher cones
Where can one find the highest concentration of Rod photoreceptors in the retina?
20-40 degrees away from fovea
What wavelength of lights do the following cones respond do:
S cones
M cones
L cones?
S cones - blue
M cones - green
L cones - red
What is the commonest colour vision deficiency?
Dueteranomaly - shfiting of M cone sensitivity peak towards the L cone curve causing red-green confusion
- check slide 34 for frequency spectrum
What is Achromatopsia?
Full colour blindness
What is anamalous trichromatism?
Colour Vision deficits can be caused by a shift in the photo-pigment peak sensitivity.
What is dichromatism?
only two cone photo-pigment sub-types are present
What is monochromatism?
there is complete absence of colour vision.
This can be caused by Blue Cone Monochromatism,
with the presence of only blue L-cones.
Or by Rod Monochromatism,
in which there is a total absence of all cone photo-receptors.
Px with blue cone monochromatism have…?
normal day light visual acuity
Px with rod Rod Monochromatism have …?
No functional day vision
What is the colour blindness test called?
Ishihara test
