Visual System Flashcards
What is the space between the lateral and medial canthus of the eye?
The palpebral fissure
What term refers to the lateral confluence of the upper and lower eyelid regions?
Lateral canthus
What term refers to the medial confluence of the upper and lower eyelid regions?
Medial canthus
What is the opening within the iris?
The pupil
Which globular nodule structure resides besides the medial canthus of the eye?
Caruncle
Which structure is the border between the cornea and sclera?
Limbus
Where is the lacrimal gland located?
Located within the orbit, latero-superior to the globe
What are the three types of tears?
Basal
Reflex
Emotional (crying)
What is the function performed by the lacrimal gland?
It continually releases fluid which cleanses and protects the eye’s surface as it lubricates and moistens it. These lacrimal secretions are commonly known as tears.
What are basal tears?
Basal tears are your basic functional tear. They are released continuously in tiny quantities to lubricate the cornea and keep it clear of dust. This is vital to ensure good visual acuity and comfort. Basal tears also fight against bacterial infection as a part of the immune system.
Which tears are referred to the increased tear production in response to ocular irritation?
A reflex tear
What does the tear pathway comprise of?
Afferent pathway
CNS
Efferent pathway
Lacrimal gland
Which afferent nerve innervates the cornea?
Sensory nerve fibres via the ophthalmic branch of trigeminal nerve.
How is the efferent pathway mediated?
Mediated by the parasympathetic nerve, innervating the lacrimal gland.
How are tears drained by the lacrimal system?
The tear film drains through the two puncta, tiny openings on the upper and lower medial lid margins
Flows through the superior and inferior canaliculi converging into a single common canaliculus, draining into a tear sac –> Into the nasal cavity through the tear duct.
Which two superior and inferior eyelid structures are responsible for tear film drainage?
The puncta
How are tears drained from the puncta to the tear sac?
Tears flow through the superior and inferior canaliculi to the tear sac
What is the fate of tears within the tear sac?
Exit the tear sac through the tear duct into the nasal cavity.
What are the four primary functions of the tear film?
1) Maintains smooth cornea-air surface (Clear vision)
2) Oxygen supply to cornea
3) Removal of debris
4) Bactericide
How many layers comprise the tear film?
Composed of three layers:
1) Lipid layer (Superficial)
2) Water layer
3) Mucin layer
Which glands along the lid margins produce the superficial lipid layer of the tear film?
Meibomian glands
What is the purpose of the mucinous layer corneal surface?
Maintains surface wetting.
What provides oxygen supply to the anterior segment of the eye (cornea)?
The tear film considering there are no blood vessels supplying the cornea
What is the function of the lipid layer of the tear film?
Responsible for protecting the tear film from rapid evaporation.
Which tear film layer forms the main bulk of the tear film?
The water layer
What are the two main functions of the water layer (tear film)?
1) Delivers oxygen and nutrient to surrounding tissue.
2) Contains factors against potentially harmful bacteria.
How do mucin molecules regulate surface wetting of the corneal epithelial surface?
The mucin molecules act by binding water molecules to the hydrophobic corneal epithelial cell surface.
Which layer in the tear film protects the tear film from rapid evaporation?
Lipid layer
Which thin transparent tissue covers the outer surface of the eye?
Conjunctiva
Describe the anatomical course of the conjunctiva?
Begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids.
What function is performed by the conjuctiva?
The conjunctiva of the eye provides protection and lubrication of the eye by the production of mucus and tears. It prevents microbial entrance into the eye and plays a role in immune surveillance. It lines the inside of the eyelids and provides a covering to the sclera
How is conjunctiva nourished with nutrients and oxygen?
Tiny blood vessels
What is the average antero-posterior diameter of the eye?
24mm in adults.
What are the three layers of the eye?
Sclera
Choroid
Retina
Which anatomical space does the eye reside within?
The orbit
What is the function of the sclera?
The outer fibrous opaque layer responsible for protecting the eye and maintaining shape
Which eye layer is pigmented and vascular?
Choroid
What is the function of the choroid?
Responsible for providing circulation to the eye, and shielding out unwanted scattered light.
What is the innermost neurosensory layer of the eye?
The Retina
How many layes form the cornea?
1) Epithelium
2) Bowman’s membrane
3) Stroma
4) Descemet’s membrane
5) Endothelium
Describe the water content of the sclera:
High water content
Describe the water content of the cornea:
Low water content
The regularity of which corneal layer contributes towards transparency?
Stroma
Which corneal structure pumps fluid out of the cornea and prevents corneal oedema?
Endothelium
What property of the cornea provides 2/3 of the eye’s focussing power?
The refractive index
What happens if the you hydrate the cornea?
it becomes white
What is the uvea?
The most vascular coat of the eyeball and lies between the sclera and retina
Where does the uvea reside?
Between the sclera and retina
What three structures comprise the uvea?
Iris
Ciliary body
Choroid
What is the function of the iris?
Controls light levels inside the eye similar to the aperture on a camera
Round opening in the centre is the pupil.
Muscles within the iris dilate and constrict the pupil size.
What is the outer capsule of the lens?
Outer acellular capsule
Which fibres comprise the core of the lens?
Regular inner elongated cell fibres- providing transparency
What term is used to describe an increase in lens opacity with age?
Cataracts
What are the four main functions of the lens?
1) Provide transparency
2) Regular structure
3) Refractive power, 1/3 of the eye focussing power (higher refractive index than aqueous fluid and vitreous)
4) Elasticity and accommodation
Which nerve transmits electrical impulses to the visual processing centres of the brain?
The optic nerve
What term describes the visual portion of the optic nerve?
Optic disc
Where is the optic blind spot located?
Where the optic nerve meets the retina, there are no light sensitive cells (Rods and Cones).
Where is the macula located?
Centre of the retina, temporal to the optic nerve
What is the macula?
A small and highly sensitive part of the retina responsible for detailed central vision
Which structure is located at the very centre of the macula?
The fovea
What is the fovea?
A small depression in the retina of the eye where visual acuity is highest. The centre of the field of vision is focused in this region, where retinal cones are particularly concentrated.
Which structure is the most sensitive part of the retina?
The fovea
Which photoreceptor cells are found at their highest concentration within the fovea?
Cone cells to perceive in detail.
What is the corresponding landmark for the physiological blind spot?
The optic disc
What is central vision?
Macular vision is responsible for detailed central fine vision and daytime colour vision.
-Reading, fascial recognition
Which photoreceptor cells are concerned with colour vision?
Cone cells
How is central vision assessed?
Visual acuity assessment
What is associated with a loss of foveal vision?
Poor visual acuity
What is peripheral vision?
Specialises in detecting shape, and movement in the environment
Night vision, navigation vision
What is associated with a loss of central vision?
Problems with reading and recognising faces
How is peripheral vision assessed?
Visual field assessment
An extensive loss of peripheral vision is associated with what?
Inability to navigate within the environment
Needs white stick despite perfect visual acuity.
Which cells comprise the outer layer of the retina?
Photoreceptors (1st order neurones)
Detection of light
Which cells comprise the middle layer of the retina?
Bipolar cells (2nd order neurones) Local signal processing to improve contrast sensitivity, regulate sensitivity.
Which cells comprise the inner retinal layer?
Retinal ganglion cells (3rd order neurone)
What is the neuroretina?
The inner thicker layer comprising of retinal ganglion cells, and photoreceptors
What is the function performed by the retinal pigment epithelium?
Transports nutrient from the choroid to the photoreceptor cells, and removes metabolic waste from the retina
How many layers form the neuroretina?
Three
1) Outer photoreceptor
2) Middle bipolar
3) Inner retinal ganglion cells
What are the two main classes of photoreceptor cells?
Rod and cone cells
Which pigment resides within the outer segment of rod cells?
Rhodopsin
Where are the mitochondria located within the photoreceptors?
Inner segments
What are rod photoreceptor cells?
100 times more sensitive to light than cones
Slow response to light
Scotopic vision
120 million rods
What are cone photoreceptor cells?
Less sensitive to light, faster response.
Responsible for day light fine vision and colour vision (photopic vision)
6 million cones
What are the outer segments comprised of?
Discs
What are the two components of rhodopsin?
Opsin and Retinal
Upon exposure to photons of light, rhodopsin exists in which form?
Trans-form upon the conversion of cis-retinal to trans-retinal
What happens to rhodopsin upon light exposure?
Photon interactions result in isomerisation of retinal from the cis to trans-form, subsequently degenerating the rhodopsin molecule into it’s constituent components, opsin and trans-retinal.
A process known as bleaching
What is the fate of deactivated photo-pigments in the outer segment?
Phagocytossed by the retinal epithelial cells
Regenerated inside the retinal epithelial cells and transported back to the photo-receptors.
What type of vision are rod photoreceptors responsible for?
Scotopic vision (night vision)
More photoreceptors –> More rhodopsin –> Higher spatial and temporal summation –> Recognises motion
Describe the density and distribution of rod photoreceptors:
Widely distributed throughout the retina, with the highest density outside the macula, progressively decreasing towards the periphery.
N.B: Absent within the macula
Where are cone photoreceptors located?
Distributed exclusively within the macula
Where can one find the highest concentration of rod photoreceptors in the retina?
20-40 degrees away from the fovea
What are the three types of cone cells?
S-cones
M-cones
L-cones
What is the peak light sensitivity of rod cells?
498nm
Which cone cells are sensitives to short wavelengths (blue)?
S-cones
Which cone cells are sensitive to medium wavelengths (green)?
M-cones
Which colour are L-cones sensitive too?
Red colour
Which cone cells are stimulated by yellow light?
M- and L-cones equally
Combination of green and red light
What is the commonest form of colour vision deficiency?
Deuteranomaly (Daltonism(
What is deuteranomaly?
Caused by the shifting of the M-cone sensitivity peak towards that of the L-cone curve, causing red-green confusion.
Cannot perceive the colour red
What is full colour blindness called?
Achromatopsia
What is the prevalence of colour vision deficit within both females and males?
8% males
0.5% females
What is anomalous trichromatism?
Colour vision deficits caused by a shift in the photo-pigment peak sensitivity.
What causes colour vision deficits?
Caused by the absence of one or more of the 3 photo-pigments.
What is monochromatism?
There is a complete absence of colour vision
What can cause monochromatism?
The presence of only blue L-cones
Normal day light visual acuity
What is rod monochromatism?
There is a total absence of all cone photo-receptors.
No functional day vision
What test is conducted to detect colour blindness?
Ishihara test
