Structure and function of the eye Flashcards
Give 3 reasons for tear production?
Basal Tears
Reflex Tears – in response to irritation: Afferent – Cornea – CN V1, Efferent – Parasympathetic, Neurotransmitter – Acetylcholine
Crying (Emotional) Tears
Describe the lacrimal system?
Tear produced by the Lacrimal Gland
Tear drains through the two puncta, opening on medial lid margin
Tear flows through the superior and the inferior canaliculi
Tear gathers in the Tear Sac
Tear exits the Tear Sac through the tear duct into the nose cavity
What is the function and structure of the tear film?
Tear film maintains smooth cornea-air surface
Oxygen Supply to Cornea – Normal cornea has no blood vessels
Removal of Debris (Tear film and Blinking)
Bactericide
• Three Layers
– Superficial Oily Layer to reduce tear film evaporation (produced by a row of Meibomian Glands along the lid margins)
– Aqueous Tear Film (Tear Gland)
– Mucinous Layer on the Corneal Surface to maintain surface wetting
Describe the conjunctiva?
The conjunctiva is the thin, transparent tissue that
covers the outer surface of the eye. It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids. It is nourished by tiny blood vessels
What are the 3 coats of the eye?
Sclera – Hard and Opaque
Choroid – Pigmented and Vascular
Retina – Neurosensory Tissue
What is the sclera?
Commonly known as “the white of the
eye,” is the tough, opaque tissue that serves as the eye’s protective outer coat. High water content
What is the cornea?
Front-most part of Anterior Segment Continuous with the Scleral Layer Transparent Refraction – 2/3 of power Convex curvature Higher refractive index than air Physical and Infection Barrier
What are the 5 layers of the cornea?
1 – Epithelium
2 – Bowman’s Membrane
3 – Stroma – regularity contributes towards transparency. Corneal nerve endings provides sensation and nutrients for healthy tissue. No blood vessels in normal cornea
4 – Descemet’s Membrane
5 – Endothelium – pumps fluid out of corneal and prevents corneal oedema, Doesn’t regenerate and dysfunction may cause cloudiness and oedema.
What is the UVEA?
Vascular coat of eye ball and lies between the sclera and retina. Uvea is composed of three parts. • Iris • ciliary body • choroid
What is the choroid?
Lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.
What is the iris?
It controls light levels inside the eye similar to the aperture on a camera.
The round opening in the centre of the iris is called the pupil.
The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil
size.
What is the structure and function of the lens?
Outer Acellular Capsule with regular inner elongated cell fibres
Transparency, Refractive Power, Accommodation
What is the Lens zonules?
Lens is suspended by a fibrous ring known as lens
zonules, consists of passive connective tissue
What is the macula?
The macula is located roughly in the centre of the retina, temporal to the optic nerve.
It is a small and highly sensitive part of the retina responsible for detailed central vision. The fovea is the very centre of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading.
What divides the anterior and posterior segments of the eye?
The lens
What is introccular fluid?
Prouduced by cilary body, flows anteriorly
into the Anterior Chamber, supplies nutrients
Trabecular Meshwork drains the fluid out of the eye
Normal Intraocular Pressure – 12-21mmHg
What can Glaucoma lead to?
Retinal Ganglion Cell Death and Enlarged Optic Disc Cupping
What are the two types of glaucoma?
Primary Open Angle Glaucoma due to trabecular Meshwork dysfunction
Closed Angle Glaucoma
• Increased pressure pushing the iris/lens complex forwards, blocking the trabecular meshwork – vicious cycle
• Risk factors - small eye (hypermetropia), narrow angle at trabecular meshwork
• May present with sudden painful red eye with acute drop in vision
• Can be treated with peripheral laser iridotomy to create a
What are the two types of vision?
Central Vision
Detail Day Vision, Colour Vision – Fovea has the highest concentration of cone photoreceptors. Assessed by Visual Acuity Assessment
Loss of Foveal Vision – Poor visual acuity
Peripheral Vision
Shape, Movement, Night Vision, Navigation Vision
Assessed by Visual Field Assessment
Extensive loss of Visual Field – unable to navigate in environment,
What are the 3 layers of the retina?
Outer Layer – Photoreceptors (1st Order Neuron) – Detection of Light
Middle Layer – Bipolar Cells (2nd Order Neurons) – Local Signal Processing to improve contrast sensitivity, regulate sensitivity
Inner Layer – Retinal Ganglion Cells (3rd Order Neurons) – Transmission of Signal from the Eye to the Brain
What distinguishes the fovea?
Fovea forms the pit at the centre of the macula due to absence of the overlying ganglion cell layer
Fovea has the highest concentration of photoreceptors for fine vision
What are the two types of photoreceptors?
Rod Photoreceptor
Longer outer segment with photo-sensitive pigment
100 times more sensitive to light than cones
Slow response to light
Responsible for night vision
120million rods
Cone Photoreceptor
Less sensitive to light, but faster response
Responsible for day light fine vision and colour vision
6 million cones
What is the relationship between the two visons and the two types of photoreceptors?
Central and day vision is mainly cone vision whereas peripheral and night vision is mainly rod vision
Describe the distribution of rod and cone cells?
Rods are distributed quite well all around the eye but are more numerous as you go closer to the optic nerve. Cones are practically non existent except in the centre.
What is the test for colour bindness?
Istihara test
Describe light dark adaption?
Dark Adaptation Increase in light sensitivity in dark Biphasic Process • Cone adaptation 7 minutes • Rod adaptation 30 minutes – regeneration of rhodopsin
Light Adaptation
Adaptation from dark to light, Occurs over 5 minutes, Bleaching of photo-pigments, Inhibition of Rod/Cone function
Pupil Adaptation (minor) Constriction of pupil with light
Define emmetropia?
Adequate correlation between axial length
and refractive power. Parallel light rays fall on the retina (no accommodation)
Define ametropia and what are the 4 types?
Mismatch between axial length and refractive power Parallel light rays don’t fall on the retina (no accommodation) – Nearsightedness (Myopia) – Farsightedness (Hyperopia) – Astigmatism – Presbyopia
Describe myopia?
Parallel rays converge at a focal point anterior to the retina
Causes
– excessive long globe (axial myopia) : more common
– excessive refractive power (refractive myopia)
Blurred distance vision
Squint in an attempt to improve uncorrected
visual acuity when gazing into the distance
Headache
Describe hyperopia?
Parallel rays converge at a focal point posterior to the retina
Causes
– excessive short globe (axial hyperopia) : more common
– insufficient refractive power (refractive hyperopia)
Visual acuity at near tends to blur relatively early,
asthenopic symptoms: eyepain, headache in frontal
region, burning sensation in the eyes,
blepharoconjunctivitis
Describe astigmatism?
Parallel rays come to focus in 2 focal lines rather than a single
focal point
Cause : refractive media is not spherical–>refract differently along one meridian than along meridian perpendicular to it–>2 focal points
Asthenopic symptoms ( headache , eyepain) blurred vision, distortion of vision, head tilting and turning
Treated with cylinder lens with or without concave or convex lens. may need surgey.
How do you adapt for near vision?
Pupillary Miosis (Sphincter Pupillae) to increase depth
of field
Convergence (medial rectus from both eyes) to align
both eyes towards a near object
Accommodation (Circular Ciliary Muscle) to increase
the refractive power of lens for near vision
Describe presbyopia?
Naturally occurring loss of accommodation (focus for near objects)
Onset from age 40 years
Distant vision intact
Corrected by reading glasses (convex lenses) to increase refractive power of the eye
What are the 4 types of optical corrections?
Spectacle lenses, contact lenses, intraocular lenses, surgical correction
