Structure And Function Of The Eye

Question 1

Label the basic anatomy of the eye and describe the 3 layers of the coat of the eyes.

Answer 1

Basic anatomy: pupil, iris, sclera, medial canthus, casoncle, limbus, lateral canthus, paspetral fissure, optic nerve, optic disc, vitreous, lens, Aqueous, cornea, ciliary body, retina and choroid.

Three layers of the eye coat:

1. Sclera - hard and opaque, protects the eye and maintains shape
2. Choroid - pigmented and vascularised, provers circulation to the eye and shields out unwanted scattered light
3. Retina - neurosensory tissue, converts light into neurological impulses to be transmitted to the brain via optic nerve

Question 2

Describe the lacrimal systems and types of tears

Answer 2

The lacrimal gland is located within the orbit, lateral-superior to the globe. It produces three types of tears:

1. Basal tears - produced at constant levels (even without irritation)
2. Reflex tears - in presence of irritation (made up of afferent pathway)
3. Emotional tears - crying

Tears drain through the puncta opening on the medial lid margin, tear flows through the superior and inferior canaliculi, tear gathers in the tear sac, exits through the tear duct into the nasal cavity. The cornea is most sensitive tissue in the body - innervated by the sensory fibres of ophthalmic branch of CNV

Question 3

Summarise the structure and role of the tear film

Answer 3

Tear film maintains the smooth cornea air surface, oxygen to supply cornea (since it’s got no blood vessels). The tear film is important to maintain clear vision and remove surface debris during blinking (bacteriacide)

Layers of tear film:

1. Superficial oily layer: reduces tear film evaporation and produced by meribomian glands along lid margin
2. Aqueous tear film: maintain bulk - delivers oxygen and nutrients - controls bactericide
3. Muscinous layer: maintains surface setting and ensures the tear film sticks to eye vasculture, the mucin molecules act by binding to water molecules to they hydrophobic corneal epithelial

The conjunctiva is the transparent layer on cornea - very vascularised and has goblet cells that produce mucin.

Question 4

Define the following: cornea, uvea, iris, lens and retina

Answer 4

Cornea: transparent, dome shaped window covering eye - powerful refracting surface, providing 2/3 of focal power of the eye. Low water content. 5 layers (epithelium, bowmanns membrane (Basement), storma, descements membrane (basal) and endothelium)

Uvea: vascular cost of the eyeball and lies between sclera and retina, consists of iris, ciliary body and choroid.

Iris: coloured part of eye, controls light levels inside the eye, the round opening in centre - pupil.

Lens: made up of outer acellular capsule, regular inner elongated cell fibre p, transparency. It’s main function is transparency has 1/3 refractive power, the lens can shape so focal point and refractive index changes. Lens is suspended by fibrous ring (lens zonules).

Retina: very thin layer of tissue that lines the inner part of the eye, responsible for capturing the light rays.

Question 5

Define the macular and segments of eye:

Answer 5

Macular: in the centre of the retina, temporal to the optic nerve, small highly sensitive and responsible for detailed centred vision. The fovea is at the centre of the macular, the fovea - highest conc photoreceptors (1:1 ganglion to receptor p).

Anterior segment of eye: ocular structure anterior to the lens = aqueous humour. Aqueous fluid produced by ciliary body and passes into anterior chamber and then out through the angle via trebecukar (drainage - maintains introcular pressure)

Posterior segment: located between lens and retina, vitreous humour (composed of 99% water trapped in jelly matrix). The jelly substance provides mechanical support to the eye (some collagen and GAG chains). As you hey older you lose the jelly composure and detaches from retina.

Question 6

Describe the effect of glaucoma and its types

Answer 6

Conduit of sustained raised intraocular pressure, results in gradual and accumulative damage to the optic nerve tissue > this is the retinal ganglion cell death and enlarged optic disc cupping. Patients with glaucoma lose pupillary Vision gradually and eventually blind.

1. Primary open angle glaucoma - caused by functional blockage of the trabecular mesh work
2. Closed angle glaucoma - forward displacement of the iris and lens complex, narrowing the intratecular mesh work

Question 7

Compare the difference between central and peripheral vision

Answer 7

Central - detail day vision, colour vision - fovea has the highest concentration of cone photoreceptors, reading and facial recognition assessed by visual acuity assessment.

Peripheral - shape, movement, night vision, navigation vision and assessed by field assessment.

Question 8

Describe the retinal structure

Answer 8

Consists of an outer layer - retinal pigmented epithelium, immediately in front of the choroid and inner thicker layer called neuro retina. Transports nutrients and semures debris.

Divisions:

1. Outer layer - photoreceptors (rods and cons)
2. Middle layer - bipolar cells (because their axons project in both directions)
3. Inner layer - retinal ganglion cells (transmission of signal from the eye to the brain)

Question 9

Describe the role of visual processing

Answer 9

Photoreceptors has two main classes:

1. Rod photoreceptors: layer outer segment with photosensitive pigment, slow response to light, responsible for night vision
2. Cone photoreceptors: less sensitive to light, but faster response, responsible for day light fine vision and colour vision.

The photoreceptor distribution: look at slide

Question 10

Summarise the colour pathways and its defect

Answer 10

Colour vision deficiency: tested by ishihara test - plates of circles different class and forms 2 digits.

Light - dark adaptations: dark adaptations increase in light sensitivity in dark, light adaptations - adapts from dark to light occurs over 5 mins, bleaching of photo-pigments.

Frequency spectrum - red green blindness

Question 11

Define the following: emmetropia, ametropia, myopia, hyperopia, astigmatism.

Answer 11

Emmetropia: normal vision.

Ametropia: mismatch between axial length and refractive power, parallel rays don’t fall on retina, no accommodation.

Myopia : short sightedness.

Hyperopia: long sighted ness

Astigmatism: parallel rays come to focal in 2 focal lines rather than a single point (hereditary) one of the layers of the cornea a bit too thin, blurry and confusion.

Question 12

Describe the typical optical corrections

Answer 12

Spectacle lens: mono focal, spherical, cylindracle, multifocal

Contact lens: ? Revise that specific lecture

Intraocular lenses: ? Revise that specific lecture

Surgical corrections: ? Revise that specific lecture

Accomodation: ? Revise that specific lecture