Structure and function of the eye Flashcards

Question

Describe what can be seen through an ophthalmoscope

Answer 1

See diagram

Answer 2

Where the optic nerve meets the retina there are no | light sensitive cells. It is a blind spot.

Answer 3

- Your fovea is the most sensitive part of the retina. - It has the highest concentration of cones (colour), but a low concentration of rods. - Fovea has the highest concentration of photoreceptors for fine vision - This is why stars out of the corner of your eye are brighter than when you look at it directly. - But only your fovea has the concentration of cones to perceive in detail. - Fovea forms the pit at the centre of the macula due to absence of the overlying ganglion cell layer

Answer 4

Central Vision (Cones) – Detail Day Vision, Colour Vision – Fovea has the highest concentration of cone photoreceptors – Reading, Facial Recognition – Assessed by Visual Acuity Assessment – Loss of Foveal Vision – Poor visual acuity Peripheral Vision (Rods) – Shape, Movement, Night Vision – Navigation Vision – Assessed by Visual Field Assessment – Extensive loss of Visual Field – unable to navigate in environment, patient may need white stick even with perfect visual acuity

Answer 5

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 See slides

Answer 6

Rod Photoreceptor • More sensitive to light than cones • Responsible for night vision (Scotopic Vision - Peripheral and Night Vision. Recognises motion) Cone Photoreceptor • Less sensitive to light, but faster response • Responsible for day light fine vision and colour vision (Photopic Vision - Central and Day Vision. Recognises colour and details) Photoreceptors have different frequencies

Answer 7

The highest concentration of rod photoreceptors lie 20-40 degrees away from fovea See graph

Answer 8

Ishihara test - can test for only red-green deficiencies only. The most common form of colour deficiency is red-green confusion. Patients with colour vision deficiencies will not recognise any pattern or recognise the wrong pattern

Answer 9

``` 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 – Neuro-adaptation – Inhibition of Rod/Cone function ``` ``` Pupil Adaptation (minor) – Constriction of pupil with light ```

Answer 10

Convex lens - converging lens, always have focal point after the lens Concave lens - diverging lens, always have focal point before the lens

Answer 11

Adequate correlation between axial length and refractive power Parallel light rays fall on the retina (no accommodation)

Answer 12

Mismatch between axial length and refractive power ``` Parallel light rays don’t fall on the retina (no accommodation) – Nearsightedness (Myopia) – Farsightedness (Hyperopia) – Astigmatism – Presbyopia ```

Answer 13

Parallel rays converge at a focal point anterior to the retina Etiology : not clear , genetic factor Causes – excessive long globe (axial myopia) : more common – excessive refractive power (refractive myopia) Treatment - you need a concave lens Symptoms – Blurred distance vision – Squint in an attempt to improve uncorrected visual acuity when gazing into the distance – Headache`

Answer 14

Parallel rays converge at a focal point posterior to the retina Etiology : not clear , inherited Causes – excessive short globe (axial hyperopia) : more common – insufficient refractive power (refractive hyperopia) Treatment - You need a convex lens - surgery Symptoms – visual acuity at near tends to blur relatively early • nature of blur is vary from inability to read fine print to near vision is clear but suddenly and intermittently blur • blurred vision is more noticeable if person istired , printing is weak or light inadequate – asthenopic symptoms: eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis – Amblyopia – uncorrected hyperopia > 5D

Answer 15

Parallel rays come to focus in 2 focal lines rather than a single focal point Etiology : heredity Cause : refractive media is not spherical-->refract differently along one meridian than along meridian perpendicular to it-- >2 focal points (punctiform object is represent as 2 sharply defined lines) ``` Symptoms – asthenopic symptoms ( headache , eyepain) – blurred vision – distortion of vision – head tilting and turning ``` Treatment – Regular astigmatism :cylinder lenses with or without spherical lenses(convex or concave), Sx – Irregular astigmatism : rigid CL , surgery LOOK UP

Answer 16

Adaptation for Near Vision ``` Near Response Triad – Pupillary Miosis (Sphincter Pupillae) to increase depth of field – Convergence (medial recti 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 ```

Answer 17

``` • 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 ``` ``` Treatment – convex lenses in near vision • Reading glasses • Bifocal glasses • Trifocal glasses • Progressive power glasses ```

Answer 18

Spectacle lenses – Monofocal lenses : spherical lenses , cylindrical lenses – Multifocal lenses Contact lenses – higher quality of optical image and less influence on the size of retinal image than spectacle lenses – indication : cosmetic , athletic activities , occupational , irregular corneal astigmatism , high anisometropia , corneal disease Contact lenses – disadvantages : careful daily cleaning and disinfection , expense – complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis Intraocular lenses – replacement of cataract crystalline lens – give best optical correction for aphakia , avoid significant magnification and distortion caused by spectacle lenses Surgical correction – Keratorefractive surgery :RK, AK, PRK, LASIK, ICR, thermokeratoplasty – Intraocular surgery : clear lens extraction (with or without IOL), phakic IOL

Answer 19

– Contraction of the Circular Ciliary Muscle inside the Ciliary Body – This relaxes the zonules that are normally stretched between the ciliary body attachment and the lens capsule attachment – Note that zonules are passive elastic bands with no active contractile muscle – In the absence of zonular tension, the lens returns to its natural convex shape due to its innate elasticity – This increases the refractive power of the lens Mediated by the efferent Third Cranial Nerve