Structure and function of the eye part 2 Flashcards

Question

Why is the blind spot undetected with one eye closed

Answer 1

When the world is viewed monocularly, the visual system simply fills in the missing part of the scene. To observe filling in, simply notice what happens when the pencil lies across the optic disk representation, at about 5-8 degrees in diameter, remarkably, it looks complete.

Answer 2

Your fovea is the most sensitive part of the retina. § It has the highest concentration of cones, but a low concentration of rods. § This is why stars out of the corner of your eye are brighter than when you look at the directly. § But only your fovea has the concentration of cones to perceive in detail But when you look at it directly- it will have greater definition, but a lower light sensitivity

Answer 3

Look at the star (in the centre) and try to read the letters (in the periphery) The letters will appear blurry as you are focussing on the star- due to the fovea

Answer 4

Lateral/Medial Canthus: corners of the eye; caruncle present in medial canthus Puncta: medial openings that drain fluid into the lacrimal canaliculi to the tear sac

Answer 5

The cornea is very sensitive and it is innervated by the ophthalmic branch of the trigeminal nerve (CN V)

Answer 6

``` – Detail Day Vision, Colour Vision –Fovea has the highest concentration of cone photoreceptors – Reading, Facial Recognition – Assessed by Visual Acuity Assessment – Loss of FovealVision –Poor visual acuity ```

Answer 7

``` – 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 8

Patient may lose central vision and thus have impaired visual acuity- but their peripheral vision may be fine- allowing them to navigate themselves around the room easily. Conversely, the patient's central vision may be very good but their peripheral vision is impaired, meaning that they need a walking stick to navigate the room.

Answer 9

Away from the pupil

Answer 10

Incoming light travels through the retina, then bounces back on the pigment epithelium and then hits the pigment epithelium. Photons that hit the centre will be reflected back to photoreceptors in the centre, photons that hit the curves will reflect back onto the photoreceptors at the side- allowing us to see where the objects are in space (hit different portions of the retina).

Answer 11

Dark- so absorbs a lot of light- so inside the eye not much refraction or reflection of light- also the reason why pupil is black- only when you shine a direct light into pupil and see the light coming back out- red reflex.

Answer 12

Once the light hits the photoreceptor it is not reflected anymore and so cannot come back out of the eye

Answer 13

Outer Layer –Photoreceptors (1st Order Neuron) –Detection of Light Middle Layer –Bipolar Cells (2ndOrder 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 Outer means away from the centre of the eye Inner- means towards the centre of the eye

Answer 14

– Macula –Macula Lutea (yellow patch), pigmented region at the centre of the retina of about 6 mm in diameter – 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

Answer 15

Similar to ultrasound Shine light through pupil Receive the light that is reflected back- will be reflected differently by different structures- some light waves will be in phase, others out of phase, computer uses this to compute and image of the retina in layers (light, dark, light, dark)- clinically used to assess the macula and foveal pit- can look for build up of fluid.

Answer 16

· Longer outer segment with photo-sensitive pigments · 100 times more sensitive to light than cones · Slow response to light · Responsible for night vision (scotopic vision) · 120 million rods At the lowest level of illuminations, only the rods are activated (scotopic vision). The difficulty of making fine visual discriminations under very low light is a common experience. The problem is primarily the poor resolution of the rod system (and to a lesser extent, the fact that there is no perception of color because in dim light there is no significant involvement of the cones). Although cones begin to contribute to visual perception at about the level of the starlight, spatial discrimination at light level is still poor Why we can see surroundigs relatively well at night compared to what we are actually looking at (cones in fovea not activated at night)

Answer 17

· Less sensitive to light · Faster response to light · Responsible for daylight vision and vision and colour vision (photopic vision) · 6 million cones

Answer 18

450-680nm | Blue/violet- dark red

Answer 19

Rods have the highest density just outside the macula They decrease in density the further you move away from the macula There are NO rods in the macula Cones are ONLY found in the macula The highest density of cones is in the fovea see graph of number of receptors per square metre against the angle relative to the fovea

Answer 20

``` • Scotopic(Rod Vision) Vision -Peripheral and Night Vision (More Photoreceptors, More Pigment, Higher Spatial and Time Summation) – Peripheral and Night Vision – Recognizes motion ```

Answer 21

Photopic(Cone Vision) Vision –Central and Day Vision – Central and Day Vision – Recognizes colour and details

Answer 22

–D) 20-40 degrees away from Fovea

Answer 23

Perceiving colour allows humans and many other animals to discriminate objects on the basis of distribution of the wavelengths of light that they reflect to the eye. While differences in luminance are often sufficient to distinguish objects, colour adds another perceptual dimension that is extremely useful when differences in light intensity are subtle or non-existent. Colour vision is a the result of the special properties of the cone system.

Answer 24

Unlike rods, which contain a single photopigment, the types of cones are defined by the photopigment they contain. Each photopigment is differentially sensitive to light of different wavelengths, and for this reason cones are referred to as blue, green, and red or, more appropriately, (S)-SHORT, (M)-MEDIUM, (L)- LONG wavelength cones- to describe their spectral sensitivities.

Answer 25

The nomenclature is misleading- as it seems to imply that the cones provide colour information for the wavelength of light that excites them best. In fact, individual cones, like rods, are entirely colour-blind in that their response is simply a reflection of the number of photons they capture, regardless of the wavelength of the photon (or more properly, its vibrational energy), It is impossible, therefore, to determine an extracts colour information from spectral stimuli.

Answer 26

S-cone – short wavelength – BLUE M-cone – medium wavelength – GREEN L-cone – long wavelength – RED Can use lasers to look into eye and even the brain (intrinsic imaging) Use longer wavelengths the further you want to penetrate

Answer 27

Colour Perception Test • Ishihara Isochromatic Plates can test for red-green deficiencies only • Consists of plates of circle of dots appearing randomly in size • Subjects with normal red green vision will recognize the correct pattern in the form of a 2-digit number • Patients with colou rvision deficiencies will not recognize any pattern or recognize the wrong pattern

Answer 28

Trichromatic, based on the relative levels of activity in the three sets of cones that have different absorption sepectra. The trichromatic nature of colour vision is supported by perceptual stuidies showing that any colour stimulus can be matched to a second stimulus composed of three superimposed light sources (short, medium and long wavelength).

Answer 29

Only two bandwidths of light are needed to match all the colors that can be perceived. Two most prevalent forms are: protanopia- impairment in perception of long wavelengths deuteranopia- impairment in perception of medium wavelengths Both have difficulties in the perception of red and green, and for this reason dichromacy is often called red-green colour blindness.

Answer 30

Impaired perception of short wavelengths, a condition commonly called blue-yellow colour blindness.

Answer 31

The majority of individuals with colour vision deficiencies Three light sources are needed to make all possible colour matches, but the matches are made using intensity values different to those used by most individuals. Protonamolous trichromats- higher intensity longer wavelength to stimulation to make colour matches Deuteranomalous trichromats- higher intesnsity medium wavelength stimulation to make colour matches Tritanomalous trichromats- shorter wavelength

Answer 32

Deuteranomaly: green-weak; sensitivity of green (M) cones shifted towards red (2.7% Deuteranopia: green-blind; all green sensitive (M) cones are missing Protanomaly: red-weak; red (L) cones present but have a mutation Protanopia: red-blind; all red sensitive (L) cones are missing Tritanomaly: blue-weak; blue (S) cones present but have a mutation Tritanopia: blue-blind; all blue sensitive (S) cones are missing Achromatoplasia: no colour vision whatsoever (<0.0001%) Decrease in prevalence as you go down

Answer 33

Can still distinguish between orange (dark grey) and green (light grey)

Answer 34

Anomalous trichomatism

Answer 35

Dark Adaptation – Increase in light sensitivity in dark – Biphasic Process • Cone adaptation 7 minutes • Rod adaptation 30 minutes – regeneration of rhodopsin (photopigment in rods, GPCR system)- rods higher sensitivity to light- line flat after 30 mins- no need to dark adapt more Eyes always see-even when shut- seeing black- no photons hitting the retina- photoreceptor still works and will always see 24/7. Photoreceptors in outer part of retina- highest metabolic demand- why it is susceptible to damage- and insult

Answer 36

``` – Adaptation from dark to light – Occurs over 5 minutes – Bleaching of photo-pigments – Neuro-adaptation – Inhibition of Rod/Cone function ```

Answer 37

Light sensitivity decrease in dark adaptation This suppression of light sensitivity is caused by photopigment bleaching and neuro-adaptation inhibiting rod and cone function

Answer 38

• Pupil Adaptation (minor) – Constriction of pupil with light

Answer 39

red-green confusion

Answer 40

Refraction: light passing from one medium to another change velocity and hence angle (unless perpendicular)

Answer 41

n= speed of light in a vacuum/ speed of light in the medium n=c/Vm The denominator in this case will ALWAYS be smaller and produce a unitlessvalue greater or equal to 1. This value is called the new medium’s INDEX OF REFRACTION, n. Need to assume same humidity, temperature and pollution levels of air (vacuum) All substances have an index of refraction and can be used to identify the material.

Answer 42

The cause of this is due to the fact that light BENDS when it reaches a new medium. The object is NOT directly in a straight line path, but rather it’s image appears that way. The actual object is on either side of the image you are viewing. Refraction Fact #2:As light goes from one medium to another, the path CHANGES!

Answer 43

Some of the light REFLECTS off the boundary and some of the light REFRACTS through the boundary. Angle of incidence = Angle of Reflection Angle of Incidence > or < the Angle of refraction depending on the direction of the light

Answer 44

A converging lens (Convex) takes light rays and bring them to a point Parallel rays come into lens- rays focussed onto a focal point (distance between centre of lens and focal point is the focal length)

Answer 45

A diverging lens (concave) takes light rays and spreads them outward.

Answer 46

A camera uses a lens to focus an image on photographic film. | Lens in the eye does the same to focus an image on the retina

Answer 47

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

Answer 48

Essentially a refractive error of the lens: • 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 49

• 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) Lens or cornea too convex

Answer 50

–Blurred distance vision –Squint in an attempt to improve uncorrected visual acuity when gazing into the distance - squint to apply stenopeic (or pinhole glasses) hole- try to get light light coming straight- doesn't get refracted and so will land on retina- no need for refractive power of lens- can see clearly –Headache Can see nearby object fine- rays are diverging upon entering the lens and not parallel- and so the focal point is shifted backwards

Answer 51

Divergent glasses- concave lenses- to push focal point onto retina Refractive surgery onto cornea- to flatten central bit of cornea (rather than dome shaped) Cataract operation to remove lens without replacing it- -24 short-sighted ranking, lens 24 refractive power- so removing the lens will correct the short-sightedness- but it can damage structures in the eye

Answer 52

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) Essentially, lens or cornea is flatter than normal In certain degrees of hyperopia, accommodation can compensate and bring the focus onto the retina

Answer 53

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 is tired, printing is weak or light inadequate –asthenopic symptoms: eye pain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis

Answer 54

–Amblyopia –uncorrected hyperopia > 5D (dioptres- ranking of long-sightedness) One eye emmetropic, other eye hyperopic- brain learns (continues to develop to age of 4/5)- to ignore images from the 'bad' eye- so the other eye becomes amblyopic (lazy) Hazardous- no physiological use for this eye- may not be able to see things in path Objects close blurrier than further away objects- but blurry whether long or short distance. Irreversible process

Answer 55

Same as hyopia

Answer 56

Spherical- with arches (projections) that are equal in diameter Elliptical shape that is horizontally orientated.

Answer 57

* 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)- object can land in front of or behind the retina

Answer 58

Astigmatism: parallel rays focus in 2 focal lines rather than a focal point - due to non-spherical refractive media, so refract differently along one meridan compared to that perpendicular to it; treated with cylindrical lens (width and height not same) Diameter of cornea not the same- arches unequal in diameter

Answer 59

• Symptoms –asthenopic symptoms ( headache , eyepain) –blurred vision –distortion of vision –head tilting and turning confusion seeing images as comets during night base of cornea not round- one meridian different length to other

Answer 60

–Regular astigmatism :cylinder lenses with or without spherical lenses(convex or concave), Sx –Irregular astigmatism : rigid CL , surgery Meridians in vertical plane of cylinder- straight through it Meridians on side of cyclinder (which will be concave or convex)- divereged or converged- rotate cylindrical lens until steep meridian is brought into focus

Answer 61

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 62

• 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 Lens becomes stiffer

Answer 63

``` –convex lenses in near vision • Reading glasses • Bifocal glasses • Trifocal glasses • Progressive power glasses ``` Bifocals have a lens that is divided into two parts. The lower part is used to view things near to the person’s eyes whereas the upper part is used to view things that are distant. Trifocals contain a lens that has an additional region apart from the two that are already present in bifocals

Answer 64

–Monofocal lenses : spherical lenses , cylindrical lenses –Multifocal lenses

Answer 65

–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 –disadvantages : careful daily cleaning and disinfection , expense –complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis

Answer 66

–replacement of cataract crystalline lens –give best optical correction for aphakia , avoid significant magnification and distortion caused by spectacle lenses

Answer 67

–Keratorefractive surgery :RK, AK, PRK, LASIK, ICR, thermokeratoplasty –Intraocular surgery : clear lens extraction (with or without IOL), phakic IO Laser to remove epithelium of cornea- open flap- change shape of cornea- close flap

Answer 68

* IOL: Intra ocular lens. * Same as cataract extraction. * Implantation of artificial lens. * Lose accommodation (patient will need reading glasses).

Answer 69

• Accommodation Mechanism – 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

Answer 70

Relaxation of the ciliary circular muscle

Answer 71

At lower levels of illumination But cones have a more effective adaptation method- seen in time course in response of cones in response to light flashes- the response of a cone (even to a bright light flash)- produces the maximum change in photoreceptor current, recovers in about 200ms, more than four times faster than that of rod- response of individual cone does not saturate at high levels of steady stimulation , as the rod response does (but rods still adapt over a range of luminence values) Rods produces a reliable response at 1 photon of light, whereas more than 100 photons of light are required to produce comparable responses in cones.