Vision 4: Refractive Media of the Eye Flashcards
Optical Power of the eye
67 Diopters
Optical Power of the Cornea
45 Diopters
Optical Power of the lens
- Rest: 20 Diopters
- Accomodated: 34 Diopters
Importances of Cornea
Fixed Powerful convexity:
* high curvature makes it responsible for 2/3 of optical power of the eye
Formation of Sharp retinal Image: Perfectly regular curvature
Protective function:
* Superficial reflex: Afferent= Ophthalmic div. of Trigemnal N, Efferent=Facial (bilateral), blinking of both eyes
* Absorption of some UV rays
Corneal Opacity treated by
Corneal graft
Properties of the lens
- Avascular
- Biconvex
- Transparent
Describe structure of the lens
General:
* Highly condensed proteins
* Cortex is made up of young fibers
* center (nucleus) made up of old fibers
* old fibers have higher power than young fibers
At Rest:
* It’s more flat, for farsightedness (20 D)
* The posterior is more convex than the anterior
during Accomodation:
* More convex with higher power (34 D)
Describe metabolism of the lens
- Low metabolism
- Mainly Anaerobic
- Aerobic at some level, with the presence of Glutathione (anti-oxidant)
Causes of transparency of the lens
- Avascular
- Non-innervated
- Uniform arrangement of fibers that are densly packed together
Definition and causes of Cataract
Loss of the lens transparency by:
1. Glutathione deficiency
2. Denaturation/Coagulation of lens proteins
3. Senile Cataract (most common): DM, frequent exposure to UV rays, increase temperature
Treatement for Cataract
Surgical treatement
Definiton of Accomodation
Increasing Dioptric Power of The eye Lens by increasing its Curvature (convexity), for Near vison
Mechanism of Radical Muscle fibers of Cillary muscles in accomodation
- Extend from Corneo-scleral Junction all the way to the choroids
- Their insertion is behind the suspensory ligament’s insertion
- So their contraction causes pulling of the Ligament’s Insertion foreward
- Causing the ligaments to relax, and the lens to be more convex
Mechanism of Circular Muscle fibers of Cillary muscles in accomodation
- Surround the lens in ring fashion, thus acting as a Sphincter
- when contracting, they create a smaller ring , which release the tension in the ligaments, causing the lens to be more convex
Accomodation mainly affects …… surface of the lens
Anterior surface
State changes that occur during Near response
- Accommodation: to increases Dioptric power of the lens
- Converegence of both eyes: by Medial recti muscles to focus the light waves onto fovea centralis of the retina
- Myosis (pupil constriction): by constrictor pupillae Muscle, to guarantee that light waves pass through Nucleus (old fibers) of the lens
Define Power of Accommodation
differences between the 2 powers of lens during Resting and Accommodation , it decreases by age due to decrease in elasticity
Define the Near point
It’s the Shortest distance from the eye, where person can see clearly using maximum accommodation
* Young adult= 10cm
* 60 year old= 80 cm
Define far point
the furthest point from the eye, which can be seen clearly without any accommodation
=6m (unchanged)
Range of Accommodation
Distance between far point and near point, which decreases by age
Enumerate types of errors of Refraction of the eye
- Myopia (Near sightedness)
- Hypermetropia (far sightedness)
- prespyopia
- Astigmatism
Causes, optical changes & correction for Myopia
Cause
* Longer eyeball than usual
* abnormally High curvature of cornea or lens
Optical changes:
* parallel rays: Convergence of light in front of retina
* Near Point: less than 10 cm
* far point: less than 6m
* Accommodation: Aggravate (worsens) the myopia
Correction: concave lens
Causes, Optical Changes & Correction for Hypermetropia (farsightedness)
Cause:
* Shorter Eyeball
* Low curvature (convexity) of lens or cornea
Optical Changes
* Parallel rays: light converges behind the retina
* Near point: Increases (more than 10cm)
* Far point: Normal (but under continuous accommodation/ continuous headache)
* Accommodation: Essential non stop
Correction: convex lens
Causes, Optical Changes & Correction for prespyopia
Cause: decrease in power of lens due to decrease in elasticity by age
Optical Changes:
* Near point: increases (moves away)
* Far point: unchanged
Correction: convex lens
Causes, Optical Changes & Correction for Astigmatism
Causes: Inequality of Corneal curvature
Optical changes:
* Power of lens: different at different axis, causing the person to try and focus vison on either horizontal plane or vertical plane
Correction: Cylindrical lens
