Refractory Media Of Eye S4 March 10Presentation Transcript
VISION; Refractory Media of the Eye Dr. Nisreen Abo-elmaaty Physiology Department
Visible light (of wavelength ranging from 400-700 nm) only can excite photo-receptors
Infrared rays can not excite, felt only as heat ,Ultraviolet rays are filtered out by the lens of the eye
It is the bending of light rays when they travel in 2 transparent media with 2 different refractive indices, provided that they travel at angulated interface.
Refractive Index (RI) of a medium
= velocity of light in air
velocity of light in that medium
e.g. RI of cornea is 1.38 means
= velocity of light in air / velocity of light in cornea
Parallel rays (coming from distance > 6 m) will be bent toward edges of a convex lens.
At certain curvature of the lens, these rays will pass through a single point; Focal Point.
Focal Length: is the distance between central part of lens & Focal point.
Focal Point Focal Length Dioptric power of a lens is a measure of its ability to refract light rays. Refractive power = 1 meter / focal length = ? Dioptre. Thus, Dioptric power is inversely related to focal length
The Eye Ball
24 mm in diameter
3 layers + intraocular fluids & chambers
Ocular Chambers & Fluids
The lens divides the ocular cavity into 2 chambers filled with intraocular fluids :
Posterior to lens
Filled with Vitreous humour (clear jelly-like fluid)
Anterior to lens
Divided by the iris into: anterior & posterior chambers.
Filled with Aqueous humour (clear watery fluid)
Vitreous Chamber Aqueous Chamber
Clear transparent fluid filling ant. & post. chambers.
Formed by the epithelial lining of ciliary processes of ciliary body at a rate of 1-3 μ l /min.
Relative to plasma , Aqueous H is:
- alkaline, nearly protein-free fluid
- Higher Na + & HCO 3 content
- Higher vit. C, pyruvic & lactic acids
- Lower glucose
Circulation & Drainage of Aqueous humor
Aq.H from ciliary processes -> flows between suspensory ligaments -> post.chamber -> through pupil -> ant. Chamber -> through iridocorneal angle (filtration angle) ->space of Fontana -> canal of Schlemm ->aqueous veins -> episcleral veins -> systemic veins.
Functions of Aq.H
One of refractory media of the eye
Supply of O 2 & nutrients for avascular cornea & lens.
Buffering & removal of acid products of anaerobic metabolism of cornea & lens.
Keeping intraocular pressure (IOP) constant by the balance between its formation & drainage.
Transparent gelatinous mass held together by fibrillar network. (little flow of fluid; Vitreous body).
Filling the space between lens & retina.
Separated from lens by a very narrow Retrolental space.
Enclosed in a thin hyaloid membrane , firmly adherent to retina around optic disc.
- one of the refractive media of the eye.
- support of intracocular structures (lens & retina).
- Maintenance of spherical shape of eye.
The refractive power of cornea is mainly due to its anterior
The Lens: half of bending occurs at its ant. surface & half at its posterior surface.
Refractive Media of the Eye 1.34 Vitreous Humour +20 Dioptres 1.40 Lens 1.33 Aqueous Humour +39 Dioptres 1.38 Cornea 1 Air Refractive power Refractive Index (RI) Medium
The Optical System of the Eye The divergent light rays must pass through an optical system that brings them back into focus. The cornea & lens is the optical system of the eye that focus light rays onto the retina
It is the transparent anterior 1/6 of outer layer.
Its thickness is 1mm at the periphery & 0.5 mm at centre.
Richly supplied by sensory nerve endings (branches of ophthalmic division of V cranial n).
The cornea itself is avascular, but the corneoscleral junction is richly supplied by capillaries.
Derives its nutrition from aqueous humour (glucose), tear film (O 2 ) & corneoscleral capillaries.
Functions of Cornea
1- The most powerful refractive medium of eye; 39 dioptres (2/3 of refractive power of eye) -> formation of sharp clear image.
2- Protective for sensitive intraocular structures:
- absorbs UV rays.
- initiator of Corneal reflex which is a protective reflex.
What are the causes of Corneal Transparency?
1- Anatomical Structure
Regular arrangement of collagen fibres of substantia propria & of corneal epithelium.
Non-myelinated nerve fibres running parallel to collagen fibres.
2- Mild degree of corneal dehydration
Achieved by an active endothelial pump -> maintenance of corneal fibres packed together.
Abnormalities of Cornea
1. Corneal Opacity;
- is partial loss of corneal transparency.
- is a complication of corneal ulcer.
- ttt: corneal grafting.
- the corneal curvature is not equal in all
- ttt: cylinderical or contact lens.
3. Keratoconus (conical cornea);
- congenital condition characterised by
conical protrusion of cornea.
- usually bilateral & appears at puberty.
- ttt: contact lens or corneal grafting.
Is a biconvex lens composed of a strong elastic capsule filled with viscous transparent proteinaceous fluid.
Lens diameter ~ 10 mm, its thickness ~ 4 mm.
The capsule is attached to ciliary body by suspensory ligaments (70), the tension of these ligaments is controlled by ciliary ms.
Its transparency is caused by
uniform arrangement of its fibres,
no blood vessels, dehydration
normal lens metabolism.
Is avascular; derives its nutrition & O 2 supply from aqueous humour.
Function of the Lens
The 2 nd major refractive medium of the eye; Its refractive power (20 dioptres) represents ~ 30% of total refractive power of the eye.
The important is that the lens is the only player responsible for adjustment for distance;
It can increase its refractive power, in response to nerve signals from the brain, providing the important mechanism of Accommodation.
Accommodation Reflex (Near Response)
Accommodation is the ability of the eye to keep the image of an object focused on the retina as the distance between the object & retina varies
Accommodation Reflex (Near Response) is the changes occurring in the eye as a result of retinal blurred image of the near object.
1- Constriction of the pupils.
2- ↑ Thickness of the lens (more spherical; becomes very convex (instead of moderate convexity).
3- Medial convergence of the eyes.
The lens becomes more spherical during near vision (contraction of ciliary muscle)
Area 8, Frontal eye field area Superior Colliculus, midbrain Somatic III n. supplying medial recti Convergence Edinger-Westphall n. of III n. Ciliary ganglion + Ciliary ms . +Constrictor pupillae ms . The image falls on fovea centralis ↑ curvature of lens ↑ its refractive power ↑ depth of focus ↓ excess light Prevents aberrations
Pathway of Accommodation reflex:
Amplitude of Accommodation
Is the difference between refractive power of the lens on maximum accommodation & in far vision when the lens is fully relaxed.
It decreases gradually with age due to loss of lens elasticity & weakness of ciliary ms.
The nearest point is the minimal distance from the eye at which an object can be brought into focus
recedes with age (same reasons above).
1 100 60 5 50 40 7 20 30 10 14 20 14 10 10 Amplitude of Accommodation (dioptres) Nearest point (cm) Age (years)
is the physiological progressive loss of the accommodation with age due to loss of elasticity of the lens (partly because of denaturation of lens proteins), until the lens becomes totally non-accommodating at the age of 70 years.
Corrected by wearing convex lens for near vision (e.g. reading).
Errors of Refraction
In normal eye (emmetropic), parallel rays from distant objects converge to a focus on the retina, provided that the ciliary ms. is completely relaxed.
If this does not occur -> an error of refraction
Types of refractive errors
Hypermetropia (long sight).
Myopia (short sight).
Myopia (short sight)
Hypermetropia (Long sight)
Errors of Refraction
Cylindrical lens in suitable plane Contact lens. Biconvex lens ( ↑ refractive power) Biconcave lens (diverge) ttt - Unequal curvature of cornea or lens. -short eye ball - Weak lens -long eye ball -Strong lens Cause Focus of rays in one plane is at different distance from that of rays in another plane. Focus behind retina Focus in front of retina Defect Astigmatism Hypermetropia Myopia
During near vision, which of the following is false?
The tension in the suspensory ligaments is increased
The tension in the lens capsule is decreased
The lens becomes more round
There is an increased signal to ciliary muscle
Control of Pupillary Diameter
The diameter of the pupil varies between 1.5 mm (bright light) – 8 mm (darkness).
This achieved by the presence of 2 smooth muscles;
1-Constrictor pupillae ms .; its fibres arranged in a circular manner around pupil. Receives motor postgang.para-sympathetic supply along short Ciliary nerves (pregang.from EW n of III relay in ciliary ganglion).
2- Dilator pupillae ms .; its fibres arranged radially around pupil.
Receives post gang. sympathetic from SCG (pregang. arise in 1, 2
Stimulus : exposure of one eye to light.
Response : Ipsilateral constriction of pupil ( Direct Light Reflex) & Contralateral pupillary constriction ( Indirect or consensual Light Reflex ).
Pathway : light -> retinal nervous elements -> optic nerve -> optic chiasma -> optic tract -> ipsilateral pretectal nucleus in midbrain -> along tectonuclear tract around aqueduct of Sylvius -> to EW nuclei (parasymp n of III) of both sides -> motor pregang. to ciliary ganglion -> post ganglionic along short ciliary nerves -> + bilateral constrictor pupillae ms. -> bilateral constriction of pupils.
Non-reactive to light but reactive to accommodation.
Due to damage of tectonuclear tract (by syphilis or syringomyelia of aqueduct of Sylvius).
Reverse Argyll-Robertson Pupil
Non-reactive to accommodation but reactive to light.
Due to lesion in occipitotectal tract.
The Visual Field
Blindness of ipsilat. Eye
(anopia of left eye .
b -> bitemporal hemianopia (heteronymous hemianopia) c ->homonymous hemianopia Lesion area 17(d) -> homonymous hemianopia of opposite side with macular sparing