Mod7-Obj4: Major structures of the eye and its physiology Flashcards
Vision is our…
Dominant sense
- 70% of the body’s receptors are in the eye
- Visual processing by half cerebral cortex
Accessory structures
- Eyebrows
- Eyelids (Palpebrae)
- Conjuctiva
- Lacrimal apparatus
- Extrinsic eye muscles
Eyebrows
- Protect the eye from perspiration
- Shade from direct sunlight
Eyelids (Palpebrae)
- Muscular folds of facial skin fringed with hair
- Eyelashes are richly innervated (triggers blinking to prevent damage)
- Glands secret oily lubricant that lubricates eyelid & prevents eyelids from sticking together
Conjunctiva
- Transparent mucous membrane lining the eyelids and white of the eye
- Produces lubricating fluid to prevent drying out and friction
Lacrimal (tear) Apparatus
- Secretes dilute saline solution (tears) containing mucus, antibodies &lysozyme
- Blinking spreads tears
- Tears provide nutrients and oxygen, cleanse, moisten and protect the surface of the eye
Movement of the eyeball
- Six extrinsic muscles which are innervated by cranial nerves III, IV and VI
- These enable the eyes to be fixed on stationary or moving objects and keep both eyes fixed on the same object at the same time
Lens
- Lens is bioconvex, transparent and flexible
- Changes shape to focus light
- Spherical unless and outside force is applied
- Held in place by ciliary zonules called suspensory ligaments attached to ciliary muscles
Ciliary muscle
Circular muscle surrounding the lens
Suspensory ligaments
Extend from muscle to lens to hold in place
Ciliary muscles contract
flatten the lens
Suspensory ligaments relax and the lens becomes spherical
Ciliary muscles relax
lens become round
Suspensory ligaments tighten, pull at the circumference of the lens. causing it to flatten
The lens and ciliary zonules divides the eye into 2 segements
- ) Anterior segment
2. ) Posterior segment
Anterior segment of the lens and ciliary zonules
In front of the lens
-Anterior chamber: Between cornea and iris
-Posterior chamber:
Between iris and lens
The anterior segment of the lens and ciliary zonules
Contains aqueous humour that supplies nutrients and oxygen to the lens and cornea
Posterior segement of the lens and ciliary zonules
Behind the lens
Contains vitreous humour (gelatinous)
-Maintains the shape of the hollow eyeball and holds the retina into place
3 layers of the eyeball
- ) Fibrous layer (outer layer)
- ) Vascular layer (middle layer), three regions
- ) Retina (inner layer): nervous layer
Fibrous layer of the eyeball
outer layer
- Sclera: Tough opaque connective tissue
- Cornea: Transparent region
Vascular Layer (middle layer), supplies blood to the other two layers
- Choroid: Highly vascularised area
- Ciliary body: Forms muscular rings around lens
- Iris: Adjustable diaphragm that controls diameter of pupil (coloured area)
Retina (inner layer): Nervous layer
- Photoreceptors (cones & rodes)
- Fovea centralis (cones for visual acuity)
- Optic disc (no photoreceptors)
Rods and cones
Photoreceptors
Photoreceptors: light detecting cells (converting it to an action potential)
-Light reflects off objects and travels through our eyes to the photoreceptors of the retina
Rods (photoreceptors)
Receptors for dim light and peripheral vision
Cones (photoreceptors)
Receptors for bright light, colour and visual acuity
Focusing light on the Retina: 2 processes
- ) Refraction
2. ) Accommodation
Refraction (focusing light on the Retina)
Bending of light when it changes speed passing from one medium to another
Accomodation (focusing light on the Retina)
Changing the shape of the lens to direct light rays to the retina
Refraction of light
Refractions explains why these objects appear the way they do (why they appear bent)
Focussing light on the Retina: Refraction involving 4 changes
(Bending of the light, multiple
refractions before it reaches the retina)
- ) Air to cornea
- ) Cornea to aqueous humour
- ) Aqueous humour to lens
- ) Lens to vitreous humour
Focussing light on the Retina: Refraction
The amount of refraction is constant for changes except for the amount of refraction through the lens because the lens is highly elastic and changes shape for focussing (changes the bending of the light ray)
Combined effects of the refractions…
occur to focus the image on the retina
Focussing light on the Retina…Refraction of distant vision
flattened lens
An object that is 6 metres or more away from the eye reflects light rays that are virtually parallel to each other. These light rays will focus on the retina with the lens flattened (ciliary muscles relaxed, suspensory ligaments tightened)
Distance vision
Sympathetic activation
- Nearly parallel rays from distant object
- Lens is flattened for distant vision. Sympathetic input relaxes he ciliary muscle, tightening the ciliary zonule, and flattening the lens
Focusing light on the Retina: Close vision
rounder lens
Objects that are nearer than 6m reflect light rays that are on divergent paths. These requirer greater refraction to bring them into focus at the retina
- Lens is less thickened (more round) to increase refractory power=accomodation
- This is achieved through contraction of the ciliary muscle
Close vision
Parasympathetic activation
- Divergent rays from close object
- Lens bulges for close vision. Parasympathetic input contracts the ciliary muscles, loosing the ciliary zonule, allowing the lens to bulge
Focussing for close vision
- Convergence of the eyeballs: turning both eyeballs medially e.g.) looking at the tip of your nose
- Constriction of pupil: by iris sphincter muscle to decrease the diameter of the pupil. This prevents most divergent light rays from entering eye and causing blurred vision
Converting light to neural impulses
Once an object is focussed light rays are directed on the retina where photoreceptors (rods and cones) are located
Rods contain rhodopsin
- Light promotes the breakdown of rhodopsin causing the rod cell membrane to depolarise, which produces and action potential
- The visual pigments of cones operate in similar fashion
Rhodopsin
Sensitive to light, found in rods
