Week 7.1- The eye Flashcards
What is the pupil?
The black centre of the eye
What is the iris?
The colour of the eye
What is the sclera?
The white part of the eye
What is the lower eyelid?
The lower skin of the eye
What is the upper eyelid?
The upper skin of the eye
What are the 3 layers of the eyeball?
1) Fibrous tunic
2) Vascular tunis
3) Retina
What does the fibrous tunic contain?
1) Sclera
2) cornea
What does the vascular tunic contain?
1) Iris
2) Ciliary body
3) Choroid
What does the retina contain?
1) Pigmented layer
2) Neural layer
What is the sclera / functions?
- Forms white of eye
- Tough fibrous coat
- Function:*
- Gives shape to eyeball
- Protects inner parts
- Continues as cornea anteriorly:*
- Transparent, non-vascular
- Covers iris
- The principle refractive medium of the eye
What is the choroid / functions?
- Pigmented and highly vascularised
- Functions:
1) Nourishes retina
2) Pigment absorbs light rays so it does not bouce around the eye
3) Posterior choroid continues anteriorly
4) Anterior choroid – ciliary body and iris
What is the ciliary body?
Alters shape of lens (through ciliary muscle) and allows eyes to focus on an object
What is the iris?
- Coloured part of eye
- Diaphragm between lens and cornea
- Alters pupil size
What regulates pupillary diameter?
- Iris circular and radial muscles
What happens to the pupil when there is bright light?
Pupil constricts as circular muscles of iris contract
What happens to the pupil when there is dim light?
- Pupil dilates as radial muscles of iris contracts
What is the retina / functions?
- Contains nervous tissue and has pigment
- Contains photoreceptors
- Role in image formation
- Retina ends anteriorly at border with ciliary body as a jagged scalloped edge called the ora serrata
What is the ora serrata?
- The junction between the retina and the ciliary body
- It is the area where the retina transitions from a non-photosensitive area to a multi-layered photosensitive region
What is the macula lutea?
- Small spot in centre of retina
- Contains a depression (fovea)
- Point of highest visual acuity
- High cencentration of cone photoreceptors
Where/ what is the optic disc?
- Location in retina where ganglion neurons bend posteriorly to form optic nerve
- Blind spot
What are lens / functions?
- Transparent, located posterior to the iris
- Focuses light rays on retina for image formation
- Part of the refractory apparatus of the eye
- Enclosed by a fibrous capsule
- Suspensory ligaments attach lens to ciliary body
What does the lens divide?
The eye into anterior and posterior
What is the difference of the anterior vs posterior cavities of the eye?
Anterior cavity:
- Filled with aqueous humor secreted from blood capillaries in the ciliary body
- Maintains intraocular pressures
- Supplies nutrients to lens / cornea
Posterior Cavity:
- Filled with vitreous humor (jelly like substance)
- Contributes to intraocular pressure
- Holds retina flush against choroid
What is the conjunctiva / functions?
- Thin mucous membrane
- Lines eyelid and covers the front surface of the eyeball, except the cornea
- Function: Produces lubricant and keeps front of eyeball moist
What is the formation of an image from the retina with Refraction of light rays?
- light rays bend as they pass through different mediums
- Cornea – principal refraction medium of the eye, this refraction is fixed
- Lens – also refracts light, its refraction can be varied and this helps focusing of near and distant objects
What is the formation of an image from the retina with the accomendation of the lens?
- Process by which the lens changes shape to focus on a near object
- Reflex action
- Compare status of the eye during image formation for a distant object versus a near object
Describe the lens with a distant object
1) Light rays arriving at cornea are parallel
2) Ciliary muscles are relaxed
3) Suspensory ligaments are tight
4) Lens is stretched by the tight ligaments
5) Lens has a tall, thin shape
Describe the lens with a near object
1) Light rays arriving at cornea are divergent
2) Ciliary muscles contract
3) Suspensory ligaments are loose
4) Lens is short and bulges out becoming more convex
5) Lens bends light more to form
What is the formation of an image from the retina with convergence of the eyes?
- During near vision both eyes rotate medially due to extrinsic muscle eye movements
- Six extrinsic eye muscles enable the eye to follow moving objects
- Ensures image falls on fovea (sharpest vision)
What is the formation of an image from the retina with constriction of the pupil?
- Occurs during accommodation for near vision
- limits amount of light that passes through the periphery of the bulging lens
- light passes through the center and thickest part of the lens
- prevents the formation of a blurred image on the retina
What is the photoreceptor function?
- Translates the image on the retina into nerve impulses
What are the 2 types of photoreceptors?
1) Rods
2) Cones
What is the difference of rods and cones?
Rods:
- 20 times more numerous than cones
- Dim light, high sensitivity and peripheral vision receptors
- Does not provide sharp images (low acuity)
Cones:
- Less numerous than rods
- Operates in bright light, low sensitivity, mostly central receptors
- Provides high clarity (high acuity) and colour vision
What is located in the innermost zone of the retina?
Photoreceptors
What only has cones?
Fovea centralis
What increase and decreses with further distance from the fovea?
Rods increase, cones decreases
Photopigement - rods
- Only one pigment (grey colour) (Rhudopsin)
- In dim light, we see fuzzy images in grey
-
What is rhudopsin composed of ?
- Pigment (retinal) – derived from Vitamin A
- Protein (opsin)
- When light strikes rhodopsin, the pigment breaks down and this causes a nervous impulse in the neuron
Cone photopigment facts
- 3 types of cones (for primary colour)
1) red light
2) blue light
3) green light - The activation of more than one type of cone leads to intermediate colours
- in bright light we see sharp and colour images
- Colour blindness results in the absence or deficiency of one of the three types of cones
Describe the pathway of the nervous impulses from the photoreceptors of the retina to the brain explaining why the left side of the brain interprets images from the right side of an object
- Each eye has 2 visual fields
1) medial
2) lateral - The retina of each eye is also divided in two halves:
1) nasal
2) temporal - Rays of light from the right side of an object refracts on the nasal half of the right retina and the temporal half of the left retina
- Rays of light from the left side of an object refracts on the nasal half of the left retina and the temporal half of the right eye
What are tears / functions?
- watery solution containing:
- Salts
- Mucous
- Lysozyme (bactericidal enzyme)
Functions:
- Clean, lubricate and moisten eyes
- Protect against bacterial infections
What is glaucoma?
- Excessive intraocular pressure
- If untreated in results in retinal degeneration and may cause blindness
What is myopia?
- Short-sightedness
- Corrected with a concave lens
What are the effects of hyperopia?
- Far sightedness
- corrected with a Convex lens
What is astigmatism?
- Distortion of curvature of cornea or lens or both
(multiple focal points)
what is Cataract?
- Loss of lens transparency
- Enhanced in diabetes and exposure to Xrays
