Lecture 1
THE EYE
Summary
Parts of the eye
Layers of the eye
Chambers of the eye
The retina
The visual pathway
Parts Of The Eye
THE EYEBALL
It is a sphere with a diameter of about 24 mm.
There are 3 layers of the wall of the eye that extend around the eye:
1. The outer layer:
Sclera
– the tough, fibrous layer,
cornea
-
transparent
2. Intermediat layer:
Uveal tract
–
the vascular layer
3. The outer layer:
Retina
– the sensory layer
There are also numerous cavities and structures within the eye.
These can be split into three chambers:
1. Anterior chamber
Aqueous humour
2. Posterior chamber
Aqueous humour
3. Vitreous chamber
Vitreous humour
The outer Fibrous coat
THE SCLERA
= the whites of the eyes
Tough, fibrous collagen outer layer of the eye
Forms the posterior 5/6 of the outer coat of the eyeball
Becomes transparent at the cornea which covers the iris/pupil
(therefore the two are continuous)
Functions:
Maintains eye shape
Maintains intraocular pressure
Protective against infection and trauma
The limbus: is a zone marking the junction between the cornea and the
sclera.
The Cornea
It is clear, dome-shaped
at the very front of the eye covering the
lens/pupil/iris is essentially the window to the eye.
Forms the anterior 1/6 of the outer coat of the eyeball
The cornea is comprised of
five layers
: the epithelium, Bowman's layer, the
stroma, Descemet's membrane, and the endothelium.
The first layer, the
epithelium
, is a layer of cells covering the cornea. It
absorbs nutrients and oxygen from tears and conveys it to the rest of the
cornea. It contains free nerve endings.
Function:
1. It functions like a window that controls and focuses the entry of light
into the eye.
2.
The cornea contributes between 65- 75 percent of the eye's total
focusing power (a power of about 42 diopters). When light strikes
the cornea, it bends—or refracts--the incoming light onto the lens
then retina.
The middle vascular Coat ( the uvea)
This is the pigmented and vascular layer of the eye
3 parts (from posterior to anterior):
Choroid
Ciliary body which is made up of
1.
Ciliary processes
2.
Ciliary muscles
Iris
The choroid makes up the bulk of the uveal tract:
Mostly located posteriorly.
Connective tissue and vascular layer that gives nutrition to
the outer layers of the retina and the macular area including
the fovea.
Ciliary body anteriorly has 2 functions:
1. Ciliary processes which release aqueous humour
Also give rise to the suspensory ligament which holds lens in
position
2. Ciliary muscles which adjust the shape of the lens
This is required in the accommodation reflex
Controlled by CNIII
The Iris
is the anterior most part with a central opening called the
pupil.
The iris is the colored part of the eye which helps regulate the amount of
light entering the eye. When there is bright light, the iris closes the pupil to
let in less light. And when there is low light, the iris opens up the pupil to let
in more light.
The pupil is controlled by sphincter (parasympathetic) and dilator
pupillae (sympathetic) muscles.
RETINA
Is the innermost, light-sensitive layer of tissue of the eye
This is the sensory interface of the eye.
Very thin transparent membrane lining the inside of the eye.
It starts at the ciliary body by an area called the ora serrata and ends
around the optic disc.
Grosss anatomy
• Three distinct regions of retina:
1.optic disc
2.macula lutea
3.peripheral retina
• retina extends from optic disc to ora serrata
• The retina is transparent made up of 10 layers within it.
Normal retina contains neuroretina and retinal pigment epithelium. The
neuroretina consists of outer and inner segments of photoreceptors (rods
and cones), external limiting membrane, outer nuclear layer, outer
plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell
layer, nerve fiber layer and internal limiting membrane.
Macula lutea
lies at the middle of the posterior part of the retina
The macula lutea is comparatively dark area 5.5 mm in diameter ,
situated at the posterior pole of the eyeball , temporal to optic disc
also called as yellow spot or area centralis
Primary function :- photoptic vision
Fovea centralis is central depression in macula
It is most sensitive part of retina
Foveola Central floor of fovea
Umbo is the tiny depression in centre of foveola
Optic disc
:
The optic disc (optic nerve head) is the location where ganglion cell axons
exit the eye to form the optic nerve, Diameter: 1.5mm
There is a blind spot due to the absence of photoreceptors (There are no
light sensitive rods or cones to respond to a light stimulus at this point).
Colour:
Optic disc is orange pink in colour.
Pallor occurs in optic atrophy
Contour:
well defined circular area.
Blurred margins in papilloedema
Cup :The optic cup is the white, cup-like area in the center of the optic
disc. The ratio of the size of the optic cup to the optic disc (C/D)of about 0.4
Chambers Of The Eye
There are 3 chambers :
1. Anterior chamber
Directly posterior to cornea and anterior to iris
2. Posterior chamber
Posterior to iris and anterior to lens
3. Vitreous chamber
Posterior to the lens
The anterior and posterior chambers are both are filled with aqueous
humour (i.e. are aqueous compartments)
Secreted in posterior chamber by the ciliary body epithelium
Reabsorbed via 2 routes:
85% drain at the iridocorneal angle in the anterior
chamber via the trabecular meshwork into the scleral
venous sinus
15% reabsorbed by the ciliary body
Functions of aqueous humour
1. Nourishment to eye
2. Maintenance of intraocular pressure between 10-21mmHg
Clinical significance of aqueous humour:
Glaucoma occurs due to failed reabsorption
The vitreous chamber
is the largest of the three chambers
Contains vitreous humour which is a transparent, colorless,
gelatinous mass that fills the space in the eye between the lens and
the retina.
It makes up four-fifths of the volume of the eyeball
Clinical significance:
Protects the ocular structures
Visual Pathways
BINOCULAR AND MONOCULAR VISION
Monocular vision – i.e. the image seen by each individual eye
Lateral vision is better than nasal (as nose gets in way)
Binocular vision
Visual fields combine to form binocular field of view
THE VISUAL PATHWAY
The visual pathway begins with photoreceptors in the retina and ends in
the visual cortex of the occipital lobe. The photoreceptors are cells of two
types: rods and cones. Rods play a special role in peripheral vision and in
vision under low light conditions
1. An image is received at the retina
2. This image is transmitted from retina to brain along optic nerve
Optic nerves meet at the
optic chiasm
(located at the base of the
hypothalamus)
3.The nasal aspects of the optic nerves cross at the optic chiasm to
become the
optic tracts
Only the nasal hemiretinas cross over to the opposite side
the temporal hemiretina looks at the nasal hemifield (and vice
versa)
4. After crossing at the optic chiasm, nerve fibres travel within optic
tracts as they were divided
Information from right visual field (left hemiretinas) travels in left
optic tract
Information from left visual field (right hemiretinas) travels in right
optic tract
Basically remember as each paired hemiretina will go to the
ipsilateral brain
Therefore, corresponding hemifields of view are
sent to opposite parts of the brain
I.e. right hemifields (from left hemiretinas in both
eyes) are sent to the left brain and vice versa
Right hemifield contains the left hemiretinas
fibres of both eyes which both go to left side of brain
5.Each optic tract terminates in the lateral geniculate nucleus of thalamus
6.
Optic radiations
carry information from the LGN to the
primary visual
cortex
This is primarily in the occipital lobe
Rest of occipital lobe constitutes visual association area
PROTECTIVE MECHANISMS OF THE EYE
The eye is an important organ, it is protected from trauma by several
mechanisms:
The orbits:
The eye is surrounded by a bony orbit and orbital margins that protect it
from injury.
The retrobulbar pad of fat acts as a shock absorber and prevents the eye from
being compressed against the orbital bones in case of trauma
The lids:
Closure of the lids during sleep prevents corneal dryness and ulceration.
Reflex closure of the ocular surface by the tears within each blink.
Lubrication of the ocular surface by the tears within each blink.
The eye lashes and the eyebrows trap any foreign body.
The tears:
The precorneal tear film prevents corneal dryness and ulceration.
Tears have an anti- microbial function through its lysozyme content.
The cornea:
The cornea is richly supplied by sensory nerves, making it highly sensitive
to pain. In response to trauma, stimulation of the corneal nerves initiates
reflex blinking and tearing, protecting it from injury.