The Eye Flashcards
Cavities of the eye
Anterior Cavity: bound by the cornea, the iris, and the anterior lens. Filled with aqueous humour produced by the ciliary body
Posterior Cavity: bound by the posterior lens and by the retina. Filled with vitreous humour, which supports the retina
Tunics of the eye
Fibrous Layer: outermost layer, composed of the sclera and the cornea
Vascular Layer: the intermediate layer, composed of the iris, ciliary body, and choroid
Neural Layer: innermost layer, composed of the retina, which is a sensory membrane that processes light information
The cornea
covers the anterior 1/6 of the eyeball. it is a transparent covering with no vascular supply, and has five distinct layers
Cornea Layers
- Corneal epithelium
- Bowman’s membrane
- Stroma
- Descement’s membrane
- Endothelium
corneal epithelium
the outermost layer of the cornea and is a stratified squamous epithelium with abundant sensory innervation.
- these cells undergo rapid regeneration
Bowman’s membrane
underlies the epithelium, composed of collagen fibres
cornea stroma
composed of many layers of collagen fibre arranged at right angles to each other
- this arrangement allows the cornea to be transparent and transmit light
Descement’s Membrane
a thick basal lamina produced by endothelial cells
cornea endothelium
the innermost layer of the cornea and is a simple squamous epithelium
Sclera
the white part of the eye that covers the posterior 5/6 of the eyeball, and is continuous with the cornea
- composed of dense CT (substantia propria) with a supportive outer layer (episclera) and a vascular supply
Sclera functions
sclera gives the eye its shape and protects it structurally
it provides attachment site for extraocular muscles
Canal of Schlemm
at the junction of the cornea and the sclera
it is a vascular sinus
- lined by endothelium and receives aqueous humour from a trabecular meshwork. the aqueous humour flows into the veins of sclera, and the flow is facilitated by ciliary muscle
- improper drainage of the aqueous humour could result in increased intraocular pressure
Iris
the structure that lies anterior to the lens, except for the region of the pupil in the centre
- responsible for controlling the diameter of the pupil, and therefore, how much light is able to enter the eye
- gives the eye its colour
iris composition
vascular CT stroma with a discontinous layer of cells on its anterior surface
its posterior surface has a distinct scalloped pigmented epithelium
iris muscles
the two smooth muscles responsible for controlling the diameter of the pupil are the dilator pupillae muscle (DPM) and the constrictor pupillae muscle (CPM)
- DPM is a thin muscle that lies adjacent to the iris epithleium
- CPM is larger and is located around the circumference of the iris
ciliary body
a wedge shaped structure that is continuous with the choroid and is located at the level of the lens
- the stroma of the ciliary body is rich in elastic fibres and blood vessels
- responsible for regulating the flow of aqueous humour through the canal of Schlemm
ciliary muscle
contained in the ciliary body
comprised of bundles of smooth muscles organized in three planes
- responsible for the process of accommodation
- as these muscles contract, they decrease tension on the lens
with the contraction and relaxation of these muscles, the eye is able to focus on objects both near and far, respectively
ciliary epithelium
located on the posterior surface of the ciliary body and its ciliary processes, and is continuous with the iris epithelium
- it is part of the non-sensory layer of the retina, and thus is part of the neural tunic rather than the vascular tunic
- functions to produce aqueous humour and zonular fibres
ciliary processes
extensions of the ciliary body that is covered by ciliary epithelium
- zonular fibres attach to the grooves that separate the ciliary processes
zonular fibres
thin, delicate microfibrils that arise from the basal lamina of ciliary epithelium
they attach the ciliary body to the lens
- contraction of ciliary muscles alters the tension of the zonular fibres, which alters the shape of the lens
- these fibres relax when the ciliary muscle contracts, and tighten when the muscle relaxes
Lens
a biconvex transparent structure that is a critical refractive element of the eye
- it is avascular, and is nourished by aqueous humour anteriorly and vitreous humour posteriorly
- located immediately behind the iris and is held in place by zonular fibres
Pupil
region anterior to the lens, not covered by the iris
the pupil is the space where light enters the eye
the lens and age
the lens loses its elasticity with age, which results in a decreased ability for it to adjust to vision at different distances (accommodation)
outermost layer of the lens
a capsule formed by the thickened basal lamina of the lens epithelium
- the capsule is thicker on the anterior surface than on the posterior surface of the lens
- has simple cuboidal epithelium on the anterior and lateral surfaces. epithelium is only present until the equator of the lens
- functions to generate lens fibres that comprise the bulk of the lens
equator of the lens: lens fibres
at the equator of the lens, the epithelial cells divide to produce new cells that differentiate into lens fibres
- once generated, lens fibres elongate and flatten in an organization that is parallel to the path of light entering the eye
- the equatorial region also serves as the attachment site for zonular fibres from the ciliary body
choroid
the third part of the vascular tunic, lies between the sclera and the retina
- functions to provide oxygen and nourishment to the outer layers of the retina
- consists of three layers, from outer to inner: stroma, choroicapillary (vascular layer), and Bruch’s membrane
choroid stroma
composed of loose CT with some blood vessels and many melanocytes present
- outermost choroid layer
vascular layer of the choroid
consists of many capillaries that nourishes the outer layers of the retina
has several small RBCs and is the middle layer
Bruch’s membrane
very thin layer that the choroid shares with the retina
difficult to appreciate unless being viewed at a higher magnification
inner most layer
pigmented melanocytes
contains the dark pigment melanin, which functions to reduce uncontrolled reflection within the eye
- once produced, melanin is stored in organelles called melanosomes within melanocytes
Retina
there are three distinct regions along the surface of the retina that vary is structure and function:
- anterior portion
- the ora serrata
- the sensory portion
anterior portion of the retina
is non-sensory, not responsive to light
- lines the posterior portion of the iris and ciliary body
the ora serrata
multilayered sensory retina collapses into a single layer of columnar epithelium over the ciliary body
- is the boundary between sensory and non-sensory portions, and is a small depression located posterior to the ciliary body
the sensory portion of the retina
AKA photosensitive retina
responsive to light, lining the remainder of the eye
what happens when light enters the pupil
it is refracted by the lens towards the retina, and must pass through all of the retina layers to reach the rods and cones (photoreceptors). here, the light is transduced into neural signals
- visual signals are then transmitted from the rods and cones back in the opposite direction, towards the ganglion cells
- visual integration occurs along the pathway before reaching the ganglion cells
- ganglion cell axons transmit the neural signals to the brain via CNII at the optic disc
retina layers
from outer to inner:
1. pigmented epithelium
2. rods and cones outer/inner segment
3. external limiting membrane
4. outer nuclear layer
5. outer plexiform layer
6. inner nuclear layer
7. inner plexiform layer
8. ganglion cell layer
9. nerve fibre layer
10. internal limiting membrane
retina layers function
layer function to convert light into neural signals, integrate and transmit them to the optic nerve, where they continue to the brain for visual recognition
retina regions
fovea and optic disc
the fovea
a small depression in the retina located in the centre of a larger shaped area called the macula densa
the fovea is the area of highest visual acuity in the retina. here, the bipolar and ganglion cells are shifted to the periphery of the central region, called the foveola. displacement of these cells allows for an uninterrupted pathway for light travelling to the foveola
macula densa
located almost at the centre of the retina at the posterior of the eye
has an abundance of cones, and functions in sharp detailed vision (visual acuity)
visual acuity
the cones are photoreceptor cells responsible for colour and vision, and function best with bright light, producing detailed, high acuity vision
- with the high concentration of cones in the macula densa, and the displacement of bipolar and ganglion cells, there is an uninterrupted path for light to travel to the cones, resulting in high visual acuity
the optic disc
the site where the optic nerve (CNII) exits the eye. axons from ganglion cells travel through the retina as the nerve fibre layer of the retina
- ganglion cell axons converge at the optic nerve
retinal blood vessels are present in the nerve fibre layer and maintain the inner retinal layers
visual blind spot
the optic disc lacks any retinal neurons, including photoreceptor cells
- lack of rods and cones results in a small visual blind spot in each eye
