Lec 18: Vision Flashcards
Accessory Structures of the Eye
Eyebrows, eyelids, conjuctiva
Eyebrows
overlie the supraorbital margins and:
- shade the eye
- protect the eyes from perspiration
Eyelids (palpebrae)
- separated by the palpebral fissure
- lacrimal caruncle (corner of eye) contains sebaceous &sweat glands
- eyelash follicles richly innervated - trigger reflex blinking
- tarsal glands lubricate eyelid and eye with an oily secretion
Conjuctiva
transpaerent mucous membrane that lines eyelids
Bulbar Conjunctiva
Cover the white of the eye which produces lubricating mucus to prevent drying of eyes
Lacrimal Apparatus
- The lacrimal gland produces and secretes tears (lacrimal secretion)
- Tears enter the conjunctival sac via excretory ducts of the lacrimal gland
- Tear flow down eyeball
- Then enter lacrimal canaliculi called lacrimal puncta
- drain into lacrimal sac
- tears empty via the nasolacrimal duct into the inferior meatus of nasal cavity
Eye muscle
Lateral Rectus
Moves eye laterally (VI abducens)
Medial Rectus
Moves eye medially (III oculotomotor)
Superior Rectus
Elevates eye and turns it medially (III oculotomotor)
Inferior Rectus
Depresses eye and turns it medially (III ocutomotor)
Inferior oblique
Elevates eye and turns it laterally (III ocutomotor)
Superior oblique
Depresses eye and turns it laterally (IV trochlear)
The eyeball is a fluid-filled sphere composed of three layers:
fibrous, vascular, and inner (retina)
- Lens divides the eye into anterior and posterior segments
Fibrous layer
- dense CT that is avascular
- Sclera (majority)
- Cornea (anterior 1/6 of fibrous layer)
Sclera
- protects and shapes eyeball; anchoring site for extrinsic eye muscles
- continuous with dura mater where it is pierced by optic nerve
- seen anteriorly as the “white of the eye”
Cornea
- transparent layer (largely collagen & glycosaminoglycans); allows light entry and is important in light refraction
- corneal endothelium: simple squamous; sodium pumps to maintain corneal clarity
- lots of nerve endings; in a vulnerable location but capable of regeneration and repair; no blood vessels so no access to immune system (corneal transplants)
Vascular Layer (pigmented)
Choroid, ciliary body, iris
Choroid
- immediately deep to sclera
- vascularized; nourishes all eye layers
- contains melanin to absorb light so as to minimize scatter
Ciliary Body
anterior 1/6
- structure surrounding the lens that connects the choroid and iris. It contains ciliary muscles, (smooth muscles) which control the shape of the lens
- B on diagram
Iris
- eye colour; continuous with ciliary body posteriorly
- central opening is the pupil
- have 2 layers of smooth muscles to allow constriction (circular; PNS) versus dilation (radial; SNS)
- Only a brown pigment, but varying amounts give different colour possibilities
Inner Layer (retina)
Innermost layer of the eyeball; it contains millions of photoreceptors (rods, cones) that convert light energy, other neurons involved in processing light responses, and gilia.
Inner Layer
2 layers
Which one is involved in vision
-Outer pigmented layer
-Inner neural layer
Only neural layer is involved
Inner Layer
Pigmented Layer
absorbs light, cells can be phagocytic, stores vitamin A
Inner Layer
Neural Layer
composed of photoreceptors (rods and cones), bipolar cells, ganglion cells
Inner Layer
Signals
Produced in response to light and spread from the photoreceptors to the bipolar cells and then to the innermost ganglion cells, where action potentials are generated.
Inner Layer
Ganglion cells axons
Make a right-angle turn at the inner face of the retina, then leave the posterior aspect of the eye as the thick optic nerve.
Inner Layer
Retina
Also contains other types of neurons-horizontal and amacrine cells-which play a role in visual processing
Inner Layer
Optic Disc (blind spot)
Where the optic nerve exits the eye. It is a weak spot in the fundus (posterior wall) of the eye because it is not reinforced by the sclera
Inner Layer
Why is optic disc called the blind spot?
It lacks photoreceptors, so light focused on it cannot be seen.
Inner Layer
Why do we not notice the blind spots?
The brain uses a sophisticated process called filling in to deal with the absence of input.
Inner Layer
The quarter-billion photoreceptors found in the neural layer are of 2 types:
-Rods
-Cones
Inner Layer
Rods
dim light & peripheral vision, more numerous, don’t give sharp images;
specialized for light detection in low light environments
Inner Layer
Cones
bright light, high resolution, colour vision;
specialized for resolving spatial differences and detecting color
Inner Layer
Macula lutea (yellow spot)
An oval region that’s lateral to the blind spot of each eye and located precisely at the eye’s posterior pole with the fovea centralis in the its center; contains mostly cones
Inner Layer
Fovea centralis
a small depression within the neurosensory retina where visual acuity is the highest. The fovea itself is the central portion of the macula, which is responsible for central vision; contains only cones
Inner Layer
For us to visually comprehend a scene that’s rapidly changing
Our eyes must flick rapidly back and forth to provide the foveae with images of different parts of the visual field.
Inner Layer
Aqueous Humour
supplies nutrients and oxygen to lens and cornea and carries
away metabolic wastes
Inner layers
Vitreous Humour
*forms in the embryo and lasts our lifetime (unlike aqueous humor)
* found in the posterior segment
* transmits light, holds two retinal layers firmly together, intraocular pressure
Internal Layer
The - and - focus light on retina
Cornea and lens
Inner Layer
Cornea
anterior 1/6
- avascular
- transparent (smaller collagen fibers with low water content)
- stratified squamous epithelium
- inner surface is simple squamous
- allows light into eye
- focuses eye
- transplants easily because no antibodies
Inner Layer
Pathway for light
cornea
↓
aqueous humor
↓
lens
↓
vitreous humor
↓
neural layer of retina
↓
photoreceptors
Inner Layer
Light is refracted three times:
- cornea
- entry to lens
- leaving the len
Inner Layer
normal (emmetropic eye) vision far point
is 6 m;
with distant vision, light from an object at far point approaches as nearly parallel rays, allowing it to be precisely focused on the retina – ciliary muscles are relaxed (SNS)and lens is fla
Inner Layer
Close vision
light diverges as it approaches the
eye and so needs to be focused by the lens – ciliary muscles contract (PNS) and lens bulges
Inner Layer
Three Events of Near Vision
- Accomodation of lens
- Constriction of pupils
- Convergence of eyeballs
Inner Layer
Accomodation of lens
to allow image to be focused on the retina (near point of vision, with maximal lens bulging, is 10 cm from the eye (closer for children,
gradually farther as we age)
After age 50 eye can not accomodate = reading glasses
Inner Layer
Constriction of pupils
PNS-mediated and involves contraction of sphincter pupillae muscles of iris to keep most divergent light rays from entering and causing
image to be blurry
Inner Layer
Convergence of eyeballs
to keep object focused on retinal foveae – medial rectus muscle and oculomotor cranial nerve
Inner Layer
Most refractive problems are related to eyeball shape
- Myopia
- Hyperopia
Inner Layer
Myopia
- Myopia: nearsightedness; eyeball is long and distant objects focus in front of
retina; but can focus close objects on the retina making close vision OK
Inner Layer
Hyperopia
- Hyperopia: farsightedness; eyeball is too short and distant objects focus
behind the retina; but their ciliary muscles contract almost all the time, moving
the focus to the retina for distance; but unable to refract enough to bring close
objects onto the retina
Inner Layer
Concave lense
Concave lenses diverge
the light
Inner Layer
Convex lens
Convex lenses converge
the light
Photoreceptors
Receptive
embedded in pigmented layer
Photoreceptors
Cilium
connects outer segment to inner segment
Photoreceptors
Outer Segments
contain visual pigments (rhodopsins) that change shape as they absorb
light; visual pigments are embedded in disc
membranes
Phototransduction
- Photoreceptors hyperpolarize (-70 mV) when exposed to light and this actually
acts as a signal - Photoreceptors remain depolarized (-40 mV) in the dark
Light and Dark Adaptation
Coming from dark into bright light:
- initially both rods and cones are stimulated (all we see is bright white light)
- rhodopsin in rods bleaches as quickly as it is formed and transducin leaves the disc
membranes, rendering the rods non-functional - cones gradually take over (5-10 minutes to max acuity)
Light and Dark adaptation
Coming from light to dark
- initially all looks dark because cones are no longer stimulated and the rods had been turned off
- rhodopsin starts to accumulate in the rods and transducin moves back into the disc
membranes - slower process – takes 20-30 min to have max vision in very dim light
From the Retina to the Visual Cortex
Key Points regarding Visual (and
related) Pathways
- axons of retinal ganglion cells form the
optic nerve - most fibers of the optic tracts continue to
the lateral geniculate body of the
thalamus - other optic tract fibers end in superior
colliculi (initiating visual reflexes) and
pretectal nuclei (involved with pupillary
reflexes) - optic radiations travel from the thalamus
to the primary visual cortex
Closer Look
1.Medial half of each retina receives
light rays from the lateral field of view.
2. Lateral half of each retina receives
light from the central part of the visual
field (and there is overlap here).
3. Medial fibers of the optic nerve
decussate at the optic chiasm
4. Each optic tract leaving the optic
chiasm contains fibers from the lateral
aspect of the eye on the same side
and the medial aspect of the opposite
eye
5. Each optic tract carries information for
the same half of the visual field
Depth Perception
- The visual field of each eye is
about 170 degrees - There is considerable overlap in
the middle area, but each eye
sees on a slightly different angle - The primary visual cortex fuses
the images from both eyes
giving depth perception - Depth perception is lost if you
are looking with only one eye
