Transcript How We See PP

• (1). Visible Light and the Electromagnetic
Spectrum
• wavelengths: 400 – 700 nm
• (2). Light passes through a prism
• Bent or refracts!
• Into various wavelengths – color bands
• Red –longest ; blue – shortest
• (3). Light refracted and focused on the retina
• (4). Photopigments - Rods and Cones
• 1. Fibrous
– Sclera and Cornea
• 2. Vascular
– Choroid layer
– Pupil and Iris
– Ciliary Body
• Ciliary muscles and suspensory ligaments
• 3. Inner (Retina)
– Pigmented and Neural layer
• Photoreceptor cells
(2). Cornea
 Refractory power
Fibrous tunic
Sclera ----
cornea
External: Stratified
Squamous
Internal: Simple
Squamous
ORA SERRATA
IRIS
SUSPENSORY
LIGAMENTS
CILIARY BODY
CHOROID LAYER
(1). Pupil and Iris
Iris (smooth muscle)
Sphincter pupillae
Close vision and bright light
Parasympathetic
Dilator pupillae
Distant vision and dim light
Sympathetic
(2). Why is the Iris colored?
(3). Part of the Choroid layer
or tunic
Uvea
(1). Lens Anatomy
Epithelium (cuboidal cells –lens fiber)
 Crystalline
 Lens aging and cataracts
(2). Refraction power
 Very elastic – high
(3). Role of Ciliary body
 Ciliary muscles
 Suspensory ligaments
Lens Adaptation
• Distant vision
 Emmentropic – 20 ft.
 Ciliary muscles are relaxed
– lens is flat
• Close vision
 Most difficult – less than 6 ft.
 Lens must make changes
• Accommodation
• Pupil constriction
• Eyeball convergence
Aqueous Humor
 Constant refractory
power
 Produced by choroid
structures
 Canal of Schlemm
 Intaocular pressure –
Glaucoma
Vitreous Body
 Embryonic production
 Constant refractory
power
1. Rods and Cones
2. Bipolar Cells
3. Ganglion Cells
4. Optic Nerve
1. Blind Spot – Optic Disc
2. Fundus
Light Reaches the Retina
• (1).
 Pigmented epithelium
 Absorb light
• (2).
 Transparent
 Photoreceptor cells (rods and
cones)
• Synapse with bipolar cells
and ganglion cells
• Ganglion cells form optic
nerves – blind spot/optic
disc
• Rods
– Dark vision (fuzzy)
– Most numerous - located in the periphery of the retina
– 100 cones plus to each bipolar cell
• Cones
– Color vision (one per ganglion cell)
– Located in maculae lutea and fovea
• Fovea cones only / maculae mostly cones
– Around this area the retinal area is then
• Allows for direct –bright light
Visual Pigments
(1). Rods and cones - visual pigments.
•“rhodopsin”
• cones contain (3) visual pigments
(2). Pigments are produced
• retinal (vitamin A) with opsin (protein)
(3). Rod and cone pigments are determined by the
types of opsin proteins
(4). Formation of 11-Cis retinal (stored in eye)
accumulates in the dark
• Bleaching of Rhodopsin and regeneration
• Converted into a nerve transduction
Phototransduction
• In the dark
• cGMP binds to cation channels
• Channels opened (Ca and Na)
• Membrane potential - -40 mV
• Release of glutamate - IPSP in
bipolar cell
• In the light
• Light destroys cGMP
• Cation channels closed -70 mV
• Stops release of glutamate
• No IPSP in bipolar cell
• Allows for EPSP in ganglion cells
• Action potential takes place
Myopic –
nearsighted
•Eyeball too long
Hyperopic –
farsighted
•Eyeball too short
1. Retinal ganglion – synapse with
bipolar cells (form the optic
nerves)
2. Cross at the optic chiasma
3. Move around the hypothalamusSynapse at the lateral geniculate
body (lateral fibers) and Some
medial fibers in the pretectal
nucleus.
4. Other medial fibers synapse in the
lateral geniculate body and
proceed through the cerebral
white matter onto the visual
cortex.
• (1). Retinal Processing
– Light hyperpolarizes the photoreceptor cells
– Bipolar neurons – Rods
– Bipolar neurons – cones
• (2). Thalamic Processing
– LGN of the thalamus relays info on movement
• Depth perception; emphasize cone inputs; contrast sharpening
• (3). Cortical Processing
– Primary or striate visual cortex
• Incoming – LGN of thalamus
– Dark and bright (contrast) information; object orientation
• Provides form, color and motion inputs to the visual association
area
– Visual Association areas- (pre-striate cortices)
• in occipital lobe area
• Continue processing info about color, form and movement
• Let’s us understand the ‘what’ and ‘where’