Photoreception - Vision
Vision
Accessory structures of the eye
• Eyelids (palpebrae) separated by the
palpebral fissue
• Eyelashes
• Tarsal glands
• Lacrimal apparatus
• The eye can only
perceive a small portion
of the spectrum of
electromagnetic waves
Vision
• In order to see an object:
- 1- the pattern of the object must fall on the vision
receptors (rods and cones in the retina) 
accommodation
- 2- the amount of light entering the eye must be
regulated (too much light will “bleach out” the
signals)
- 3- the energy from the waves of photons must be
transduced into electrical signals
- 4- The brain must receive and interpret the signals
External Features and Accessory Structures of the
Eye
External Structures of the Eye
• Conjunctiva covers most of eye
• Cornea is transparent anterior portion
Lacrimal Apparatus
• Secretions from the lacrimal gland contain
lysozyme
• Tears form in the lacrimal glands, wash across
the eye and collect in the lacrimal lake
• Pass through the lacrimal punctae, lacrimal
canaliculi, lacrimal sac and nasolacrimal duct
The eye
• Three layers
– Outer fibrous tunic
• Sclera, cornea, limbus
– Middle vascular tunic
• Iris, ciliary body, choroid
– Inner nervous tunic
• Retina
The Sectional Anatomy of the Eye
Internal Structures of the Eye
• Ciliary body
– Ciliary muscles and ciliary processes, which attach to
suspensory ligaments of lens
• Retina
– Outer pigmented portion
– Inner neural part
• Rods and cones
Sectional Anatomy of the Eye
Regulation of the Amount of Light Entering the
Eye
• The iris controls the
amount of light
entering the eye cavities
• The contraction of
radial or circular
smooth muscles located
within the iris permit
changes in the pupil
diameter
The Pupillary Muscles
Retina
• Retina contains rods and cones
– Cones densely packed at fovea (center of the macula
lutea)
• Retinal pathway
– Photoreceptors to bipolar cells to ganglion cells, to
the brain via the optic nerve
• Axons of ganglion cells converge at blind spot
(optic disc)
– Horizontal cells and amacrine cells modify the signal
passed along the retinal neurons
The Organization of the Retina
Retinal structure
• Three cell layers:
-- outer layer: photoreceptorsrods and cones
-- middle layer: bipolar
neurons
-- inner layer: ganglion cells
The Organization of the Retina
Eye Anatomy
• Ciliary body and lens divide the anterior cavity of
the eye into posterior (vitreous) cavity and
anterior cavity
• Anterior cavity further divided
– anterior chamber in front of eye
– posterior chamber between the iris and the lens
The Circulation of Aqueous Humor
Fluids in the eye
• Aqueous humor circulates within the eye
– diffuses through the walls of anterior chamber
– passes through canal of Schlemm
– re-enters circulation
• Vitreous humor fills the posterior cavity.
– Not recycled – permanent fluid
Eye Abnormalities
• Glaucoma
• Cataract
Lens
• Posterior to the cornea and forms anterior boundary of posterior
cavity
– Posterior cavity contains vitreous humor
• Lens helps focus
– Light is refracted as it passes through lens
– Accommodation is the process by which the lens adjusts to
focus images
– Normal visual acuity is 20/20
Image Formation
Accommodation
• It is the process of adjusting the shape of the
lens so that the external image fall exactly on
the retina
Accommodation
Accommodation Abnormalities
Accommodation Abnormalities
• Myopia
• Hyperopia
• Astigmatism: the cornea is irregular 
irregular pattern of vision
• Presbyopia: stiffening of the lens occurring
with aging  increased difficulty with near
vision
Visual Physiology
• Rods – respond to almost any photon
• Cones – specific ranges of specificity
Rods and Cones
Photoreceptor Structure
• Outer segment with membranous discs
• Narrow stalk connecting outer segment to inner segment
• Light absorption occurs in the visual pigments
– Derivatives of rhodopsin
Phototransduction - General
• Photons hit the pigment of
a photoreceptor   
enzymes are activated in
the cell which modify its
state of polarization  the
signals are sent to visual
area of the occipital lobe of
the brain through the optic
nerve
Photoreception - In More Detail
Photoreception
Bleaching and Regeneration of Visual Pigments
Color sensitivity
• Integration of information from red, blue and green cones
• Colorblindness is the inability to detect certain colors
Retinal Adaptation
• Dark adapted – most visual pigments are fully
receptive to stimulation
• Light adapted – pupil constricts and pigments
bleached.
The Visual Pathway
• Large M-cells monitor rods
• Smaller more numerous P cells monitor cones
Convergence and Ganglion Cell Function
Seeing in Stereo
• Vision from the field of view transfers from one
side to the other while in transit
• Depth perception is obtained by comparing
relative positions of objects from the two eyes
Neural processing
• The bipolar neurons and ganglion cells process
the signal
• In the fovea where the acuity is the highest: 1
cone  1 bipolar cell  1 ganglion cell
• At the periphery: many rods  1 bipolar cell
… acuity is much decreased
• Other cells in the retina participate in signal
processing
The Visual Pathways
Visual Circadian Rhythm
• Input to suprachiasmic nucleus affects the
function of the brainstem
• Circadian rhythm ties to day-night cycle, and
affects metabolic rates