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Photoreception - Vision
Basic Structure
• 70% of all sensory receptors in the body
• Optic tracts contain over a million nerve fibers
• Eye is sphere that is approximately 1inch in
diameter
• Anterior 1/6th of surface is visible
• The rest is enclosed and protected by a fat pad
Vision
Accessory structures of the eye
• Eyelids (palpebrae)- separated by the palpebral fissue and
meet at the medial and lateral canthi. Medial has a fleshy
elevation called the caruncle which contains sebaceous and
sweat glands. Connective tissue sheets called tarsal plates where
the orbicularis oculi and levator palpebrae superioris muscles run
within eyelid. Orbicularis muscle encircles the eye- when it
contracts eye closes, levator palpebrae muscle causes the eyelid
to raise. Eyelid muscles are reflexive and blink every 3-7 seconds.
• Eyelashes- follicles of the eyelash hairs are richly innervated by
nerve endings, when they touch anything blinking reflex is triggered.
Accessory structures of the eye cont.
• Eyebrows-shade the eye from sunlight and perspiration that may trickle
into eyes- orbicularis muscles of the eye cause the eyebrows to move.
• Tarsal/meibomian glands- embedded in tarsal plates where their ducts
open at the eyelid edge just posterior to the eyelashes. Produce oily
secretion that lubricates the eyelid and eye: preventing them from
sticking together.
• Conjunctiva- delicate mucus membrane that lines the eyelid, blood
vessels are clearly visible through this structure. Covers all of the white
areas of the eye. Prevents foreign objects from penetration beyond the
conjuctival sac (where contacts and medications enter the eye). Vitamin A
helps to maintain this structure.
• Lacrimal apparatus- consists of lacrimal gland and ducts that drain
secretions and drain into the nasal cavity. Releases tears into superior
part of the conjuctival sac through small excretory ducts. Blinking
spreads tears. Lacrimal secretions contain mucus, antibodies, and
lysozymes.
External Features and Accessory Structures of the
Eye
• Facts about extrinsic eye muscles
• -Eye muscles are among the most precisely and
rapidly controlled skeletal muscles in the entire
body.
• - Have a high axon to muscle ratio
• Movements:
• -Saccades are small jerky movements allowing
one to see a whole field of vision in a short time.
• -Slow scanning movements allow us to follow an
object or focus on one object as our head moves.
• Six strap like muscles that originate from the bony orbit and
insert into the outer surface of the eyeball which control
movement and maintain shape.
• Four of the extrinsic muscles (superior, inferior, lateral, and
medial) originate from a common tendinous ring call the
annular ring at the back of the orbit and run straight to
their insertion on the eyeball.
• Superior oblique- originates in common with rectus muscle
and runs along the medial wall of the orbit
• Trochlea-fibrocartilaginous loop that inserts at the
superolateral aspect of the eyeball.
• Inferior oblique muscle- originates from the medial orbit
surace and runs laterally and obliquely to insert on the
inferolateral eye surface.
• Three Tunics
– Outer fibrous tunic
The eye
• Sclera (white of eye), cornea (lets light
enter eye)
– Middle vascular tunic
• Iris (anterior portion of uvea, contains
pupil-controls amount of light), ciliary
body ( muscles that control lens shape,
rich in blood vessels), choroid (blood
vessels feed the eye tunics, contains
melanocytes to absorb light)
suspensory ligaments (holds lens in
position)
– Inner Sensory nervous tunic
• Retina (pigmented epithelial cells,
absorbs light and prevent it from
scattering in eye, act as phagocytes and
store vitamin A. Neural layer (contains
millions of photoreceptors and
transduce light into energy, neuronsphotoreceptors, bipolar, and ganglion)
The Sectional Anatomy of the Eye
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 Organization of the Retina
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
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
Eye Abnormalities
• Glaucoma
• Cataract
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
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
Visible light
• 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
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