The Physiology of Vision
Anatomy of the eye
• 1- sclera: is the outer protective layer.
• 2- cornea : anterior , modified part of the sclera, light
rays enter through it.
• 3- choroid : deep to the sclera , rich in blood vessels.
• 4-retina : lines the post. Two thirds of the choroid ,
formed of neural tissue rich in receptors.
• 5- cilliary bodies : thickened anterior parts of the
choroid , contain circular & longitudinal muscle fibers ,
produce aqueos humor.
• 6-iris: pigmented , opaque , contains constrictors of the
pupil ( circular ) & dilators of the pupil ( radial ).
Anatomy of the eye
• 7- vitreous humor : a gelatinous substance
between the lens & retina.
• 8- aqueous humor : a clear liquid that
nourishes the cornea & lens.
• 9- canal of Schlemm : at the junction between
the iris & cornea , drains aqueous humor.
Retina
• Extends anteriorly almost reaching the cilliary body.
• Contains visual receptors ( rods & cones) + bipolar ,
ganglion , horizontal & amacrine cells.
• Rods & cones are next to the choroid . They synapse
with bipolar cells .
• Bipolar cells synapse with ganglion cells.
• Amacrine cells connect the ganglion cells to each
other.
• Axons of the ganglion cells converge to form the optic
nerve.
• Optic disk : is the point where the optic nerve leaves
the eye & blood vessels enter. It is located 3mm
medial to & slightly above the posterior pole of the
globe.
Retina
• The optic disk is a blind spot , it doesn’t contain any
visual receptors.
• Macula lutea :
• is a yellowish area near the posterior pole. It marks the
location of the fovea centralis.
• Fovea centralis :
• a thin , rod-free , cone-packed area.
• Each cone synapses with a single bipolar cell , which in
turn synapses with a single ganglion cell.
• Contains NO blood vessels.
• Maximum visual acuity.
Retinal blood supply
• Retinal vessels : supply the bipolar & ganglion
cells.
• Choroid plexus : supplies the rods & cones.
Neural pathways.
• Axons of ganglion cells pass caudally , converge & form
the optic nerve.
• The optic nerve passes to the optic tract & ultimately
reaching the lateral genicualte body of the thalamus.
• Fibers from each nasal (medial) hemiretina decussate
in the optic chiasm .
• Fibers from the temporal ( lateral) hemiretina do not
decussate.
• In the geniculate body , fibers from one nasal
hemiretina synapse with the temporal fibers of the
other retina to form the geniculocalacrine tract.
• The geniculoclacrine tract passes to the primary visual
receiving area of the occipital lobe ( broadman’s 17 )
Receptors
• Each rod & cone is divided into :
1- an outer segment :
-made of modified cillia that form saccules & disks
.
- The saccules & disks contain photosensitive
compounds.
2- an inner segment :
-contains a nuclear region.
-rich in mitochondria.
3- a synaptic zone.
Receptors
• Rods have a thin , rod-like ( 27lf !) appearance
in the outer segment.
• Cones have thick inner segments & conical
outer segments ( 27lf marra thanya! )
• Rods predominate in the extrfoveal portions
of the retina.
Duplicity theory
• Rods :
- are extremely sensitive to light ( low threshold of
stimulation )
- Are scotopic ( scoto = no light ) : they are used in
night vision.
- Are incapable of resolving details or colors.
• Cones :
• - have a much higher threshold , & much greater
acuity.
• Are photopic ( photo = light ) : used for light &
color vision.
Principles of optics.
• Light rays are refracted when passing from
one medium to another due to the difference
in density.
• Parallel light rays striking a biconcave lens are
refracted to a point behind the lens , called
the principal focus.
Principles of optics
• Principle focal distance : is the distance between
the lens & principle focus.
• Biconcave lenses diverge light , biconvex ones
converge it.
• Diopters : are used to measure the refractive
power of a lens in meters
• They are the reciprocal of the principle focal
distance in meters.
• 1 Diopter = 1/ PFL
• The human eye has a refractive power of 6 m at
rest.
Principles of optics
•
•
•
•
Diopteric power of the eye :
Cornea = 40-45 D
Lens = 15-20 D
Accommodation +14 ( depending on age )
Accommodation.
• Is an active process that requires muscular
actions.
• At rest , objects closer than 6 m to the eye appear
blurred.
• This blurriness is diminished by accommodation.
• So accommodation is : the process by which the
curvature of the lens is increased in order to
focus on a near object.
• When looking at a near object , the cilliary
muscles contract , the distance between the
edges of the cilliary bodies decrease & the lens
ligaments relax . Thus , the lens becomes more
convex.
Accommodation
When the cilary muscles
are relaxed, the zonalus
pulls tight and keeps the
lens flattened for distant
vision
The elastic lens is
attached to the circular
cilary muscles by the
zonalus which is made
of inelastic fibres
When the cilary muscles
contract, it releases the
tension on the zonalus
and the elastic lens returns
to a more rounded shape
suitable for near vision
Near point
• Is the nearest point to the eye where an
object can be brought into clear focus by
accommodation.
• Recedes throughout life due to the gradual
hardening of the lens.
• This leads to presbyopia ( loss of lens elasticity
)
Near response
• Is a three-part response that consists of :
1- accommodation.
2- convergence of the visual axes.
3- pupillary constriction.
Pupillary reflexes.
• Pupillary light reflex :
When light is directed into the eye, its pupil
constricts.
• Consensual light reflex :
When light is directed into one eye, the pupil of
the other also constricts due to complicated
mechanisms.
• Pathway :
Retina ---- optic tract ---- superior colliculus ---occulomotor nucleus ---- pupillary muscles.
Argyll – Robertson pupils
• Seen in some pathologies such as
neurosyphyllis .
• Characterized by loss of light reflex but not
accommodation reflex.