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

The elastic lens is attached to the circular

cilary muscles by the zonalus which is made

of inelastic fibres

When the cilary musclesare relaxed, the zonaluspulls tight and keeps thelens flattened for distant

vision

When the cilary musclescontract, it releases the

tension on the zonalusand the elastic lens returns

to a more rounded shapesuitable 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.