eye

MAIN STRUCTURES OF THE EYE

Sclera

Definition

White outer layer of eye.

Functions:

  • protection

  • maintains shape

  • muscle attachment


Cornea

Definition

Transparent front part of eye.

Functions:

  • allows light entry

  • major light refraction

  • fixed lens

IMPORTANT:
About 2/3 of refraction occurs here.


Iris

Definition

Coloured circular muscle.

Function:

  • controls pupil size


Pupil

Function

Controls amount of light entering eye.

Bright light:

  • constricts

Dim light:

  • dilates


Lens

Functions

  • fine focusing

  • accommodation

Held by:

  • suspensory ligaments

Controlled by:

  • ciliary muscle


AQUEOUS & VITREOUS HUMOUR

Transparent fluids in eye.

Functions:

  • maintain shape

  • provide nutrients

  • support structures


RETINA

MOST IMPORTANT STRUCTURE.

Functions

  • contains photoreceptors

  • converts light into electrical signals


OPTIC NERVE

Formed by:

  • retinal ganglion cell axons

Function:

  • carries visual information to brain


FOCUSING LIGHT

Light refracted at:

  1. cornea

  2. entering lens

  3. exiting lens

Lens provides:

  • precise focus onto retina


ACCOMMODATION

VERY IMPORTANT CONCEPT.

Definition

Changing lens shape to focus on objects at different distances.


DISTANCE VISION

Light rays:

  • almost parallel

Need:

  • less refraction

Ciliary muscles:

  • relaxed

Lens:

  • flatter


NEAR VISION

Light rays:

  • diverging

Need:

  • more refraction

Ciliary muscles:

  • contract

Lens:

  • rounder/thicker


REFRACTIVE ERRORS

Problems focusing light correctly onto retina.


PRESBYOPIA

Age-related hardening of lens.

Result:

  • reduced accommodation

  • poor near vision

Treatment:

  • bifocals/reading glasses


RETINA LAYERS & CELLS

The retina converts light into neural signals.


CHOROID

Vascular layer behind retina.

Functions:

  • nutrient supply

  • oxygen supply

  • absorbs excess light


RETINAL PIGMENT EPITHELIUM (RPE)

Functions:

  • absorbs light

  • supports photoreceptors

  • forms blood-retina barrier


PHOTORECEPTORS

Two types:

  • rods

  • cones


RODS

Functions:

  • dim light vision

  • night vision

Features:

  • very sensitive

  • monochromatic

  • poor acuity

Pigment:

Rhodopsin


CONES

Functions:

  • colour vision

  • high acuity vision

  • daytime vision

Three types:

  • red

  • green

  • blue

Pigment:

Iodopsin


FOVEA

Area of highest visual acuity.

Contains:

  • many cones

  • almost no rods


IMPORTANT COMPARISON

Rods

Cones

Dim light

Bright light

Monochrome

Colour

Poor acuity

High acuity

Peripheral retina

Fovea


Many rods → one ganglion cell

One cone → one ganglion cell at fovea


CONVERGENCE

Rods

Many rods connect to one ganglion cell.

Result:

  • better sensitivity

  • worse detail


Cones

Less convergence.

Result:

  • better visual acuity


VISUAL ACUITY

Definition

Ability to detect fine detail.

Highest at:

  • fovea


PHOTOTRANSDUCTION

MOST IMPORTANT EXAM SECTION.

Definition

Conversion of light into electrical signals.

Occurs in:

  • photoreceptors


RHODOPSIN

Contains:

  • opsin protein

  • retinal molecule


LIGHT ACTIVATION

Light converts retinal:

cis \rightarrow trans

This changes rhodopsin structure.


RESULT OF PHOTOTRANSDUCTION

Light →

rhodopsin activation →

changes Na⁺ permeability →

graded potential in photoreceptor →

action potentials in optic nerve →

brain interprets signal


BLIND SPOT

Occurs where:

  • optic nerve leaves eye

No photoreceptors present.


VISUAL PATHWAY

VERY IMPORTANT SEQUENCE.

Pathway

Retina →

optic nerve →

optic chiasm →

lateral geniculate nucleus (LGN) →

optic radiation →

primary visual cortex (V1)


OPTIC CHIASM

Partial crossing of fibres.

Allows:

  • visual field processing by opposite hemisphere


LGN

Located in:

  • thalamus

Function:

  • relay station for visual information


PRIMARY VISUAL CORTEX (V1)

Located in:

  • occipital lobe

Processes visual information.


TWO VISUAL STREAMS

Ventral stream

“What” pathway.

Functions:

  • object recognition

  • form recognition


Dorsal stream

“Where/how” pathway.

Functions:

  • movement

  • spatial awareness

  • eye/arm control


BINOCULAR VISION

Both eyes provide slightly different images.

Brain combines them for:

  • depth perception


RETINAL DISPARITY

Difference between images from left and right eye.

Used to judge:

  • distance

  • depth

Closer objects:

  • greater disparity


VISUAL DISEASES

SCLERITIS

Inflammation of sclera.

Symptoms:

  • pain

  • redness

  • tearing

  • reduced vision


CORNEAL ULCER

Open sore on cornea.

Causes:

  • infection

  • trauma

  • contact lens overuse

Can cause:

  • blindness


COLOBOMA

Hole in iris.

Congenital developmental problem.


CATARACT

Clouding of lens due to protein clumping.

Causes:

  • ageing

  • trauma

  • congenital disease

Treatment:

  • lens replacement surgery


AGE-RELATED MACULAR DEGENERATION (AMD)

Affects:

  • macula/central retina

Causes:

  • central vision loss


RETINAL DETACHMENT

Retina separates from underlying tissue.

Medical emergency.


STRABISMUS

Misalignment of eyes.

Can cause:

  • amblyopia

  • loss of depth perception


NYSTAGMUS

Involuntary repetitive eye movements.


MOST IMPORTANT EXAM CONTENT

Prioritise:

  • eye structures + functions

  • accommodation

  • rods vs cones

  • fovea

  • convergence

  • phototransduction

  • rhodopsin

  • visual pathway

  • optic chiasm

  • LGN

  • V1

  • retinal disparity

  • cataracts

  • AMD

  • retinal detachment


ENTIRE VISION TOPIC IN ONE FLOW

Light enters eye →

cornea refracts light →

lens fine focuses image →

light reaches retina →

photoreceptors convert light into electrical signals →

retinal ganglion cells form optic nerve →

signals travel through optic chiasm + LGN →

visual cortex processes image →

brain interprets colour, motion, depth and form.