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.