Eye and vision

Question 1

Vision - what it does

Answer 1

• Detection of light photons on a 2D sensory surface
• Transformation of 2D images to 3D
• Information segregation (depth, colour, form, and motion)
• Create a unified perception
• Pattern recognition
• Localisation

Question 2

The five P’s

Answer 2

Remember the 5Ps: 
– Pain
– Photophobia
– Poor vision
– Pus
– Pupillary abnormality

Question 3

Tear production

Answer 3

1. Lacrimal gland produces and secretes tears
2. Tears flow down and across eyeball
3. Tears enter lacrimal canaliculi and drain into lacrimal sac
4. Tears empty into nasal cavity via nasolacrimal duct

Question 4

Parts of the skull

Answer 4

Frontal
Maxilla
Sphenoid
Ethmoid
Lacrimal
Zygomatic

Question 5

Anatomy of the eye

Answer 5

• Two cavities
– Anterior segment (aqueous humor)
• medium for diffusion of gases & nutrients
• maintains intra-ocular pressure
• recycled & regenerated

– Posterior segment (vitreous humor)
• maintains intraocular pressure
• provides structural support
• formed in embryo, lasts for life

• Three tunics (layers)
– Fibrous
– Vascular
– Neural

Question 6

Clinical pearl: Glaucoma

Answer 6

• Aqueous humor
– Produced by ciliary body
– circulates around anterior structures (lens & cornea) – drains into venous blood via scleral venous sinus

• Glaucoma
– Failure to drain aqueous humor
– ↑↑ intra-ocular pressure, compression of retina and optic nerve
– Blindness over time (degradation of retina)
– Symptoms include seeing halos around lights, blurred vision

Question 7

Layers of the eye I: Fibrous Tunic

Answer 7

• Sclera
– White of the eye
– Tough fibrous layer
– Structural support
– Anchor point for extrinsic muscles

• Cornea
– Anterior, transparent portion of sclera
– No blood vessels
– Highly innervated
– Epithelium heals from the limbus

Question 8

Layers of the eye II: Vascular Tunic

Answer 8

Middle zone
– Choroid
• Rich in blood vessels
• O2 & nutrients to retina
• Melanocytes – stops light scattering

– Iris
– Ciliary body
• Coordination of lens shape

Question 9

Iris and the pupillary reflex

Answer 9

• Controls amount of light into the eye (pupil)
• Two rings of muscle
– Pupillary constrictors (longitudinally arranged) – Pupillary dilators (radially arranged)

Controlled by ANS
Low light: SNS - pupil dilates

High light: PNS - pupilconstricts

Question 10

Ciliary body

Answer 10

• Lens attached to ciliary body muscles via suspensory ligaments
• Contraction and relaxation of ciliary body alters lens shape
• Under ANS control

Question 11

Layers of the eye III: Neural Tunic

Answer 11

Retina
– Neural layer:
• Rods and cones - light detection 
• Horizontal cells - signal processing 
• Bipolar cells - signal processing 
• Amacrine cells - signal processing 
• Ganglion cells - signal processing 

  – Pigmented layer
• Stray light absorption
• Phagocytosis of dead
photoreceptors (endocytosis)
• Nutrient delivery from choroid

Optic nerve (axons of ganglion cells)

Question 12

The fovea

Answer 12

Fovea:
• Contains just cones (light passed directly to photoreceptors)
• Point of highest visual acuity

Macula:
• Area immediately surrounding fovea
• Cones, bipolar and ganglion cells

Question 13

Blind spot

Answer 13

Optic disc = blind spot
• Point on the retina where optic nerve and major blood vessels exit.
•No rods or cones, no visual processing

Question 14

Macular degeneration

Answer 14

– Progressive, painless loss of foveal vision.
– Build up of waste products, blood vessels grow into retina or pigment epithelium cells die.
– No cure.

Question 15

Retinal detachment

Answer 15

– Trauma or sudden jerk of head.
– Retinal tearing or separation of pigmented and neural layers (detachment).
– Surgical reattachment if diagnosed early

Question 16

Extraocular muscles

Answer 16

LR6, SO4 and all the rest CN3

• 2 recti
– Medial
– Lateral 
– Superior 
– Inferior

• 2 obliques
– Superior
– Inferior

Question 17

Refraction and projection

Answer 17

• Refraction – bending of light rays passing through different mediums (e.g. from viewed object, through the eye).
• Cornea and lens – primary refractive structures that bend incoming light rays.
• Cornea – most contribution to total refractive ability.
• Lens – adjustable contribution of refractive strength.

Question 18

Lens function in near and far vision - distant

Answer 18

Distant light source

• parallel rays of distant light source
• sympathetic stimulation
• relaxed ciliary muscle
• flattened, weak lens
• taut, suspensory ligaments

Question 19

Lens function in near and far vision - near

Answer 19

Near light source

• parasympathetic stimulation
• contracted ciliary muscle
• rounded, strong lens
• slackened suspensory ligaments

Question 20

Accommodation…

Answer 20

Accommodation: increasing the refractive power of the lens to focus a near-source image on the fovea

Question 21

Refractive errors of vision - Presbyopia:

Answer 21

Age-related reduction in accommodation ability of lens

Question 22

Refractive errors of vision - Myopia

Answer 22

(near sightedness) and Hyperopia (far sightedness) alter how and when accommodation is used.

Question 23

Myopic (nearsighted) eye

Answer 23

Eyeball too long or lens too strong

Question 24

Hyperopic (farsighted) eye

Answer 24

Eyeball too short or lens too weak

Question 25

Neural Retina

Answer 25

Neural portion of retina consists of 3 main layers of excitable cells:
– Photoreceptor layer (outermost) contains rods and cones.
– Middle layer contains bipolar cells.
– Inner layer contains ganglion cells.

• Rodsandcones absorb light.
• Bipolar cells connect photoreceptor cells with ganglion cells.
• Axons of ganglion cells join to form optic nerve.

Question 26

Photoreceptors

Answer 26

• Sensitivity of photo receptor type linked to its functional role (night vision vs acuity).
• Acuity dictated by extent of convergence onto ganglion cells.
– Many vs few receptors per ganglion cell

Question 27

Photoreceptor structure & function

Answer 27

Rods and cones consist of three parts:

– Outer segment:
• Light-absorbing photopigments

– Inner segment:
• Metabolic structures

– Synaptic terminals:
• Release of neurotransmitter onto
bipolar cells
• Complex pattern of NT release,
depends on dark or light exposure of outer segment

Light detected by photopigments
– Two parts: Opsin (transmembrane protein) & retinal (derived from vitamin A)
– Retinal changes confirmation when light is absorbed
– 4 types of Opsinà7 million colours!!

Question 28

Head trauma & visual dysfunction

Answer 28

Two streams of information processed:
– What (ventral) for colour and forms
– Where/how (dorsal) for motion & disparity

Damage to regions of occipital lobe, parietal and temporal lobes, or along any section of the neural pathway can have wide-ranging effects on visual processing and perception!