The visual system

Question 1

What are the 3 coats of the eye?

Answer 1

• Outer = cornea, sclera
• Middle (Uvea) = iris, ciliary body, choroid
• Inner = retina

Question 2

Describe the sclera, what is its function

Answer 2

• Protection

- Tough outer collagen layer

Question 3

Describe the cornea, what is its function

Answer 3

• Transmission of light
• Important for refraction
• must be transparent, smooth spherical surface
• allows attachment of extra-ocular muscles
• trigeminal nerve (400 X that of skin)
• epithelium must last a lifetime
• endothelium continuously replaced by Limbal stem cells

Question 4

Describe the iris, what is its function

Answer 4

• Contains iris colour
• contains pupil
• sphincter muscle
• dilator muscle
• pupil reflexes, light/dark, near

Question 5

Describe the ciliary body, what is its function

Answer 5

-Glandular epithelium
Production of aqueous humour
Nutrients for cornea and lens
Maintains intraocular pressure (15 mmHg)

-Smooth muscle
Controls accommodation (12 dioptre)
Adjustment of the lens in the eye so that clear images of objects at different distances are formed on the retina

Question 6

What does parasym/sym innervation to the iris do?

Answer 6

Sphincter muscle: parasympathetic innervation = lets less light in

Dilator muscle: sympathetic innervation = more light is allowed in

Question 7

Describe the choroid body, what is its function?

Answer 7

Nutrition of outer retina (photoreceptors)
Heat sink
Absorbs stray photons

Question 8

Describe the retina, what is its function?

Answer 8

• Complex layered structure, 10 layers
• light sensitive at the back of the eye
• contains the ‘specialised cells’ that are responsible for convertinf light energy into cell membrane potential for neurotransmission

1. Retinal pigment epithelium
2. Photoreceptor
3. Ganglion cell
4. Nerve fibre

Question 9

Describe cones

Answer 9

Cones

• colour vision R, B, G
• daytime vision
• fast response
• contains iodopsin

Question 10

Describe rods

Answer 10

Rods

• monochromatic vision
• night time vision
• more photopigment discs than cones
• slower reaction than cones
• contains Rhodopsin (opsin and retinal)
• mostly peripheral vision

Question 11

What happens when photoreceptors are struck by a photon?

Answer 11

Photoreceptors contain rhodopsin (rods) or iodopsin (cones), this is the photosensitive pigments that have the cis isomer of retinal bound

-When struck by a photon, the cis retinal is converted into the trans form and activation of chemical signalling cascades involving cGMP, resulting in hyperpolarisation of the cell

Question 12

Describe the aqueous humour what is its function?

Answer 12

• Mainly composed of water and electrolytes
• Anterior chamber
• Ciliary epithelium
• Circulates around lens, through pupil, exits through trabecular meshwork
• Maintains intraocular pressure needed to inflate the eye and lens
• provides nutrients for the lens and central cornea as they do not have their own blood supply

Question 13

Describe the Crystalline lens

Answer 13

• Transparent biconvex structure
• functions - maintain its own clarity. to refract light. to provide accommodation.

Fibres
Anterior epithelium
Elastic capsule
Zonular fibres

Question 14

Describe the eyelids

Answer 14

Eyelids
-Protective function
Anterior skin
Eye lashes
Meibomium glands (lipid)
Posterior mucous membrane
Firm tarsal plate
Orbicularis oculi (closure)
Levator palpabrae superioris (LPS) and sympathetic muscle (opening)

Question 15

Tears

Answer 15

• 3 layers
  1. Anterior lipid (meibomium glands) - oil layer, line upper and lower eyelids
  2. Middle aqueous (lacrimal glands) water layer, under orbital rim bone below eyebrow
  3. Posterior mucous (microscopic goblet cells in the conjunctiva)
• Protective
• Antibacterial
• Smooth surface
• Nutrition to cornea

Question 16

Describe the blood supply to the eye

Answer 16

1. Internal carotid artery
   =Ophthalmic artery, gives off central retinal artery (most important as supplies internal surface of the retina, occlusion=blindness), branches into superior and inferior, temperal and nasal branch retinal arteries

Optic nerve
Ciliary arteries (anterior and posterior)
Lacrimal artery
Ethmoid and eyelid

2. External carotid
   Facial artery
   angular artery supplies medial lid and orbit

Question 17

Describe venous drainage of the eye

Answer 17

Venous drainage of the eyeball is carried out by the superior and inferior ophthalmic veins. These drain into the cavernous sinus, a dural venous sinus in close proximity to the eye.

Question 18

Describe the path of a photon in the eye

Answer 18

Tear film (transmission)
Cornea (transmission and refraction)
Aqueous (transmission)
Lens (transmission and refraction)
Vitreous (transmission)
Inner retina (transmission)
Outer retina (transduction)
RPE/choroid (absorption)

Question 19

Why is it important that the extra-ocular muscles move the eyes in synchrony

Answer 19

The extra-ocular muscles move the eyes in synchrony to maintain binocular single vision and allow depth perception (stereopsis)

Question 20

Which two layers refract light?

Answer 20

• Lens

- Cornea

Question 21

What is the fovea centralis?

Answer 21

-Depression in the centre of the retina containing a high concentration of only cones

Question 22

Describe the extra ocular muscles of the eye

Answer 22

-4 Recti
medial rectus - adduction
lateral rectus - abduction CN6
superior rectus - eleveation, intorsion, adduction
inferior rectus - depression, intorsion, adduction
-2 Obliques,
superior oblique - intorsion, depression, abduction CN4
inferior oblique - extorsion, elevation, abduction

(down and out) if CN3 damaged

Question 23

Describe innervation of the extra ocular muscles

Answer 23

• lateral rectus CN6
• superior oblique CN4
• the rest CN3

(down and out) if CN3 damaged

Question 24

What is the optic chiasm?

Answer 24

• Panoramic defence

- mechanism for the fusing of two images to prevent double vision

Question 25

What are the dorsal and ventral streams?

Answer 25

DORSAL - ‘where’ pathway
puts objects in spatial location
from primary visual cortex in occipital lobe to the parietal lobe (spatial

VENTRAL - ‘what’ pathway
object and visual identification and recognition
- has strong connections to the medial temporal lobe (which stores long-term memories), the limbic system (which controls emotions), and the dorsal stream (which deals with object locations and motion).
-enters via primary visual cortex

Question 26

Where are the face-selective regions?

Answer 26

• Fusiform face area
• Occipital face area
• Superior temporal sulcus

Question 27

What does the ‘vestibular’ movement of the eye do?

Answer 27

Vestibular - holds images of the seen world steady on the retina during brief head rotations of translations

Question 28

What does the ‘visual fixation’ movement of the eye do?

Answer 28

Visual fixation holds images of the seen world steady on the retina during sustained head rotation

Question 29

What does the ‘smooth pursuit’ movement of the eye do?

Answer 29

Smooth pursuit holds the image of a small moving object steady on the fovea

Question 30

What does the ‘nystagmus - quick phases’ movement of the eye do?

Answer 30

-This resets the eyes during prolonged rotation and directs the gaze towards the oncoming visual scene

(nystagmus is also a pathological condition)

Question 31

What does the ‘saccades’ movement of the eye do?

Answer 31

Saccades - brings images of objects of interest onto the fovea

Question 32

What does the ‘Vergence’ movement of the eye do?

Answer 32

Vergence - moves the eyes in opposite directions so that the images of a single object are placed or held simultaneously on fovea of each eye

Question 33

What is hemianopia?

Answer 33

Blindness is over half field of vision

eg left homonymous hemianopsia- when L side of each eye is damaged to vision is on right side in each eye

Question 34

What is a heteronymous hemianopsia?

Answer 34

A Heteronymous hemianopsia is the loss of half of the visual field on different sides in both eyes. It is separated into two categories:

Binasal hemianopsia - The loss of the fields surrounding the nose;
Bitemporal hemianopsia - The loss of the fields closest to the temples.

Question 35

What is superior and inferior hemianopsia?

Answer 35

Superior hemianopsia: the upper half of the field of vision is affected;
Inferior hemianopsia: the lower half of the field of vision is affected.