(7.2) Vision

Question 1

Briefly describe the layers that light have to travel through before being detected by photoreceptors. Each briefly describe its function.

Answer 1

1) Cornea - a refractive medium
2) Aqueous Humerus - controls intraocular pressure
3) Lens - contractions to allow accommodation by refracting light onto retina
4) Vitreous Humerus - holds retina in place

Question 2

Briefly describe the process of accommodation to adjust near vision.

Answer 2

1) Afferent info via Retinal Ganglion -> Optic Nerve -> Optic Chiasma -> Optic Tract -> Lateral Geniculate Ganglion -> Radiation -> medial Occipital Lobe
2) synapse BILATERALLY at Edinger-Westphal Nucleus with PNS pre-ganglionic fibres
3) PNS preganglionic fibres travel with Optic N (CN III)
4) synapse at Ciliary Ganglion with PNS post-ganglionic fibres, stimulating:
- Medial Rectus -> convergence of both eyes
- Constrictor Pupillae -> constricted pupils
- Ciliary Muscles -> fatten lens

Question 3

What components are involved in accommodation to adjust close vision.

Answer 3

REMOVAL of PNS stimulation, removing:

• convergence of eyes
• pupil constriction
• fat pupils

Question 4

What allows the consensual response in the contralateral eye in a light reflex & accommodation.

Answer 4

bilateral synapse at Edinger-Westphal nucleus

Question 5

What is Hypermetropia? What are the possible causes?

Answer 5

Far sighted due to image focuses BEHIND retina, possible causes:

• weak lens -> impaired dioptric strength
• eyeballs too small

Question 6

What is Myopia? What are the possible causes?

Answer 6

Short sighted due to image focuses IN FRONT of retina, possible causes:
- eyeballs too long causing normal dioptric strength too high

Question 7

What is Presbyopia?

Answer 7

Old sighted due to lens losing its elasticity, impairing dioptric strength , becoming far sighted
e.g. those who are short sighted in young life will improve over age

Question 8

What visions do Rods responsible for? How do they relay their stimuli to postsynamptic fibres?

Answer 8

• Light sensitive -> night vision

- Convergence of several rod cells to one bipolar neurone

Question 9

What visions do Cones responsible for? How do they relay their stimuli to postsynamptic fibres?

Answer 9

• Colour sensitive, day vision

- One to one synapase with bipolar cell

Question 10

What is Fovea? In terms of vision, how can you describe its quality, explain.

Answer 10

• The area where Cones concentrated at
• Takes up the biggest area in Lateral geniculate nucleus
• Maximal acuity

Question 11

Which structures detect the left visual field?

Answer 11

• Nasal hemiretina of left eye

- Temporal hemiretina of right eye

Question 12

What is the most common cause of Non-homonymous Bilateral Hemianopia? Explain why

Answer 12

Pituitary gland pathology e.g. Pituitary Adenoma

• Optic Chiasma located anterior to Pituitary Gland
• Damage axons from Nasal Hemiretinae from both eyes cross here

Question 13

What visual impairment may a lesion at Optic Chiasma cause? Explain why

Answer 13

• Tunnel Vision = Non-homonymous Bilateral Hemianopia

- Damage axons from Nasal Hemiretinae from both eyes, which cross at Optic Chiasma

Question 14

What do you call it when there’s a visual field defect that is surrounded by normal visual fields? What may be the possible causes?

Answer 14

Scotoma

• Non-pathological: Optic disc, where no photoreceptors located, retinal axons compose into Optic N exits
• Pathological: Retina/Optic N pathologies e.g. Vascular blockage, Multiple Sclerosis

Question 15

What structure do most of retinal ganglionic axons terminate? Where is it found?

Answer 15

Lateral Geniculate Nucleus found in Thalamus

Question 16

What are the two pathways in Optic Radiation? What types of vision does each responsible of?

Answer 16

• Magnocellular: detecting fast moving stimuli

- Parvocellular: detecting colour and acute vision

Question 17

What visual defect may result from a lesion at Right Optic Tract?

Answer 17

Homonymous Left Hemianopia

Question 18

What visual defect may result from a lesion at Left Geniculocalcarine Tract?

Answer 18

Homonymous Right Hemianopia

Question 19

Describe the pathway of Geniculocalcarine tract that carries fibres from the Superior retina.

Answer 19

1) arise from Dorsomedial area
1) through Parietal Lobe
2) directly via Internal Capsule
3) enters Visual cortex in Medial Occipital Lobe

Question 20

Describe the pathway of Geniculocalcarine tract that carries fibres from the Inferior retina.

Answer 20

1) arise from Ventrolateral area
2) through Temporal Lobe
3) loops around Meyer’s Loop of Lateral Ventricles
4) via Internal Capsule
5) enters Visual cortex in Medial Occipital Lobe

Question 21

What visual defect may result from a lesion at Right Meyer’s Loop of Lateral Ventricles?

Answer 21

Superior Left Homonymous Quadratanopia

Question 22

What visual defect may result from a lesion of Geniculocalcarine Tract at Left Parietal Lobe?

Answer 22

Inferior Right Homonymous Quadratanopia

Question 23

What visual defect may result from a lesion at Visual Cortex receiving Left Geniculocalcarine Tract from superior retina?

Answer 23

Inferior Right Homonymous Quadratanopia + Macular sparing

Question 24

Suggest 3 ways causing complete right side vision.

Answer 24

Pathologies in Left Eye or Left Optic Nerve:

• Enucleation (eyeball removal) of the left eye
• Left Optic Neuritis
• Trauma

Question 25

Describe the role of Superior Colliculus in vision.

Answer 25

• Receives inputs from Optic tracts -> orientate eyes towards stimuli
• Where Tectospinal tract arise -> allows head responds to visual stimuli by neck muscle movements

Question 26

What is located at the centre of retina?

Answer 26

Macula, with Fovea at its centre