Visual system

Question 1

What structures compose the external eye?

Answer 1

• upper and lower eyelids
• palpebral fissure (between upper+lower eyelids?)
• lateral and medial canthus (corners of eye)
• caruncle at medial canthus
• pupil
• iris
• limbus (border between cornea, which covers iris+pupil, and the sclera, which)
• sclera
• conjunctiva (covers sclera)

Question 2

What are the 3 kinds of tear?

Answer 2

basal, reflex and emotional (crying)

Question 3

Where is the lacrimal gland located?

Answer 3

within the orbit, latero-superior to the globe

Question 4

When is the basal tear produced?

Answer 4

• produced at a constant level all the time

- even in absence of irritation or stimulation

Question 5

When is the reflex tear produced?

Answer 5

• reflex tear is increased tear production

- in response to ocular irritation

Question 6

What structure produces tears?

Answer 6

the lacrimal gland

Question 7

What is the tear reflex pathway?

Answer 7

afferent: cornea, innervated by sensory nerve fibres via ophthalmic branch of trigeminal nerve (V1)
- -> relays signal to CNS
efferent: parasympathetic nerve, which innervates the lacrimal gland
- -> tear film drains through the 2 puncta (tiny openings on upper+lower medial lid margins)
- -> flow through inferior+superior canaliculi into the tear sac
- -> tear finally drained out of tear sac into nasal cavity through tear duct

Question 8

What are the functions of tear film?

Answer 8

N.B. healthy cornea constantly covered by tear film

• maintains a smooth cornea-air surface
• removes surface debris during blinking
• source of oxygen and nutrient supply (normal cornea has no blood vessels)
• kills bacteria

Question 9

What are the 3 layers of the tear film and their respective roles?

Answer 9

• mucin layer (binds water molecules to the hydrophobic corneal epithelial cell surface)
• water layer (main bulk of tear film)
• lipid layer (outer layer, preventing tear film from rapid evaporation)

Question 10

Where is the lipid layer of the tear film produced?

Answer 10

Meibomian glands along the eyelid margins

Question 11

What is the conjunctiva?

Answer 11

• thin, transparent tissue that covers the outer surface of the eye
• begins at outer edge of cornea, covers visible part of eye and lines inside of eyelids
• nourished by tiny blood vessels that are nearly invisible to naked eye

Question 12

What are the 3 layers of the coat of the eye?

Answer 12

• sclera: outer fibrous opaque layer; protects and maintains shape of eye
• choroid: middle pigmented vascular layer; provides circulation to eye and shields from unwanted scattered light
• retina: inner neurosensory layer; responsible for converting light to neurological impulses, which are transmitted to brain via optic nerve

Question 13

What is the sclera?

Answer 13

• ‘white of eye’
• tough, opaque protective outer coat of eye
• high water content

Question 14

What is the cornea?

Answer 14

• transparent dome-shaped window covering front of eye
• low water content
• powerful refracting surface providing 2/3 of eye’s focusing power
• https://www.notion.so/Visual-System-7b20fa8802da426abd53060670a1380f