Choroid, Iris and Lens

Question 1

Why are both layers of the iris epithelium pigmented?

Answer 1

Absorb all the stray light

Question 2

What sort of muscle is the iris dilator muscle and what is the orientation of the fibres?

Answer 2

It is a smooth muscle arranged radially and runs perpendicular to the pupil.

Question 3

What is the effect of the contraction of the iris dilator muscle on pupil size?

Answer 3

Results in pupil dilation - widens the pupil

Question 4

What is the orientation of the iris sphincter fibres and what is the effect of their contraction on pupil size?

Answer 4

It is a circular muscle and contraction leads to pupil constriction - decreases pupil size

Question 5

What neurotransmitters are involved at the iris sphincter and dilator muscles?

Answer 5

Iris Sphincter = adrenaline

Iris Dilator =

Question 6

Apart from muscle, what else do you expect to find in the iris stroma?

Answer 6

Loose connective tissue and widely spaced collagen fibres, fibroblasts and mast cells and blood vessels

Question 7

What makes up the anterior border layer of the iris? How is this different to most other biological borders?

Answer 7

Fibroblasts

-There arent any epithelial cells lining the surface

Question 8

What makes up the anterior border layer of the iris? How is this different to most other biological borders?

Answer 8

Fibroblasts

-There arent any epithelial cells lining the surface

Question 9

What makes irises different colours?

Answer 9

The varying number and thickness of melanocytes

Question 10

What is the nature and function of the pigment in the choroid?

Answer 10

To create a black box effect, minimising light scatter

Question 11

To what structure do the choroidal capillaries supply nutrients?

Answer 11

RPE

Question 12

How thick is Bruch’s membrane?

Answer 12

5 microns

Question 13

To what structures to the short posterior ciliary arteries supply blood?

Answer 13

They supply blood to the choroid and iris

Question 14

To what structures do the anterior ciliary arteries supply blood?

Answer 14

They supply blood to the superior, inferior and medial rectus as well as the conjunctiva and sclera

Question 15

What is the compostition of the lens capsule?

Answer 15

It is a smooth transparent basement membrane composed of super helical collagen fibres

Question 16

What gives the lens capsule its elasticity?

Answer 16

Due to superhelical (spring like structure) collagen

Question 17

What is the function of the lens capsule elasticity?

Answer 17

To allow the lens to change shape during accommodation

Question 18

How does the shape of the lens epithelium vary with location on the lens?

Answer 18

A the anterior pole, it is squamos, between the pole and the equator it is more cuboidal and at the equator its almost columnar

Question 19

What process accounts for the nuclear bow in the lens equator?

Answer 19

epithelial cells elongate to become lens fibres

Question 20

What happens to old lens fibres?

Answer 20

They remain in the lens, clump together and form opacities in the lens

Question 21

What structures do lens fibres form when they join together on the anterior and posterior surfaces?

Answer 21

Sutures

Question 22

Why is there a need for such a tight fitting organisation of the lens fibres?

Answer 22

So that all light passing through the lens is refracted

Question 23

Why the requirement for gap junctions in lens fibres?

Answer 23

To allow nutrients to pass from cell to cell in the lens - cytoplasmic continuity

Question 24

Whys is the stroma of the iris so insubstantial?

Answer 24

Widely spaced collagen fibres allow contraction