iris (week 12)

Question 1

iris

Answer 1

adjustable diaphragm around a central aperture/pupil

has a double pigmented epithelium

Question 2

iris location

Answer 2

anterior and continuous with ciiary body

posterior to cornea

divides anterior and posterior chambers of aqueous filled anterior segment

front of lens rests partilally on posterior of iris

Question 3

2 strutural regions of iris

Answer 3

outer ciliary zone

inner pupillary zone

where they merge = collarette

Question 4

how does iris thickness change at diff regions

Answer 4

collarette = thickest

iris root = thinnest
(where iris joins ciliary body)

Question 5

pupillary ruff

Answer 5

posterior pigmented epithelium JUST visible from front of eye

peeps out at pupillary margin

Question 6

why is the iris double pigmented epithelia

Answer 6

one layer in ciliary body contributes to black box effect

but iris main role is to stop all light entering eye from regions other than pupil = melanin absorbs all light

Question 7

iris stroma structure

Answer 7

lies anterior to double pigmented epithelium

composed of loose aerolar connective tissue
allows iris to change shape

contains vasculature of iris

Question 8

stromal cells in iris

Answer 8

`loose aerolar connective tissue
fibroblasts
loose collagen
blood vessels
melanocytes

CLUMP CELLS:
macrophage like with more melanin than melanocytes

keeps aqueous full of debris by ingesting melanocytes etc

Question 9

iris dilator muscle structure

Answer 9

runs along posterior 2/3rds of iris and radially around the pupil

dilates pupil - mydriasis

the muscle (actin/myosin) contained within anterior cell layer of double pigmented epithelium

dilator muscle = myoepithelial muscle (muscle made of epithelial cells)

Question 10

how is the iris adpted to withstand stress during dilation/constriction

Answer 10

epithelial cells are attached mechanically by interdigitating microvilli and intercellular junctions eg: desmosomes

Question 11

how is iris dilater muscle innervated

Answer 11

smooth muscle so sympathetic component of ANS
(fight flight wide eyed fear)

Question 12

why are iris dilator and sphincter muscles spindles held by junctions

Answer 12

each muscle spindle isnt long enough to reach the full length from iris root to pupil margin

multiple spindles joined by tight/gap junctions forming syncytium contracting simultaneously

Question 13

iris sphincter muscle

Answer 13

circular muscle around the pupil located within the stroma of iris

embryologically derived from the epithelial cells

Question 14

sphincter vs dilator

Answer 14

sphincter = parasympathetic
dilator = sympathetic

sphincter = gap between muscle and epithelial cell
dilator = no gap

Question 15

anterior surface of the iris stucture

Answer 15

composed of anterior border layer (not epithelial cells)

made of dense sheet of fibroblasts covering melanocyte layers
so highly textured

Question 16

why is the iris anterior surface highly textured

Answer 16

3 causes

crypts of fuchs
diamond lacuna near the collarette

pigment spots = regions with more/less melanocytes

contraction furrows = deep depressions that lie around the outer periphery of the iris

more unique than fingerprint

Question 17

main blood supply of iris and stucture

Answer 17

major iridic circle

artery running circular around iris circumferance near iris root (Like M25)

smaller spokes run off from the major iridic circle towards pupil - straight/wavy

near collarette lies a second incomplete arterial circle - minor iridic circle (mid way iris)

smaller arteries then continue to the pupil margin before veins then return directly back up the iris (straight)

Question 18

fluorescein angiography finds:

Answer 18

iris vasculature viewed in vivo using fluorescent dye

doesnt leak as no fenestrated capillaries

thicker artery walls due to stress of constriction/dilation

Question 19

how and what is the dilator muscle innervated by

Answer 19

innervated by spindal cord via superior cervical ganglion lies deep in carotid artery of neck

sympathetic so ganglia closer to CNS than effector

preganglionic fibres = spine -> SVG
post ganglionic = SVG -> dilator muscle

Question 20

how and what is the sphincter muscle innervated by

Answer 20

sphincter (and ciliary muscle) innervated by midbrains edinger westphal nucleus via ciliary ganglion

preganglionic fibres = midbrain -> CG through oculomotor nerve

postganglionic = CG -> sphincter muscle

(part of near triad so constriction involved in accomodation)

Question 21

factors determining pupil size

Answer 21

light levels (pupillary light reflex diagnosis disorders)

accomodation reflex

drugs

pain cognition