Function Of The Iris Flashcards
Which of the following is not a function of the iris
Reduce depth of focus
The pupil that is seen is actually a
Magnified virtual image of the real object
Most anterior portion of uveal tract
Iris
What does the iris separate
The anterior and posterior chambers of the eye
What is the iris surrounded by
Aqueous
First relationship with the lens
Rests lightly upon the lens
- not flat
- shallow truncated cone
Magnification of the iris
Magnified by 1/8th when viewed through cornea
Diameters of iris
12mm white to white
Thickness at the thinnest point, the iris root
500microns
How is pupil centered
Slightly nasal and inferior to iris center
Abnormally decentered pupil is
Ectopic
Optical quality in irregular or ectopic pupils
Degrades it
What does the pupillary margin rest on
Anterior surface of the lens
How does the aqueous humor flows
From posterior into anterior chamber through the pupil
Pupil fluctuation
In constant state of dynamic fluctuation, always changing size
Stimulus variables that contribute to the constant state of fluctuation of the pupil
- light level
- spectral composition
- spatial configuration: field size, spatial structure of the field
- monocular/binocular view
- non visual stimuli: pain and noise
Observer variables that contribute to the constant fluctuation of the pupil
- individual differences
- age
- day to day within observer variance
- biomechanical factors: respirations, heart beat
- cognitive factors: arousal, attentions, fright, workload, hedonistic content
Pupils in other animals
Varies in other animals based on their environment and visual needs
What part of the iris is broader in humans
Temporal iris in humans broader than nasal iris
Giant squid eyes
10.5” across and 3.5” pupil filled with water
What animal has the largest eyes
Giant squid
Largest eyes relative to body size
Tarsier
Posterior portion of the iris is derived from
Neuroepithelium
What iris structures are derived from the neuroepithelium
- dilator muscle
- sphincter muscle
- posterior pigmented epithelium
Anterior portion of the iris is derived from
Mesoderm
Iris structures derived from the mesodermal
- iris stroma
- iris vessels
- iris nerves
- chromatophores
The different embryonic tissue can cause
Anterior and posterior portions are torn apart
4 layers of the iris
Anterior border layer
Stroma and sphincter muscle
Anterior epithelium and dilator muscle
Posterior epithelium
Which layer of the following contains both mesoderm and ectoderm
Stroma and sphincter muscle
Functions of the pupil
- control of rental illumination
- reduction in optical aberrations
- depth of focus
Need 2mm pupil for 2 out of these 3 functions
Functions of the iris
- regulates retinal illumination
- regulates glare
- regulates optical aberrations
- regulates depth of focus
- provides nonverbal communication and social signaling
- attraction/mate selection
What is probably the number one reason for the pupil changing shape as much as it does
Communication and social signaling as well as mate selection/attraction
Dilator muscle attaches to these at the collarette
Michels spur
Fuchs spur
Only elements that are capable of initiating movement in the iris
Iris muscles
All other elements of the iris except the muscles when it comes to movement
Are constructed so as to adapt to the movement of these muscles
The iris in miosis
The pupillary portion of iris gathers into pleats while the culinary portion smooths out
The iris in mydriasis
The stroma in the pupillary Zeno pulls flat while contraction furrows appear in the ciliary area
Blood flow of the iris in the miosis or mydriasis
Blood flow is not impeded by wither miosis or mydriasis
Elasticity of stroma and iris
Not elastic!
Stroma is loose but orderly mesh work of collagen fibers with aqueous fluid. It compresses and expands easily like a sponge
Iris vessels in movements
Are thin walled layer of endothelium with an outer covering of collagen fibrils. Keeps it from pinching the vessels
When was the sphincter muscle discovered
Middle of the 19th century
When was the dilator muscle discovered
Late 1950s with electron microscope
Thickest part of the iris
Stroma
Collagenous network of fibrils with scissor-like angles for folding
Stroma
Stroma and movement
No active part in iris movement
What does the stroma proved anchor for
- sphincter muscle
- dilator posterior epithelial plate
- iris nerves
- iris vessels
Protects the nerves and vessels from stretching and kinking
Stroma
Adapts to constant movements
Stroma
Folds and unfolds with minimal resistance to minimize energy expenditure
Stroma
Identity and characteristics of the iris during movements
All elements of the iris system maintain their identity and characteristics across millions of iris movements over decades of time
Constructed more densely than deeper stroma
Anterior border layer
Porous
Anterior border layer
Fibroblasts interlaced with melanocytes
Anterior border layer
More solid and dense with more layers in darkly pigmented eyes
Anterior border layer
Where do you see nevi on the iris
Anterior border layer
Compact masses of rounded cells filled with melanin
Anterior border layer
What layer may require a stronger laser in someone with dark eyes
Anterior border layer
Causes of blue eyes
Just as light scatter in the atmosphere or in water can give a blue appearance, light can scatter in uniform uniformly space stromal tissue to give a blue appearance
Newborns iris
Have a blue iris which changes darker because of the development of anterior stromal melanocytes and production of pigment granules can lead to a darker color
Which iris color is dominant
Dark, light colors are recessive
Blood flow to the iris is derived from
Tow long posterior cilioary arteries and anterior ciliary arteries
Iris bleeding
Does not bleed much when it is cut, maybe because severed ends retract slightly to heal wound
Major and Minor arterial circles
Probably not continuous. Anterior ciliary arteries probably bypass the major arterial circle to send radial branches to the iris directly
How do the anterior ciliary arteries run
Along the reclusive muscle. Two in each muscle except the lateral rectus which has only 1
Could lose blood supply is rectus muscles are severed
Surrounding sheaths of vessels
Not part of the vessel
What is unique to the iris vessels
Pervascular collagen sheaths
-they are not found anywhere else’s in the body
What does the perivascular sheaths on the blood vessels of the iris do
Allows for little dissension and folding of vessels during constant movement of iris
Light reflex of the iris
Some animals have photosensitive irises, but humans react to light stimulation of the retina rather than the iris
What is light reflex important for
To discover the integrity of the retina, optic nerves, chiasam, and optic tracts
What variables can effect the pupil
Light intensity State of light adaptation Temporal frequency Stimulus duration Retinal area Perimetric location Spectral sensitivity Spatial Frequency Motion
Scotopic vision and pupil size
Doesn’t change the pupil size that much if darkness increases if already under scotopic conditions
Mesopic light and pupil size
Causes a lot of change with changes in illumination
Photopic conditions and pupil size
Doesn’t change much with illumination if already in photopic lighting
What kind of curve do we see with pupil size and mesopic, photopic, and scotopic lighting
S curve
In _______, pupillaryreactions occur at intensities below photopic range
Dark adapted eyes
Paradoxes and peripheral more sensitive than fovea
In ________ adapted eyes, the pupillary threshold is much higher
Light
Fovea more reactive than periphery
Pupil constriction speed
Does not depend on the level of brightness
- max contraction amplitude reached
- pupil remains in spastic contraction for a few seconds
When does the pupil return to normal size
3-15 seconds
Stimulus duration and pupil
Pupil contracts more for a longer duration stimulus
Retinal area and pupil
The pupil more constricted when light covers greater area when light is equally bright
Perimetric location and pupils
Pupils sum affront impulses regardless of spatial distribution, both rods and cones
Stimulus wavelength and pupil
Experiments show that spectral sensitivities match well with pupillary reactions
Spatial Frequency and pupil
Increased SF=decreased pupil size
Motion and pupil
The pupil may respond to a motion stimulus even under isolimunant conditions
Myopia and pupils
Myopes have larger pupils
-enhanced by greater corneal refractive power and deeper anterior chambers
Myopes and accommodation
Don’t need to accommodate as much
-less than 1% variability based on ametropia
Larger pupils and images
Result in degradation and glare, common after refractive surgery
Pupils that are too small (less than 2mm)
Diffraction limit and reduced illumination can effect visual performance
Optimal pupil size
Around 2mm
Pupil size and age
DECREASES
Pupils role in retinal illumination
Minimal role
16 fold change in area
Eye can change with a factor of 10^9, those are quite different numbers
Retina is more responsible
Cat pupils
Large range of pupil size 0.5mm-13mm
What limits round pupil from getting miosis less than 1mm
Mechanical
- iris tissue is not elastic, it is folding
- containers a meshed network of fibers
Near vision triad
Convergence, miosis, and accommodation
-miosis can be stimulated with either convergence or accommodation in the absence of the other
What is the accommodative triad controlled by
Third nerve nucleus, eddinger-westphal
Amplitude for young person equal to what
Contraction from bright light
Less extensive for older who have los accommodation
Fairly linearly
What are the sphincter and dilator muscles derived from
Neural ectoderm
Iris sphincter muscle innervated by
Parasympathetic
No sympathetic innervation but beta adrenergic receptor sites can inhibit function and cause relaxation
Iris dilator muscle innervated by
Sympathetic
Some inhibtoryu effect of cholinergic net which innervated dilator muscle
How thick is the iris dilator
3-5 layers
Which iris muscle is the strongest
Equally strong
Reflex dilation
Sensory, emotional, or mental stimulus tend to elicit pupillary dilation
When are pupils smaller
During sleeep and after death
When are pupils large
During waking hours and when frightened
What animal has the most dramatic reaction in pupil reflex dilation
Cats
Alphagan and pupils
Can cause miosis under scotopic conditions. A2 adrenergic action might have inhibitory affect on A1 receptors. Some CNS effect might also be involved
CNS and pupillary reactions
Fatigue, sleep and arousal contribute to pupillary reactions causing dilation or constriction
Drugs treating the CNS and pupils
Usually have pupillary effects
Disease affecting CNS and pupils
Affect the pupils
Habituation
Some stimuli become less arousing over tine, less pupillary reaction
Sensitization
Others stimuli become more infuriating and develop greater pupillary reaction
Theories for reduction in pupil size with age
- reduced strength of dilator muscle
- greater resistance in iris stroma
- iris fatigue
- fatigue in CNS
Response to mydriatic drugs in older eyes
Still able to dilate fully
Shape of pupil and age
Can be round and smooth with advanced age
Light reflex in age
Can remain brisk into late Middle Ages
Reduction in pupil size is a _____ process
Linear
Thickness of dilator muscle and age
No change
Decreases in pupil size beings in what kind of eyes
Healthy eyes with strong levels of accommodation and brisk reactive pupils
Presentation of hornets syndrome
Small pupil
Drooping lid
Lack of sweating
Damaged sympathetic on one side
Cocaine test is usedfor what
Testing for horners syndrome
- one drop of 10% cocaine in each eye
- check in 15 minutes
- horners pupil dilates less than normal pupil
- cocaine blockers the reuptake of post ganglionic NE resulting in the dilation of a normal pupil from retention of norepinephrine in the synapse
High levels of infrared and iris
Can burn any ocular structure and have very deep penetration
What layer of the iris absorbed heat
Black pigment of the posterior iris layer
When stimulated directly by heat, what does the iris sphincter do
Contracts
What is a feature of iritis
Long lasting miosis
-mediated by prostaglandins
Microwaves and eyes
Miosis lasting several days
Ultrasonic energy and eyes
Dilation
