Chapter 10-Iris and Pupil
I. FUNCTION
Key Terms:
Aperture – a hole limiting the amount of light passing along an optical pathway placed near or
at the lens
Anisocoria – a detectable difference in pupil diameter; pupils normally same size and move in
synchrony in reflex responses and during normal hippos; normal pupils are circular but
decentered toward nose about 0.5 mm; pupils decentered more than 0.5 mm are considered
ectopic.
Blur circles – caused by out of focus objects; formed on the image plane when the object is
focused behind or in front of the image plane
Stop – all optical systems have stops, they affect image intensity, image spread, field of view
and depth of focus; effect depends on location in optical system.
Aperture stop – immediately in front of lens; limits the amount of light, but has very little effect
on the field of view, permits even the most oblique rays to enter; iris acts as an aperture stop.
Field stop – a hole limiting the amount of light passing along an optical pathway located some
distance from lens, greatly affects field of view, since many oblique light rays cannot pass
through to the lens
Diffraction – bending of light at the edges of an aperture, an optical problem
Chromatic aberration – light of different wavelengths (i.e. colors) is refracted, or bent,
differently by a lens, resulting in different focal points for each color, an optical problem
Spherical aberration – changes in focal points for light rays entering off axis in a lens, an
optical problem
Depth of field – range of object position over which acceptable focus is achieved, pertains to
object space (i.e. in your environment)
Depth of focus – range of image position over which focus is achieved, pertains to the image
space (i.e. in the retina); smaller pupil=larger depth of focus.
Pupil/Iris Aperture – dynamically reacts to ambient lighting conditions to adjust retinal
illumination
Near response triad – simultaneous constriction of pupils, vergence, and accommodation
when viewing near objects
Vergence – eyes rotate nasally to align target on fovea
Accommodation – Changes of lens shape to provide dynamic focus of objects either near of far
from the eye; the curvature of lens increases, which increases the lens power, allowing near
objects to be focused.
Pupillary constriction (also call miosis) – increases depth of focus, which demands less
accommodative precision; with bright light increase-pupil responded after about 200 ms and
constricted about 4.5 mm within 2.5 seconds.
Hippus – normal, rhythmic oscillation of pupil diameter, ±2mm; becomes larger, more rapid,
and more regular as the illumination level is increased; absence of hippos indicates a problem
with the innervation of the iris and damage to it.
Important Facts:
Basic Function of Iris and Pupil – admit light to the interior of the eye, regulate the amount of
light admitted, and affect the quality of retinal image.
Iris lies just in front of the lens.
VS 112 Ocular Anatomy
UAB School of Optometry
Page 1 of 7
-
-
Permits oblique rays to pass through the pupil to be imaged on the peripheral retina while
limiting the size of the incident ray bundle.
Color of iris is the light reflecting from melanin and blood.
Pupil – 2 to 8 mm in diameter depending on amount of light entering; admits more light
in dim environment (8 mm); admits less light in bright environment (2 mm).
Pupil diameter decreases by about 1mm for each log of intensity change in light in
daylight; at night only 1mm over a 4 log unit.
Pupil – dilates less rapidly than it constricts.
Pupil size affects image quality through the effects of diffraction and aberration;
increasing pupil size reduces image quality by increasing the effects of aberration by
exposing a larger area of the lens to off-axis rays (spherical aberration); decreasing pupil
size eliminates most of the spherical aberration, but reduces image quality by increasing
the effects of diffraction (diffraction less significant than aberration).
The pupil keeps the retina from receiving or being exposed to the full magnitude of the
ambient illumination range.
AC/A ratio- angle of convergence elicited by each diopter of accommodation; pupil size
vs accommodation (P/A) is also linear over most of the accommodative range.
Pupil dilates and constricts about two times a second over a range of about 0.5 mm.
Albinism-little or no melanin in eyes where pigment is normally found; iris is pale or
typically pink; less opaque-light goes through it, causes irregular spread of light over
retina, visual acuity is reduced.
II. STRUCTURE
Five layers of tissue:
(1) Anterior border layer – surface of the iris you see, is not continuous, but rather it has
breaks and holes (Fuchs); varies in thickness; allows aqueous to penetrate the stroma of the iris;
first light absorbing layer in the iris; iris processes-extensions from the anterior border layer run
across the apex of the anterior chamber angle to merge with trabecular meshwork, composed of
fibroblasts from the iris and endothelial cells from the trabecular meshwork.
(2) Stroma – makes up most of the iris; spongy mixture of blood vessels, collagen, fibroblasts,
melanocytes, and nerve fibers; contains open spaces filled with aqueous in vivo; open structure
allows it to be moved during change in pupil size; the blood vessels and nerve fibers run through
the spaces created by collagen fibers running radially and tangentially in stroma; clump cellfound near sphincter, acts like a macrophage and cleans up stray bits of melanin; swell as they
age.
(3) Muscular Layer – made up of circular sphincter and radial dilator; muscles are not
connected; the sphincter is thicker.
(4 and 5) Anterior and posterior iris epithelia – the posterior of the iris has two levels of
epithelial, they both contain pigment, but the anterior has less; the anterior lies directly in front of
the posterior; the anterior is the myoepithelium from which the dilator muscle arises.
Iris Root –where iris joins the ciliary body, thinnest and weakest area - most common site of
trauma (iris rips away from ciliary body).
VS 112 Ocular Anatomy
UAB School of Optometry
Page 2 of 7
Collarette – two-thirds of the way from the iris root to the pupil margin site of minor arterial
circle, separates the pupillary and ciliary regions, thickest muscular portion of iris, defined by
sphincter muscle
Pupil – an aperture, or hole, in the iris; last anatomical structure of iris to form.
Pupillary region – proximal to pupil, contains sphincter muscle.
Ciliary region – proximal to the ciliary body, contains dilator muscle.
Crypts of Fuchs – the holes in the anterior border layer, give iris its individuality and color,
blocks light, and allow aqueous to enter stroma from the anterior chamber.
Pupillary ruff or papillary frill – posterior epithelial lined with dark pigment wraps around and
forms border of pupil.
Contraction folds –deep valleys between ridges of iris tissues; prominent circular lines appear
in surface when pupil is dilated (consequence of pupil dilation), dangerous if angle of anterior
chamber is not fully open (blocks trabecular meshwork and causes elevated IOP because of
reduced aqueous flow).
III. BLOOD SUPPLY
-
meandering radial vessels from the iris root toward the pupil
-
Physical construction. related to variations in pupil size; curves in vessels straighten
when pupil is constricted; curves in vessels exaggerated-pupil is dilated; provides slackminimizes mechanical restrictions as tissue stretches and compresses.
-
Arteries-branches from the major and intramuscular arterial circles in the ciliary body.
-
Veins-drain through the ciliary processes into the vortex vein.
-
Minor arteriole circle- incomplete; formed by anastomoses among the extensively
branching vessels at the collarette.
-
Subsidiary braches from the minor arteriole circle form radial arcades running toward the
pupil margin and supplying sphincter.
-
Other branches from the minor arteriole circle are recurrent, running along the dilator
muscle toward iris root.
-
There are veins parallel to the arterial distribution, but there is no venous circle
corresponding to the minor arteriole circle.
-
The endothelium of the iris vessels overlap one another extensively and contain many
anchoring and occluding junctions; restricts movement of fluid and large molecules
through capillary walls.
VS 112 Ocular Anatomy
UAB School of Optometry
Page 3 of 7
-
The connective tissue sheath has no smooth muscle in walls. It is found in larger vessels
and there is an incomplete sheath of pericytes around connective tissue. All of this
prevents “kinks” in the blood vessels that would cut off blood supply.
IV. IRIS MUSCLES
A. Sphincter
occupies pupillary portion of iris
muscle fibers run along circular arcs parallel to pupil margin
length of fibers not known; probably ¼ of muscle’s circumference
fibers interlace and are bound by connective tissue sheaths
Constriction (miosis-pupil constriction)- produced when fibers contract (shorten and
straighten) and the circle of muscles becomes smaller.
-
Relaxtion (mydriasis-pupil dilation)- relaxation of sphincter from contracted state as the
muscle expands.
B. Dilator
defines the ciliary portion of the iris
fibers run radially along lines between the center of the pupil and iris root.
fibers are extensions of the anterior epithelial cells
fibers relatively short and bound by connective tissue
contraction=dilation – muscle pulling radially outward
relaxation=constriction-opposite of dilation
C. Sphincter and Dilator
The pupil diameter is the result of coordinated constriction and relaxation of the dilator and
sphincter muscles.
There are two ways to cause constriction of the pupil:
Constriction-contraction of sphincter or relaxation of dilator.
and two ways to dilate pupil Dilation-active contraction of dilator or relaxation of sphincter.
D. Autonomic Control of Muscles
1. Sphincter
- parasympathetic stimulation causes constriction of sphincter
- acetylcholine is neurotransmitter; receptor is muscarinic
- causes miosis
- does not open channels directly; uses a G-protein and a secondary messenger link;
activation is slower
- Edinger/Westphal nucleus-axons terminate on cells in ciliary ganglion-travel in eye in short
ciliary nerves-then to sphincter
- two neuron parasympathetic pathway
- second receptor is beta-adrenergic receptor that binds norepinephrine and inhibits muscle fiber
activation
2. Dilator
- sympathetic excitation stimulates dilation (constriction of dilator)
VS 112 Ocular Anatomy
UAB School of Optometry
Page 4 of 7
- norepinephrine is neurotransmitter
- causes mydriasis
- alpha-adrenergic receptors
- fast ligand-gated receptors at nicotinic synapses
- pathway longer though than that to sphincter
- cells in the superior cervical ganglion receive inputs from cells whose axons arise in the
brainstem and travel down spinal cord to synapse on other neurons; at first thoracic
vertebra whose axons run out to sympathetic trunk and up to ganglion
- cell bodies in superior cervical ganglion in neck-sympathetic plexus around the internal
carotid artery-enter orbit as sympathetic root of ciliary ganglion-enter eye in short ciliary
nerves
V. PIGMENTED EPITHELIUM
A. Anterior Pigmented Epithelium
1. forms the epithelial layer and the dilator muscle
2. myoepithelium- contractile filaments extend out from the cell body to form the dilator muscle
(in the ciliary portion of the iris)
3. in the pupillary portion of the iris, anterior epithelial cells lack muscular processes
4. more cuboidal and less pigmented than posterior epithelium
B. Posterior Pigmented Epithelium
1. main function is to absorb light
2. more columnar and pigmented than anterior pigmented epithelium
3. rest on anterior surface of lens (presence of junctional complexes b/w the posterior epithelial
cells provides a diffusional barrier between the iris stroma and the posterior chamber)
VI. PATHOLOGY
A. Anterior synechiae- adhesion of the anterior iris to the corneal endothelium
B.
Posterior synechiae- adhesion of the posterior iris to the anterior lens capsule
C. Iris Bombe- extensive posterior synechiae may cause aqueous buildup in the posterior
chamber and thus bulging of the pupil, may be treated by performing a partial iridectomy
(creation of drainage holes near the root of iris to allow aqueous flow between the anterior and
posterior chambers)
D. Heterochromia- difference in color between the two irises
1. -congenital heterochromia- abnormal (normally stable and of no clinical significance)
2. acquired heterochromia- pathological, may be drug induced or result of unilateral lesions in
sympathetic pathway
E. Pigmentary dispersion glaucoma – Pigment release from the posterior epithelial cells due to
cellular loss/damage can result in excessive pigmentation in the trabecular meshwork. Pigment
granules travel through the aqueous flow through the pupil into the anterior chamber and
eventually into the trabecular meshwork where in excess blocks the passage of fluid through the
meshwork.
VS 112 Ocular Anatomy
UAB School of Optometry
Page 5 of 7
VII. PUPILLARY LIGHT REFLEXES
-Under normal conditions of equal illuminations, pupils should be about the same size, due
to the strong correlation of the signals to the iris muscles.
A. Direct reflex- constriction of the pupil that is illuminated
B. Consensual reflex- simultaneous constriction of the unilluminated eye
-Indicates the presence of a reflex pathway (Figure 10.22 in our textbook).
C. Swinging Flashlight Test- detects deficits in afferent pupillary pathway ex. optic neuritis (an
infection of the optic nerve)
-Marcus Gunn’s Test – used to find defects in afferent pathways; illumination of one eye
may prove to be less effective than illumination of the other eye with respect to producing pupil
constriction.
VIII. PUPIL DEFECTS
1. Anisocoria: difference in pupil diameter between the two eyes of more than 0.5mm
A. Adie’s syndrome: condition in which pupil is dilated and pupillary light
reflexes are reduced or absent
B. Argyll Robertson pupil: condition pupil in which both the direct and
consensual light reflexes of pupil are reduced or absent but pupil constriction
to accommodative stimuli is normal
C. Horner’s syndrome: condition characterized by combination of ptosis
(drooping of the upper eyelid), miosis (pupil constriction), anhidrosis (lack of
sweating by the facial skin) produced by lesion in preganglionic portion of
sympathetic pathway to superior cervical ganglion
IX. CLINICALLY USEFUL DRUGS AFFECTING PUPIL SIZE:
1. Drugs causing constriction
A. Agonists (Stimulate Sphincter)
a. Pilocarpine
b. Metacholine
c. Physostigmine
B. Antagonists (Inhibit or block dilator)
a. Thymoxamine
b. Dapiprozole
2. Drugs causing dilation
A. Agonists (Stimulate dilator)
a. Phenylephrine
b. Hydroxyamphetamine
B. Antagonists (Inhibit Sphincter)
a. Atropine
b. Tropicamide
c. Botulinum A toxin
* Know bolded drugs
VS 112 Ocular Anatomy
UAB School of Optometry
Page 6 of 7
X. DEVELOPMENT OF IRIS
1. Formation of iris stroma
A. migration of undifferentiated neural crest cells (3 waves of migration)
a. first migration forms endothelium for cornea and future trabecular
meshwork
b. second migration forms a thin line of tissue which forms the iris
stroma and separates anterior and posterior chambers
c. third migration forms corneal stroma
2. Development of epithelium and iris muscles
A. Migrating neural cells form primitive iris stroma, cells at rim of optic cup
proliferate and push inward along posterior surface of pupillary membrane
B. At 4 mos gestation, sphincter appears; these cells move into iris along with
advancing epithelial layers
C. At 6 mos gestation, epithelium is largely complete; anterior epithelial cells
differentiate and begin to form processes of dilator
3. Development of iris color:
A. Anterior epithelium acquires pigment early in gestation, while still part of
optic cup
B. Posterior epithelium acquires pigment gradually during 5-6 month
C. Postnatal color change due to gradual accumulation of melanin in stroma and
anterior border
XI. FORMATION OF PUPIL
1. Pupil is the last anatomical structure of iris to form
2. Epithelium continues to grow inward following iris formation
3. Blood vessels penetrate to center of iris, which atrophy back to region of sphincter
A. Persistent pupillary membrane: residual portion of pupillary membrane;
strands of iris tissue crossing pupil as result of incomplete atrophy
XII. OTHER IRIS/PUPIL DEFECTS:
1. Aniridia: total absence of iris
2. Coloboma: Greek: the past taken out; a sector in eye in which tissue fails to develop
properly; may be found in optic nerve, retina, choroids, ciliary body, iris
A.Typical
B. Atypical
C. Coloboma at iris root
3. Hypoplasia: reduced amount of cell formation
4. Ectopic pupil: pupil that is out of position
Source: The Human Eye: Structure and Function by Clyde W. Oyster
VS 112 Ocular Anatomy
UAB School of Optometry
Page 7 of 7