Ch 23: The Eye Section: Special Sense Organs

Question 1

EYES: THE PHOTORECEPTOR
SYSTEM

Each eye is composed of three concentric tunics or layers:

Answer 1

A tough external fibrous layer consisting of the sclera
and the transparent cornea
■ A middle vascular layer that includes the choroid , ciliary
body, and iris; and
■ An inner sensory layer, the retina , which communicates
with the cerebrum through the posterior optic nerve.

Question 2

Not part of these layers, the lens is a perfectly transparent biconvex structure held in place by a circular system of zonular
fibersthat attach to theciliary body and by close apposition
to the posterior vitreous body.

Answer 2

Partly covering the
anterior surface of the lens is an opaque pigmented extension of
the middle layer called the iris , which surrounds a central opening,
the pupil.

Question 3

Located in the anterior portion of the eye, the iris and lens
are bathed in clear aqueous humor that fills both the anterior
chamber between the cornea and iris and the posterior chamber
between the iris and lens.

Answer 3

• *Aqueous humor** flows through the pupil that connects these two chambers. The posterior vitreous chamber, surrounded by the retina, lies behind the lens and its zonular fibers and contains a large gelatinous mass of transparent connective tissue called the
• *vitreous body.**

Question 4

In the 4-week embryo, epithelial optic vesicles bulge
bilaterally from the forebrain, then turn into the optic
stalksbearingoptic cups.

Answer 4

Inductive interactions between the optic cups and the
overlying surface ectoderm cause the latter to invaginate
and eventually detach as the initially hollow lens vesicles.

Question 5

The optic stalk develops as the optic nerve and in an
inferior groove called the choroid fissure encloses the
hyaloid vessels that supply blood for the developing lens
and optic cup.

Answer 5

In the ensuing weeks, head mesenchyme differentiates to
form most of the tissue in the eye’s two outer layers and the
vitreous. Ectoderm of the optic cup differentiates as the
retina and surface ectoderm creates the corneal epithelium.

When the lens is fully formed, the distal
hyaloid artery and vein disappear, leaving only the blood
supply to the retina.

Question 6

Fibrous Layer: Has 2 parts

1. Sclera: Mainly dense irregular connective tissue with flat bundles of type I collagen parallel to to the organ surface.

Protects delicate internals, supports shape.

Extrinsic eye muscles attachment site.

Answer 6

Tendons of the extraocular muscles which move the eyes
insert into the anterior region of the sclera. Posteriorly the sclera
thickens to approximately 1 mm and joins with the epineurium
covering the optic nerve. Where it surrounds the choroid, the
sclera includes an inner suprachoroid lamina, with less collagen,
more fibroblasts, elastic fibers, and melanocytes.

Question 7

Fibrous Layer: 2 parts

In contrast to the sclera, the anterior one-sixth of the eye—the
cornea—is transparent and completely avascular.

Protects anterior surface of the eye
refracts (bends) incoming light.

A section of the cornea shows five distinct layers:

Answer 7

■■ An external stratified squamous epithelium;
■■ An anterior limiting membrane (Bowman’s membrane),
which is the basement membrane of the external
stratified epithelium;
■■ The thick stroma;
■■ A posterior limiting membrane (Descemet’s membrane),
which is the basement membrane of the endothelium;
and
■■ An inner simple squamous endothelium.

Question 8

Vascular Tunic (Middle Layer) of eye.

Choroid

Areolar connective tissue;
highly vascularized.

Answer 8

Supplies nourishment to retina
Pigment absorbs extraneous light

Question 9

Vascular Tunic (Middle Layer) of eye.

Ciliary body

Ciliary smooth muscle and ciliary processes;
covered with a secretory epithelium.

Epithelium secretes aqueous humor

Answer 9

Holds suspensory ligaments that
attach to the lens and change lens
shape for far and near vision.

Question 10

Vascular Tunic (Middle Layer) of eye.

Iris:

Two layers of smooth muscle (sphincter pupillae
and dilator pupillae) and connective tissue, with a
central pupil.

Answer 10

Controls pupil diameter and thus
the amount of light entering the
eye.

Question 11

Retina (Internal Layer) of the eye.

Pigmented layer

Pigmented epithelial cells

Answer 11

Absorbs extraneous light.
Provides vitamin A for
photoreceptor cells.

Question 12

Retina (Internal Layer) of the eye.

Neural layer

Photoreceptors, bipolar neurons, ganglion cells,
and supporting Müller cells.

Answer 12

Detects incoming light rays; light
rays are converted to nerve signals
and transmitted to the brain

Question 13

The basal cells have a high proliferative
capacity important for renewal and repair of the corneal surface
and emerge from stem cells in the corneoscleral limbus that
encircles the cornea.

Answer 13

As another protective adaptation, the corneal epithelium
also has one of the richest sensory nerve supplies of any tissue.

Question 14

has one of the richest sensory nerve supplies of any tissue.
Th e basement membrane of this epithelium, often called
Bowman’s membrane,

Answer 14

contributes to the stability and strength of the cornea, helping to protect
against infection of the underlying stroma.

Question 15

The stroma, or substantia propria, makes up 90% of
the cornea’s thickness and consists of approximately 60 layers
of parallel collagen bundles aligned at approximately right
angles to each other and extending almost the full diameter
of the cornea.

Answer 15

Between the collagen lamellae are cytoplasmic extensions
of flattened fibroblast-like cells called keratocytes.

The ground substance around these cells contains proteoglycans
such as lumican, with keratan sulfate and chondroitin
sulfate, which help maintain the precise organization and
spacing of the collagen fibrils.

Question 16

The posterior surface of the stroma is bounded by another
thick basement membrane, called Descemet’s membrane,
which supports the internal simple squamous corneal endothelium.

Answer 16

This endothelium maintains Descemet’s membrane and
includes the most metabolically active cells of the cornea.

*responsible for regulating the _proper hydration stat_e
of the corneal stroma to provide maximal transparency and
optimal light refraction.

Question 17

Encircling the cornea is the limbus, a transitional area
where the transparent cornea merges with the opaque sclera.

Here Bowman’s membrane ends
and the surface epithelium becomes more stratified as the conjunctiva that covers the anterior part of the sclera.

Answer 17

Also at the limbus Descemet’s membrane and its simple
endothelium change into a system of irregular endothelium-
lined channels called the trabecular meshwork.

they allow slow, continuous drainage of aqueous
humor from the anterior chamber. This fluid moves from
these channels into the adjacent larger space of the scleral
venous sinus, orcanal of Schlemm.

Question 18

Vascular Layer
The eye’s more vascular middle layer, known as the uvea, consists
of three parts:

Answer 18

from posterior to anterior:

the choroid,

the
ciliary body,

and the iris

Question 19

Located in the posterior two-thirds of the eye, the choroid consists
of loose, well-vascularized connective tissue and contains
numerous melanocytes.

These form a characteristic
black layer in the choroid and prevent light from entering the
eye except through the pupil. Two layers make up the choroid

Answer 19

The inner choroido-capillary lamina has a rich microvasculature
important for nutrition of the outer retinal layers.

■■ Bruch’s membrane, a thin extracellular sheet, is composed
of collagen and elastic fibers surrounding the
adjacent microvasculature and basal lamina of the retina’s
pigmented layer.

Question 20

The ciliary body, the anterior expansion of the uvea that encircles
the lens, lies posterior to the limbus.
Like the choroid, most of the ciliary body rests on the sclera.
Important structures associated with the ciliary body include
the following:

Answer 20

Ciliary muscle makes up most of the ciliary body’s
stroma and consists of three groups of smooth muscle
fibers. Contraction of these muscles affects the shape of
the lens and is important in visual accommodation.

Ciliary processes: Cells of this dual epithelium have extensive basolateral folds with Na+/K+-_ATPase activi_ty and are
specialized for secretion of aqueous humor.

The ciliary zonule: is a system of many radially oriented
fibers composed largely of fibrillin-1 and 2 produced by
the nonpigmented epithelial cells on the ciliary processes. The fibers extend from grooves between the ciliary processesand attach to the surface of the lens,
holding that structure in place.

Question 21

The iris is the most anterior extension of the middle uveal layer
which covers part of the lens, leaving a round central pupil.

The posterior surface of the iris has a two-layered epithelium
continuous with that covering the ciliary processes, but
very heavily filled with melanin. The highly pigmented posterior
epithelium of the iris blocks all light from entering the
eye except that passing through the pupil.

Answer 21

Myoepithelial cells form a partially pigmented epithelial layer and extend contractile processes radially as the very thin dilator pupillae muscle.
Smooth muscle fibers form a circular bundle
near the pupil as the sphincter pupillae muscle.

The dilator and sphincter muscles of the iris have sympathetic and parasympathetic
innervation, respectively, for enlarging and constricting the pupil.

Melanocytes of the iris stroma provide the color of one’s
eyes.

Question 22

Lens
The lens is a transparent biconvex structure suspended immediately
behind the iris, which focuses light on the retina.

Answer 22

the lens is a unique avascular
tissue and is highly elastic, a property that normally decreases
with age. The lens has three principal components:

Question 23

Lens:

A thick (10-20 μm), homogeneous lens capsule composed
of proteoglycans and type IV collagen surrounds
the lens and provides the place of attachment
for the fibers of the ciliary zonule.

subcapsular lens epithelium consists of a single layer
of cuboidal cells present only on the anterior surface
of the lens. Area here allows for _growth of the
lens and continues at a slow,_decreasing rate near the
equator of the lens throughout adult life.

Answer 23

Lens fibers are highly elongated, terminally differentiated
cells that appear as thin, flattened structures. Its cytoplasm
becomes filled with a group of proteins called crystallins,
and the organelles and nuclei undergo autophagy. Lens
fibers are packed tightly together and form a perfectly
transparent tissue highly specialized for light refraction.

Question 24

The lens is held in place by fibers of the ciliary zonule,
which extend from the lens capsule to the ciliary body. Together with the ciliary muscles….

Answer 24

this structure allows the process of visual accommodation, which permits focusing on near and far objects by changing the curvature of the lens.

In the fourth decade of life presbyopia
normally causes the lenses to lose elasticity and their ability to undergo
accommodation.

Question 25

The vitreous body occupies the large vitreous chamber behind
the lens. It is 99% water gel like.

Its only…

Answer 25

T cells in the vitreous body are a small mesenchymal population near the membrane
called hyalocytes, which synthesize the hyaluronate and collagen,
and a few macrophages.

Question 26

The retina, the innermost tunic of the eye, develops with
two fundamental sublayers from the inner and outer layers of
embryonic optic cup.

Answer 26

1. The outer pigmented layer is a simple cuboidal epithelium
   attached to Bruch’s membrane and the choroidocapillary
   lamina of the choroid.
2. The inner retinal region, the neural layer, is thick and
   stratified with various neurons and photoreceptors.

Question 27

Retina Pigmented Epithelium:

This cellular region also contains numerous
phagocytic vacuoles and secondary lysosomes, peroxisomes,
and abundant smooth ER (SER) specialized for retinal

(vitamin A) isomerization.

Some functions of the Retina Pigmented Epithlium are:

Answer 27

1. The pigmented layer absorbs scattered light that passes
   through the neural layer, supplementing the choroid in
   this regard.
2. With many tight junctions, cells of the pigmented epithelium
   form an important part of the protective blood-retina
   barrier isolating retina photoreceptors from the highly
   vascular choroid and regulating ion transport between
   these compartments.

Question 28

More Retina Pigmented Epithelium Functions:

3. After Phagocytosis of coponents,

The cells play key roles in the visual cycle of retinal
regeneration, making
all-trans-retinal released from photoreceptors and produce
11-cis-retinal that is then transferred back to the photoreceptors.

Answer 28

5. Cells of pigmented epithelium remove free radicals by
   various protective antioxidant activities and support the
   neural retina by secretion of ATP, various polypeptide
   growth factors, and immunomodulatory factors.

Question 29

Neural retina functions as
an outpost of the CNS with glia and several interconnected
neuronal subtypes in well-organized strata. Nine distinct layers comprise the neural retina.

1. Near the pigmented epithelium, the outer nuclear layer
   (ONL) contains cell bodies of photoreceptors (the rod and
   cone cells).

Answer 29

2. The inner nuclear layer (INL) contains the nuclei of various
   neurons, notably the bipolar cells, amacrine cells, and
   horizontal cells, all of which make specific connections
   with other neurons and integrate signals from rods and
   cones over a wide area of the retina.
3. Near the vitreous, the ganglionic layer (GL) has neurons
   (ganglion cells) with much longer axons. These
   axons make up the nerve fiber layer (NFL) and converge
   to form the optic nerve which leaves the eye and
   passes to the brain.

Question 30

Neural Retina other layers

4. The outer plexiform layer (OPL) includes axons of the
   photoreceptors and dendrites of association neurons in the
   INL.
5. The inner plexiform layer (IPL) consists of axons and
   dendrites connecting neurons of the INL with the ganglion
   cells.

Answer 30

The rod and cone cells, named for the shape of their outer
segments, are polarized neurons with their photosensitive portions
aligned in the retina’s rod and cone layer (RCL) and their
axons in the IPL.

All neurons of the retina are supported physically by glial
cells called Müller cells.

Question 31

Müller cells also
organize two boundaries that appear as very thin layers within
the retina:

Answer 31

1. The outer limiting layer (OLL) is a faint but well-defined
   series of tight and adherent junctions that form at the level
   of the rod and cone inner segments between the photoreceptors
   and Müller cell processes.
2. The inner limiting layer (ILL) consists of terminal expansions
   of other Müller cell processes that cover the collagenous
   membrane of the vitreous body.

Question 32

Rod Cells: They
are extremely sensitive to light, responding to a single photon,
and allow some vision even with light low levels, such
as at dusk or nighttime.

They consist of some parts:

Answer 32

They have outer segement, flattened membranous discs, Proteins called rhodopsin which intiates visual stimulus. Between this outer segment and the cell’s inner segment is a constriction,

the connecting stalk, which is part of the modified primary cilium arising from a basal body.

Question 33

Cone Cells:

in the human retina produce color
vision in adequately bright light. There are three morphologically
similar classes of cones, each containing one type
of the visual pigment iodopsin

Answer 33

Each of the
three iodopsins has maximal sensitivity to light of a different
wavelength, in the red, blue, or green regions of the visible
spectrum, respectively. By mixing neural input produced by
these visual pigments, cones produce a color image.

Question 34

Phototransduction: of rods and cones have rhodopsin,

Each of these visual
pigments contains a transmembrane protein, the opsin, with a
small, light-sensitive chromophore molecule bound to it. The
vitamin A derivative called retinal acts as the chromophore of
rhodopsin in rods.

Answer 34

Phototransduction is adjustment between dark and bright light.
basically similar process in both rods and cones.

transducin, a protein acitvated by light. Will make Chromophor stay attached to Opsin to see in bright light.

In Bleaching, The, chromophore dissociates from opsin, These are adjustments from bright to dim light, is when vitamin A (retinol) acts like chromophore and attachs to rhodopsin to see better in the dark.

Question 35

Specialized Areas of the Retina

The blind spot of the retina, or optic disc, lacks photoreceptors
and all conducting neurons.

Answer 35

Near the optic disc, within the portion of retina directly
opposite the pupil, lies a specialized area about 1.5 mm in diameter
called the fovea centralis, where visual
acuity or sharpness is maximal.

Surrounding the fovea centralis is the macula lutea. Its carotenoids have antioxidant properties and filter potentially damaging shortwavelength light, thus helping to protect the cone cells of the fovea.

Question 36

Within the GL of the entire retina a subset of ganglion cells
serve as nonvisual photoreceptors. These neurons contain
11-cis-retinal bound to the protein melanopsin and serve to
detect changes in light quantity and quality during each 24-hour
dawn/dusk cycle.

This is for…

Answer 36

Signals from these cells pass via axons of the
retinohypothalamic tract to the _suprachiasmatic nuclei and the
pineal gland,_where they help establish the body’s physiologic
circadian rhythms, so you can secret melatonin to go to sleep on time.

Question 37

Accessory Structures of the Eye

The conjunctiva is a thin, transparent mucosa that covers the
exposed, anterior portion of the sclera and continues as the
lining on the inner surface of the eyelids.

Answer 37

Mucous secretions from conjunctiva cells are
added to the tear film that coats this epithelium
and the cornea.

Question 38

Eyelids are pliable structures containing skin,
muscle, and conjunctiva that protect the eyes.

dense fibroelastic plate called the tarsus
that supports the other tissues in the eyelid.

Answer 38

Oils in the sebum produced by these tarsal
glands, also calledMeibomian glands, form a surface layer on
the tear film, reducing its rate of evaporation, and help lubricate
the ocular surface.

Question 39

The lacrimal glands produce fluid continuously for the tear film
that moistens and lubricates the cornea and conjunctiva and supplies
O2 to the corneal epithelial cells.

Answer 39

Tear film moves across the _ocular surfac_e and collects in
other parts of the bilateral lacrimal apparatus: flowing through
two small round openings (0.5 mm in diameter) to canaliculi at
the medial margins of the upper and lower eyelids, then passing
into the lacrimal sac, and finally draining into the nasal cavity via
the nasolacrimal duct.