Thirty Seven Flashcards
Describe the gross anatomy of the eye. What objects does light pass through in the eye? What do these things have in common? What do the eyelids do? What are the three layers of the globe?
2) Gross Anatomy. The eye is a globe is about 17mm in diameter in adults (full sized by 7
year-old but fully mature by 2 year-old). The path of light within the eye makes up the
globe’s optic components (cornea, aqueous humor, lens, vitreous humor and retina), which
are all transparent to light in the visible spectrum. These optical components are all
avascular, have a uniform cellular matrix, and share a common index of refraction (except
the lens).
Protecting the eye from dust and desiccation is the job of the eyelids. However, the lids do a
very poor job of blocking light, reducing the light flux into the eye by only a factor of ten.
The lids are not avascular and do not have a uniform cellular matrix so image formation on
the retina is blocked.
Three layers of the globe are: A) the outer or fibrous layer, B) the middle or vascular layer,
and C) the inner or retinal layer.
What are the components of the fibrous layer of the eye? Describe the components. What are some characteristics?
A) The fibrous layer consists of the sclera and cornea. The sclera is posterior and forms the
“white of the eye”. Its strength helps to maintain the shape of the eyeball and it gives
attachment to the external ocular muscles. The cornea is the avascular, translucent anterior
part of the fibrous layer. Its translucency is due to a number of anatomical factors:
non-keratinizing epithelium; absence of blood vessels and pigment; cellular components with
a uniform, regular arrangement and the same index of refraction; and the arrangement of its
collagen fibrils. The outer layers of the cornea are easily replaced if damaged. Damage to
the deeper layers, however, results in scar formation. Since the cornea is avascular, it can be
transplanted into allogeneic recipients without immunological rejection.
What are the components of the middle or vascular layer of the eye? What are some other names for it? What is it responsible for? Describe the components. What are their fncitons?
B) The middle or vascular layer, also referred to as the choroid or uvea, is responsible for focusing light and adjusting its intensity. It consists of three parts: the choroid proper, the ciliary body, and the iris. The choroid is a highly vascular and pigmented membrane lining the posterior five-sixths of the eye deep to the sclera. Anteriorly, it becomes the ciliary body that is involved in the production of aqueous humor, providing some constituents of the vitreous humor, and adjusting the shape of the lens. The ciliary body is attached to the lens
by the suspensory ligament and contains smooth muscles that are under the influence of parasympathetic impulses. Contraction of the ciliary muscles results in decreased tension on the suspensory ligament and, in turn, on the lens. This allows the lens to increase its thickness thereby focusing the light rays from a near object onto the retina (accommodation)
The iris projects inwardly from the anterior part of the ciliary body and its free margin forms
the rim of the pupil. The size of the pupil is regulated by constrictor and dilator smooth
muscles in the iris. The constrictor muscle is under the influence of parasympathetic
impulses, whereas the dilator is under sympathetic control.
What are the two striata of the inner or retinal layer? How many layers are in each? How are the layers numbered? What layer does light reach first? What direction does neural info go? How can the two strata become detached? What happens in they do? How can this be treated?
C) The inner or retinal layer is located between the choroid and the vitreous body and may be
divided into two strata, pigmented and neural. The pigmented or external stratum,
non-nervous in nature, is composed of a single layer of pigmented cells. The neural or
internal stratum is transparent and consists of nine layers. Therefore, altogether the retina is
comprised of 10 layers that are numbered from external to internal. Layer 2 of the retina is
composed of photoreceptors, the rods and cones, which transduce the light rays while layer 9
includes the ganglion cell axons that become the optic nerve. Hence, the light rays pass from
the internal to the external layers whereas the neural signals of the visual world pass from the
external to internal layers.
On occasion following head trauma, the two strata of the retina become locally detached
from one another (retinal detachment). If so, the neural retina becomes anoxic and may die if
not treated immediately. A variety of physical interventions (sclera staples or buckles) can
move the retina closer to the sclera for reattachment, but more commonly, lasers are now
used to burn small spots of retina against the sclera as a ‘spot weld’ to move the neural retina
back in place near the pigmented retina, which is also near the choroid.
Explain the production and drainage of aqueous humor and the two types of glaucoma.
Clinical note: It is thought to be secreted by the epithelium lining the ciliary body and is
similar in composition to protein-free plasma. Upon its formation, it enters the posterior
chamber of the eye, flows through the pupil into the anterior chamber, and drains into a
trabecular meshwork (the spaces of Fontana) located at the junction of the iris, cornea, and
sclera. These trabeculae then empty into the sinus venosus sclerae (canal of Schlemm), a
large, branching circumferential vessel that drains into the episcleral veins.
The balance between aqueous humor production and drainage controls the pressure
in the globe that maintains its shape. An imbalance can cause glaucoma, abnormally high
intraocular pressure that damages the retina by compressing its vasculature. 95% of
glaucoma cases are ‘open angle’ (overproduction of aqueous humor) and 5% of glaucoma
cases are ‘closed angle’ (restricted drainage).
What is the cornea like and what does it do?
Light entering the eye passes through a number of structures before it reaches the retina.
These structures are the cornea, aqueous humor, lens, and vitreous body, the eye’s internal
refractive media. The cornea has a marked curvature and a different refractive index than air.
Thus, the cornea bends the light rays. In fact, the cornea is the main refractive structure of
the eye. Then the light enters the aqueous humor, which has about the same index of
refraction as the cornea.
What is the lens like? Explain accomodation.
After passing through the cornea, the anterior chamber, and the pupil, light rays strike the
lens. Although the lens is not as refractive as the cornea, it is essential for focusing because
its refractive power can be altered. The lens is a 1 cm biconvex disc suspended from the
ciliary body by the suspensory ligament. This consists of delicate but strong fibrils that
attach to the lens capsule near the equator.
When the eye is at rest, the suspensory ligament is taut and exerts a pull on the lens capsule
thereby keeping it relatively flat. When vision is shifted from a far to a near object, reflex
contraction of the ciliary muscle causes the ciliary body to move forward and decreases the
tension on the suspensory ligament. This allows the inherently elastic lens to bulge and
increase its antero-posterior diameter thereby shortening the focal distance between the lens
and retina.
What is presbyopia? What causes it? What are cataracts? What causes floaters?
Clinical note: With age, the lens becomes harder and its power of accommodation is
decreased, a condition termed presbyopia. The lens absorbs ultraviolet light but it is
unknown whether this absorption relates to the opacification of the lens that is called a
cataract.
The vitreous body is a clear, gelatinous substance that fills the posterior four-fifths of
the eyeball. It not only supports the structures within the eye but also provides a transparent
medium. However, particles may float in the vitreous (muscae volitantes, commonly called
floaters) , which sometimes can be seen.
What is mydriasis and how does it occur? What is miosis and how does it occur? What are 3 symptoms of Horners syndrome? What is the cause? Where is the lesion ?
Mydriasis: iris dilator under sympathetic control (sup. cervical ganglion)
Miosis: iris sphincter under parasympathetic control (CN III)
(Edinger Westfall nucleus)
– PTOSIS • drooping eye lid – MIOSIS • small pupil – ANHYDROSIS • no sweating (on face)
Cause
• unilateral deficit in sympathetic drive
- Lesion
- above superior cervical ganglion
What two parts of the retina are structurally and functionally different than the rest of it? What are they called? What components do they contain? What is their function? Why is the blindspot not noticed? Why is injury to the optic nerve irreparable? What is a scotoma? What is age related macular degeneration? What is the most common cause of blindness? How is it caused? What is papilledema?
Two parts of the retina that
are structurally and functionally
different from the rest of the retina are
the central area and the optic disc.
These regions can be viewed within the
fundus using an ophthalmoscope. The
central area contains the macula lutea
and the fovea centralis. At the fovea,
the inner layers of the retina are
displaced, forming a pit or foveola.
Only cones are present in the floor of the
foveola. This is the area of most acute
vision and, therefore, a line connecting
it with the viewed object is the visual
axis. Blood vessels specifically avoid
the central retina where the retina is
sensitive to the highest acuity.
Unfortunately, during diabetes, blood
vessels invade this region and interfere
with high acuity (diabetic retinopathy,
the most common cause of blindness).
The optic disc or papilla is the area at which the unmyelinated optic nerve fibers exit from the
retina. At this point, about 1.5 mm of the outer layers of the retina are interrupted; hence, it is
the blind spot. After traversing the outer layers of the retina, the optic nerve fibers pass through
that part of the sclera referred to as the lamina cribrosa because of its sieve-like structure.
Within, or immediately beyond the lamina cribrosa, the optic nerve fibers become myelinated.
Using an ophthalmoscope, a clinical condition can be observed called papilledema (optic disk
swelling). Because the optic nerve is continuous with the cerebral spinal fluid, increased
pressure of CSF may swell the optic disk leading to nerve damage.
The blind spot is not obviously noticeable, even when one eye is covered. The reason is that the
central visual system ‘fills in’ the missing space with the visual pattern that surrounds it, rather
than give the patient a perception of a black hole in their visual world. In addition, the blind
spot of each eye is fortunately not located in the same part of the visual field. Centrally the
central visual system uses the other eye’s input to create the illusion of a continuous visual
environment. The same is true when a small region of one retina is damaged (a scotoma); the
central visual system can compensate and prevent the sensation of blindness.
Clinical note: Since the neural layers of the retina develop from the forebrain, the optic nerves
are CNS structures and, unlike all other cranial nerves, are supported by oligodendrocytes
throughout their lengths. Hence, injury to the optic nerve is considered irreparable because
axonal regeneration does not readily occur in the absence of neurolemmal cells.
What is hyperopia? What is myopia? What is presbyopia? What is astigmatism? What are the 3 components of the accomodation reflex?
a. Hyperopia (“farsightedness”) is a condition in which the eyeball is shortened. The rays thus focus behind the retina. The lens can accommodate to some extent, but when close objects are beyond the ability of the eye to accommodate only corrective biconvex converging lenses will produce a clear retinal image.
b. Myopia (“nearsightedness”) occurs when the eyeball is lengthened. It is thought to be due to both genetic and environmental factors. Myopes have focal points anterior to the retina when viewing distant objects. Corrective lenses in this case are biconcave diverging.
c. Presbyopia is an acquired condition (beginning around the age of 40) in which an individual loses the power for close vision because of lens inelasticity. Whereas a 15 year old can increase the lens strength up to 12 D for a total of 72 D (60 + 12), a 70 year old can only increase his/her lens 1 D for a total of only 61 D. Thus, light rays from close objects converge behind the retina. Eventually we all compensate for this condition by either moving the objects of interest farther from our eyes or by corrective biconvex lenses.
d. Astigmatism is a defect in which the corneal radius of curvature is not perfectly symmetrical and blurred vision is the result. This condition can be corrected with diametrically opposite (spherical) lenses.
Accommodation Reflex. This reflex has three components: 1. pupillary constriction of the iris muscles, which reduces light aberration, 2. ocular convergence, when the eyes are positioned so that a close object is imaged on both foveae, resulting in binocular vision, and 3. ciliary muscle
contraction, which changes the shape of the lens to increase its diopter strength. This tends to diminish in old age as the lens becomes less elastic (see Presbyopia above).
What is the purpose of retinal specialization? How does it work? How many photoreceptors are there? Optic nerve fibers per eye? What is the ratio of receptors to ganglion cells in the fovea? In the entire retina?
RETINAL SPECIALIZATION REDUCES SIZE OF OPTIC NERVE
127 million photoreceptors
Ratio of receptors to ganglion cells
Fovea (within 2.2°) 1:2
Entire retina 125:1
1,200,000 optic nerve fibers/retina
