Lecture 1 (Dr. Wajida Saad Bunyan)
Anatomy and Physiology of the Eyeball
The eye consists of three layers: the fibrous, vascular, and nerve
layers, and additionally, the ocular media.
A. Fibrous Layer
: The fibrous layer is the outer layer of the eye,
consisting of two parts, the cornea and the sclera.
The cornea is an a vascular, transparent organ, the clear, dome-like
structure in front of the visible iris. It is a convex structure with a
horizontal diameter of 12 millimeters and a vertical measurement of 10-
11 millimeters. The average dioptric power is 43-45. Thickness is 0.5
millimeters centrally, and thicker in the periphery to 1 millimeter. Two-
thirds of light refraction for vision is performed by the cornea. The
refractive power of the corneal structure is about plus-45 diopters.
The opaque sclera forms the posterior five-sixths of the globe. The tissue
is avascular and appears white. It is approximately 1 millimeter thick but
thins at the equator to approximately 0.6 millimeters. The thinnest part of
the sclera is 0.3 millimeters at the insertion point of the rectus muscles.
The firmness, strength, and elastic properties of this tissue maintain the
shape of the globe (with the intraocular pressure) as well as provide a
rigid base for insertion of the extraocular muscles.
The conjunctiva is a mucous membrane that covers the anterior sclera
and continues to the back surfaces of the lids to form a conjunctival sac.
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There are three portions of the conjunctiva:
the bulbar conjunctiva, which covers the white sclera,
the palpebral conjunctiva, which covers the backside of the eyelid,
and the fornix, which is the point at which the bulbar and palpebral
conjunctivae meet.
Ocular media: The ocular media is the transparent optical surfaces and
liquids within the eye, through which light ray pass before reaching the
retina. In addition to the cornea above, this includes the anterior chamber,
the crystalline lens, and the vitreous.
1. Anterior chamber and angles: The anterior chamber, a fluid filled
space, is bounded anteriorly by the cornea and posteriorly by the iris.
It is filled with aqueous humor, which is clear, colourless and has a
watery consistency (99 percent water). Produced by the ciliary processes.
the aqueous humor
travels from the posterior chamber from behind
the iris, through the pupil into the anterior chamber, bathing the cornea
and anterior segment. The aqueous humor exits through the angle, which
is a drainage structure where the cornea and iris meet. The angle is
created by the iris root and peripheral cornea where all these outflow
passages lie. The angle is made up of the trabecular meshwork and the
canal of Schlemm, then continues through a network of veins draining
aqueous out of the eye. Assessment of the angle and structure is
performed with a gonioscope. The normal opening of the angle is 30
degrees. If this angle continues to close to zero degrees, angle-closure
glaucoma can result.
The crystalline lens is a biconvex, transparent structure that sits behind
the iris and in front of the vitreous. This is the second major refracting
structure in the eye, adding approximately 15-20 diopters to the total
refractive power of the eye. Transparency is maintained by being
avasular, containing no nerves or connective tissue. It is flexible and is
suspended in position by zonules that attach it to the ciliary body. The
components of the lens are the capsule, epithelium, and lens substance
(cortex and nucleus).
The lens changes shape in order to change the dioptric power of the eye
when changing focus from distance objects to near, maintaining a clear
image on the retina (accommodation).
The vitreous makes up the largest volume (approximately 80 percent) of
the eye, and is a clear, jelly-like substance. The total volume of vitreous is
formed during embryologic development and does not replace itself.
Anteriorly, the vitreous face sits behind the posterior lens capsule, and is
bounded by the retina posteriorly. The composition of the vitreous is a
framework of collagen, mucopolysaccharide, and hyaluronic acid.
The vitreous functions to maintain the transparency of the optical media
and to provide a constant internal pressure for support of the internal
structures of the eye.
B
.The vascular layer
is also known as the uveal tract and consists
of three parts, the ciliary body, the iris, and the choroid. It is the middle
layer of the eye and is situated between the sclera and retina.
The function of the vascular layer (uveal tract) is to:
1) produce aqueous humor in the ciliary processes,
and 2) alter the shape of the crystalline lens in order for the eye to focus.
C. Nerve layer (receptor cells):
The retina is the transparent,
innermost layer of the eye and is a direct extension of the brain. Although
there are 10 distinct layers of the retina, there are two segments of the
retina: the outer retinal pigment epithelium (RPE) and the inner neural
retina.
Light must travel through most of the retinal layers in order to stimulate
the second layer of photoreceptors, the rods and cones. Once the
photoreceptors change the light signals into electrical impulses, they are
amplified and then integrated through the circuits of bipolar, horizontal,
amacrine, and ganglion cells. The impulses converge onto bipolar cells
and again onto ganglion cells. The axons of ganglion cells merge and exit
at the optic disc.
The macula area receives the sharpest formed images. Its center is
slightly depressed to the fovea and lies 3 millimeters temporal to the optic
nerve.. There are 120 million rods and 6 million cones.
Cones are densest in the macula, responsible for color and central vision,
work best in bright light, capture detail and color, and require direct
stimulation.
Rods are more numerous in the periphery of the retina, function best in
dim illumination.
PUPILLARY PATHWAYS
The pupil is a circular hole in the center of the iris that regulates light
entering the eye. It appears black because the tissues inside the eye
absorb most of the light entering the pupil. The diameter of the average
adult pupil is 3-4 millimeters under average lighting conditions.
The function of the pupil is analogous to the shutter of a camera: In
darkness, the iris dilator muscle causes the pupil to open, allowing
additional light to reach the retina. In bright light, the iris sphincter
muscle constricts, causing the pupil to get smaller, limiting the amount of
light that reaches the retina. Constriction also occurs during
accommodation.
The optic nerve (CN II) is responsible for the afferent limb of the
pupillary reflex; it senses the incoming light and dilates the pupil.
The oculomotor (CN III) nerve is responsible for the efferent limb of
the pupillary reflex; it drives the muscles that constrict the pupil.
EXTRAOCULAR MUSCLES
Horizontal recti
Medial rectus: This is the strongest extraocular muscle because it is the
heaviest muscle and has the most anterior insertion on the globe. It arises
from the annulus of Zinn and is innervated by CN III.
Action — adduction: It rotates the eye medially towards the nose. In
primary position the muscle plane coincides with the visual axis.
Lateral rectus: Arises from the annulus of Zinn and spans the superior
orbital fissure. It is innervated by CN VI.
Action — abduction: It rotates the eye laterally toward the temple. In
primary position the muscle plane coincides with the visual axis.
Vertical recti
The superior rectus arises from the upper part of the annulus of Zinn
and travels forward above the globe. It is innervated by the superior
division of the oculomotor Cranial Nerve III.
Primary action — elevation: In the primary position
Secondary action — intorsion.
The inferior rectus is the shortest of the recti muscles. It arises at the
annulus of Zinn travels downward, then forward, under the globe. It is
innervated by CN III.
Primary action — depression: This increases as the eye is turned out and
is nil when the eye is adducted. The inferior rectus is the only depressor
in the abducted position of the eye.
Secondary action — extorsion.
Oblique Muscles
The superior oblique is the longest and thinnest eye muscle. It arises at
the annulus of Zinn and travels forward along the medial superior side of
the orbit. It is innervated by the cranial nerve IV (trochlear)
Primary action — intorsion: It moves the eye downwards or laterally or
rotates it inwards (i.e. makes twelve o’clock on the cornea move towards
the nose).
Secondary action — depression: This increases as the eye is adducted.
The superior oblique is the only muscle which can depress in the
adducted position. Its action is practically nonexistent when the eye is
abducted.
Inferior oblique: This is the shortest of the six extraocular muscles and
the only muscle that does not originate from the annulus of Zinn. It arises
from a rounded tendon in a depression on orbital floor near orbital rim
(maxilla), just behind the orbital margin and lateral to orifice of the naso-
lacrimal duct. It is innervated by the inferior division of CN III.
Primary action — extorsion: The inferior oblique makes the eye look
upward or rotates it laterally.
Secondary action — elevation. This increases as the eye is turned in. The
inferior oblique is the only elevator in the adducted position.
VISUAL PATHWAY
The visual pathway is made up of axons, which connect the retina to the
occipital lobes of the brain at the level of the visual cortex. The pathway
starts at the retina, then proceeds from the orbit through the optic disc,
along the optic nerve to join with the optic nerve of the other eye at the
optic chiasm. Information then passes to the lateral geniculate body, and
finally to the occipital cortex. Any disruption of this path from the eye to
the brain will result in a visual field defect. Images arrive at the retina
inverted (upside down) and also reversed left to right. Objects viewed in
space superiorly will be imaged on the inferior retina, while objects
viewed to the right in space will be imaged on the left retina.
The retina has 10 layers, but only three layers of nerve cells specifically
convert light energy into electrochemical signals. Objects are transmitted
as light to the photoreceptor (rods and cones) layer of the retina.
Photoreceptors send signals to the optic nerve via the retinal nerve
fibers.
There are no photoreceptors at the optic disc (termed “the physiologic
blind spot”); therefore, no light can be detected.
The axons of the ganglion cells create the retinal nerve fiber layer. They
pass through the optic disc to become the optic nerve. The nerve fiber
layer is distributed across the retina in a very specific pattern as it moves
toward the optic disc. The nerve fiber layer that originates in the superior
retina which corresponds to inferior visual field enters the disc superiorly.
Similarly, the nerve fiber layer that originates in the inferior retina which
corresponds to superior visual field enters the optic disc inferiorly.
The optic chiasm is the area of the visual pathway where the optic
nerves of each eye join. Optic nerves meet near the center of the skull at
the optic chiasm, just above the pituitary gland. Nerve fibers that
originate nasal to the fovea cross to opposite half of the brain, while
fibers that originate temporally continue along the pathway without
crossing. Visual fields now split into two distinctive halves. Fibers from
the nasal retina (which sees the temporal visual field) cross to opposite
side of brain. Fibers from the temporal retina (which sees the nasal visual
field) do not cross but stay on the same side of the brain. Ten percent of
nerve fibers that represent the entire retina will leave the optic tract and
terminate in the brainstem. These fibers are associated with pupillary
function. The remaining 90 percent of the fibers travel to the lateral
geniculate body.
Lateral geniculate body: Axons leave the optic tract and synapse with
cells that ultimately are going to the occipital cortex. As the axons leave
the lateral geniculate body, they fan out into optic radiations. As the
fibers leave, they travel to the same side in the occipital lobe.
Thank you