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Lab 6
VISION, EYEBALL MOVEMENT
AND BALANCE SYSTEMS I
The 5 L’s
Lower retina (upper visual field)
Lateral part of LGN
Loop of Meyer = Lower part
(temporal) of optic radiations
Lingual Gyrus
In the conscious visual path, impulses travel
from receptors in the retina to the visual cortex
for perception of light stimuli. Visual impulses
traverse rod and cone, bipolar, and then ganglion
cells in the retina. Axons of ganglion cells exit the
eyeball in the optic nerve and traverse the optic
tract (after 50% decussation in the chiasm). The
optic tract terminates in the lateral geniculate
nucleus of the thalamus. Thalamic neurons then
send axons through the optic radiations
(retrolenticular fibers) to the primary visual
(calcarine) cortex.
The eyeball consists of 3 concentric coats:
(1) external or fibrous coat: sclera (posteriorly) and cornea (anteriorly)
(2) middle or vascular coat: choroid, ciliary body, iris
(3) internal or retinal coat: outer layer of pigmented cells and inner layers of neural elements posterior to the ora serrata
The four
refractive
media
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Light
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-adapted for ion-pumping
-pumps fluid from the corneal stroma, preventing
corneal opacification from excessive hydration
- the main layer of the cornea and
consists of some 70 broad sheets
of tightly bound, parallel collagen
fibers embedded in an extracellular
matrix
-the direction of the collagen fibers
differs in each layer for maximum
mechanical strength
- the absence of blood vessels in
the cornea and the regular
arrangement of the collagen fibers
make the cornea translucent,
allowing it to transmit light
-stratified non-keratinized
squamous
The anterior and posterior aqueous chambers communicate with each other via the aperture of the pupil. Aqueous humor resembles cerebrospinal
fluid in composition and is secreted by the epithelial cells of the ciliary processes in the posterior chamber. It flows from there to the anterior
chamber through the pupil and provides nutrients to the structures that it bathes. The following figure indicates the path for the flow of the
aqueous humor
Aqueous
Humor
Glaucoma results
from abnormal
drainage of the
aqueous humor and
is characterized by a
rise in the intraocular
pressure. If not
treated, blindness can
result from damage to
the nerve cells of the
retina.
Lens
Epithelium
Lens Capsule
After filtering through a network of spaces (spaces of Fontana ) lined by endothelium (trabecular meshwork), which runs around the
circumference of the root of the iris at the periphery of the anterior chamber, the aqueous humor enters the canal of Schlemm . This
canal runs around the whole circumference of the limbus within the sclera and drains into venous vessels. It should be
mentioned that there is normally a resistance to the flow of aqueous humor through the trabecular meshwork.
Consequently, continued secretion and resorption of the aqueous humor is responsible for the normal intraocular pressure
(10-22 mg Hg).
The axons of the retinal ganglion cells all collect at the optic disc where they penetrate the sclera to form the optic nerve.
This disc is also called the blind spot
The pigmented, vascular layer, the choroid (the middle layer of the posterior 2/3 of the eyeball) .
extends from the ora serrata (the anterior margin of the neural retina) to the optic nerve
contains blood vessels and lymphatics supporting the retina.
It will appear as a dark brown sheet which blends with the sclera in its outer portion
The inner portion is attached to the pigmented epithelium of the retina.
Separation of the retina occurs between the rods and cones and the pigment epithelium
The blood supply to the neural retina arises from the ophthalmic artery ( central artery of the retina and the ciliary arteries ).
The central artery of the retina branches out from the region of the optic disc to serve inner portions of the
retina (four branches go to the four quadrants of the retina).
These branches are end arteries and, if occluded, will result in death of the ganglion cells and therefore total
blindness from that quadrant of the eye.
The posterior ciliary arteries penetrate the sclera around the optic disc and form the choroidocapillary plexus
in the choroid.
Materials from this plexus provide nutrients to the outer portions of the neural retina (the rods and cones).
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outer coat: sclera posteriorly
cornea anteriorly
middle coat: choroid layer, containing pigmented cells
ciliary body (with unstained ciliary muscle)
iris
inner coat: retina (in general, deep to the pigmented layer)
Vitreous body –
Few scattered spindle shaped cells
Fine highly dispersed collagen fibers
ECM rich in Hyaluronic acid
LENS is held in position by a suspensory ligament composed of zonular fibers . The lens is a soft, biconvex structure and has
an outer capsule of hyaline material;
the lens epithelium (below the capsule), which is a layer of cuboidal epithelial cells;
the center region composed of tightly packed cells, which have lost their nuclei and are, chock full of special transparent proteins (crystallins).
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Pretectal Area
Occulomotor N. Edinger Westphall
Parasympathetic fibers
from ciliary ganglion
Cilliary Body
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note the thickened, black circular ciliary body, which
extends from the base of the iris to the ora serrata
(saw-toothed, black margin) at which point it is
continuous with the choroid (the pigmented,
vascular middle layer).
The ciliary body contains the ciliary muscle (smooth
muscle), which you will see in microscopic views.
Contraction of this muscle decreases the tension on the
suspensory ligament of the lens and allows the lens to
round up and become more spherical.
The iris is a sheet-like diaphragm anterior to the lens
and separates the anterior and posterior chambers of
the eyes. The pupil is the circular aperture, which can be
constricted or dilated by the actions of the circular fibers
of the constrictor pupillae (parasympathetic
innervation) and radial fibers of the dilator pupillae
(sympathetic innervation) muscles respectively (you will
see muscles in the eyeball slide).
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GROSS RELATIONS
Haines 3-7
Haines 2-26
Lower visual field
Upper visual field
Calcarine
Cortex
(V1, Area
17) is
along
banks of
calcarine
sulcus
V2 and V3
surround
V1
Haines 3-4
optic tract - ends at lateral geniculate nucleus (not visible)
optic nerve
optic radiations
Haines 4-12
LGN is on
Haines 4-13
Haines 4-13
Conscious Visual Path
Haines 4-1
Haines 5-36
Haines 4-34
Haines 5-26
LGN
Rostral Midbrain Level
Optic Radiations are thalamic fibers traveling to the primary visual cortex
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Optic Radiations