Transcript (Eye socket).

THE VISUAL SYSTEM
This is the highly specialized system
responsible for the reception and processing of
visual stimuli and initiation of appropriate
responses to visual sensation. It is made up of
three major components viz.
1. The eye and associated structures, which
accommodate the peripheral visual receptor
organs. These are located in the bony
compartment called Orbit (Eye socket).
2. The visual neural pathway for transmission of
visual information.
3. The visual cortex for processing of visual
information.
THE ORBIT (EYE SOCKET)
• The orbits are two pyramidal-shaped cavities in
the anterior aspect of the skull. Each orbit is
composed of:
• An apex, which is directed posteriorly and opens
into the cranial cavity.
• A base (the orbital margin) which is directed
anteriorly and exposes the eyeball.
• Four walls - superior, lateral, inferior and medial.
• The orbit serves as a bony protective cage for
the eyeball and its associated structures.
• The orbital margin is formed by the frontal,
zygomatic and maxillary bones while the four
walls of the orbit are formed by the frontal,
ethmoidal,
maxillary,
zygomatic,
palatine,
lacrimal and the sphenoidal bones.
CONTENTS OF THE ORBIT:
•
A.
B.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
The contents of the orbit could be organized
into the following groups:
The eyeball, its coverings, tunics and contents
The Extraocular Muscles, which include;
Superior Rectus muscle
Lateral rectus muscle
Inferior rectus muscle
Medial rectus muscle
Superior oblique muscle
Inferior oblique muscle
Levator palpebrae superioris
Superior tarsal muscle
Inferior tarsal muscle
Orbitalis muscle (The orbital muscle of Muller)
C.
1.
2.
3.
4.
5.
6.
Nerves: These include:
Ophthalmic nerve
Optic nerve
Oculomotor nerve
Trochlear nerve
Abducent nerve
Ciliary ganglion
D.
Vessels:
• Ophthalmic artery and its branches
• Ophthalmic vein and its tributaries
• Infraorbital vessels
E.
Other structures:
• Lacrimal gland
• Lacrimal sac
• Nasolacrimal duct
• Extraocular fat
•
•
•
•
•
•
•
•
THE EYE (EYEBALL)
The eye is the highly developed peripheral
photoreceptive organ of the visual system
It is enclosed in a protective bony cavity of the
skull referred to as the orbit.
It occupies the anterior half of this cavity.
It is invested by a thin fascial sheath called
Tenon Capsule.
Deep to the capsule, the eye is further enclosed
in three concentric coats, which consist of:
The outer schlerocorneal layer,
The musculovascular layer and
The retinal layer. This layer accommodates the
photoreceptor cells (Rods & Cones) of the
eyeball
Histological layers of the Retina: (See Diagram)
• Pigmented epithelium
• Outer and inner segments of photoreceptor cells
• External limiting membrane
• Outer nuclear layer of the nuclei of Photoreceptor
cells (Rods and Cones)
• Outer plexiform layer (A synaptic layer)
• Inner nuclear layer of bipolar neurons and other
retinal interneurons
• Inner plexiform layer (A synaptic layer)
• Ganglionic layer which is made up of the cell
bodies of the ganglionic cells
• Nerve fiber layer, consisting of the axons of
ganglion cells
• Internal limiting membrane derived from the
vetreous membrane (Basal lamina) and end feet of
Muller cells and astrocytes.
•
•
•
•
Photoreceptor cells of the Neural Retina
These are the highly differentiated,
specialized neurons of the visual system,
referred to as Rods and Cone
Rods
These are used for peripheral vision and for
vision in the dark (Deem light).
A retinal rod is cylindrical in shape,
measuring about 50 microns with a diameter
of 2 microns.
Each retina contains about 130 million (20
times the number of Cones).
Cones
• Cones are used for color vision and visual
acuity.
• A retinal cone is cone-shaped, measuring
about 40 microns with a diameter of 3-5
microns.
• Each retina contains about 7million cones.
• Cones share the same morphological
subdivisions with rods
• Cones are of three types, based on the
color of light absorbed by their pigments.
• The 3 colors are Red, Blue and Green.
•
•
•
Other Retina Cells include:
Bipolar Retinal Neurons
Ganglion Neurons
Retinal Interneurons which include:
1. Amacrine cells (inner part)
2. Horizontal cells (Outer part)
3. Interplexiform cells (Inner part)
•
The Glial cells which include:
1. Muller cells
2. Astrocytes and
3. microglial cells
Retinal Blood supply and Venous Drainage:
• The retina is nourished from two sources:
• The central artery of the retina, supplies
the neural layers up to the outer border of
the inner nuclear layer of the retina.
• A companion vein of the same name
drains blood from this region.
• Capillary plexus of the choroid part of the
middle tunic supplies the posterior part of
the retina up to the outer border of the
inner nuclear layer.
• This region is devoid of capillary network.
• THE VISUAL PATHWAY
• Nerve
impulse
generated
from
phototransduction of light energy in the
retina is conducted to the visual centres of
the brain along the visual pathway.
• The first order neurons of the visual
pathway are the ganglionic cells of the
retina.
• About a million axons of ganglionic cells
converge on the optic disc of each retina to
form the Optic nerve.
•
• Each optic nerve, which consists of
myelinated fibers traverses the optic canal
at the apex of the orbit and passes
posteromedially to meet its fellow from the
opposite
side
just
beneath
the
hypothalamus.
• At this point, fibers from the nasal halves
of the retina cross the midline at the Optic
chiasma.
• Thereafter, nasal fibers join uncrossed
temporal fibers of the opposite side to form
the Optic tract
•
•
•
Each optic tract passes posteriorly curving
around the rostral part of the midbrain and
terminates at the Lateral geniculate body
Second order neurons of the visual
pathway are located in the lateral
geniculate body Lateral geniculate
neurons give rise to axons, which form the
Geniculocalcarine tract.
This tract projects to the Primary visual
area (Brodmann’s area 17) on either side of
the calcarine sulcus on the medial surface
of the occipital lobe.
•
•
•
•
•
Consequences of Lesions along the
Visual pathway (See Diagram):
Blindness in one eye.
Bitemporal Hemianopsia.
Nasal hemianopsia of one eye.
Left homonymous hemianopsia.
Left homonymous hemianopsia with
macular sparing.