Download 19. Visual (2)

THE EYE
The globe is spherical in shape . The
eyeball consists of 3 concentric layers
of tissue.
1- The outermost is a fibrous and
protective. It is formed of transparent
cornea through which light enters
the eye. The sclera to which is
attached the extraocular muscles. It
is an opaque white coat .
2- Middle vascular and muscular coat
Anteriorly:
The iris ( smooth muscle ), some of the
muscle fibers of it are arranged in a
circular fashion and innervated by
parasympathetic neurons which act to
constrict the pupil ( central aperture )
thus reduce amount of light falling on
the retina. The other fibers are radial
and innervated by sympathetic neurons
to dilate the pupil.
In the middle :
The ciliary body and its ciliary muscle. This muscle is
innervated by parasympathetic neurons and its
contraction alters the shape and focal length ( power)
of the lens ( accommodation ).
The central aperture of the ciliary body is occupied
by the transparent and biconvex lens which focus
light upon the retina.
The lens is held by a suspensory ligament .
Relaxation of suspensory ligament increase the
convex of lens. The lens and suspensory ligament
divide the lumen of the eyeball into anterior and
posterior part.
Posteriorly :
The choroid and its cells which contain dark
pigments that absorb light and thus reduces
reflection within the eye .
3. Nervous coat : Retina .
Retina
It consists of a non-neural and a neural
portion. The non-neural part is the pigment
epithelium. It is a single layer of light
absorbing cells adjacent to the choroid .
The neural part contains photoreceptors;
neurons and neuroglia and a rich capillary
network.These photoreceptors transduce light
energy into electrical energy. They are Rods
and Cones. The rods are 20 times more than
cones.
20
Rods are sensitive to light . They are important
for vision in dim lighting conditions. They are
predominate in the peripheral parts but their
numbers decrease towards the macula lutea
( the surrounding 1cm to fovea centralis ) ,
where Cons are more .
Cons are responsible for colour vision and
due to their arrangement and neuronal
connections , they confer high visual activity .
At the fovea centralis only cons are present
Also, the neurons and capillaries, through
which light has pass are displaced. So,
cons are directly exposed to light to provide
for maximal visual acuity .
The retina contains the 1st neuron of the
central visual pathway which is formed of
the bipolar cells and also the 2nd neuron
which is formed of the ganglion cells. The
axons of the ganglion cells form the optic
nerve .
The retina also contains interneurons as
horizontal cells and amacrine cells . These
modulate transmission between the
photoreceptors and bipolar cells and
between the bipolar and ganglion cells .
Medial to macula lutea is a region where
retinal axons accumulate to leave the eye in
the optic nerve. This area known as the
optic disc ( blind spot ) . Photoreceptors are
absent from this region.
Refracting media
1- lens
2- Aqueous humour :
It is present in the anterior part in
front of the lens . It is a thin watery
fluid . It is secreted from the ciliary
body . It is reabsorbed into the ciliary
body through canal of Schlemm
where it is returned to venous blood
.
3- Vitreous humour ( body ) :
It lies behind the ciliary body . It is a
gelatinous material
N.B.
The image is centered near the
posterior pole of the eye along the
line of the visual axis in the fovea
centralis and the surrounding 1 cm
The formed image is
inverted in both lateral and
vertical dimensions.
The objects that lie in the
left half of the visual field
form an image upon the
nasal ( right ) half of the
left retina ( of the same
side ) and the temporal
( right ) half of the right
retina and vice versa .
Central visual pathway
The optic nerve enters the cranial cavity
through the optic canal . The 2 optic nerves
converge to form the optic chiasma on the
base of the brain . It lies immediately rostral
to the tuber cinereum of the hypothalamus
and between the terminating internal carotid
arteries .
In the chiasma , the axons derived from the
nasal portions of the 2 retinae decussate and
pass into the contralateral optic tract , while
those from the temporal hemiretinae remain
ipsilateral .
The optic tracts pass round the cerebral
peduncle to terminate mainly in the lateral
geniculate nucleus of the metathalamus.
From the lateral geniculate nucleus , 3rd
thalamocortical neurons project through the
retrolenticular part of the internal capsule and
form the optic radiation , which terminate in
the primary visual cortex on the medial
surface of the hemisphere above and below
the calcarine sulcus .
As the thalamocortical fibers leave the lateral geniculate nucleus they pass
around the lateral ventricle .
Those representing the lower part of the visual field coursing superiorly to
terminate in the visual cortex above the calcarine sulcus .
Those which represent the upper part of the visual field sweep into the
temporal lobe ( Meyer’s loop ) then below the calcarine sulcus .
Surrounding the primary visual cortex , the rest of the occipital lobe constitutes
the visual association cortex . It concerned with Interpretation of visual images ,
recognetion and depth perception ( stereoscopic vision ) and colour vision .
Each optic nerve carries information
concerning both halves of visual field .
Due to decussation of fibers from the
nasal hemiretinae at the optic chiasma ,
each optic tract , lateral geniculate nucleus
and visual cortex receives information
relating only to the contralateral half of the
visual field.
This combination of images from both
eyes is necessary for stereoscopic vision
.
The upper half of the visual field forms
images upon the lower halves of the
retinae. And the lower visual field upon
the upper hemiretinae
So, There is both lateral and vertical
inversion in the projection of the visual
field upon the visual cortex such as , the
upper left quadrant of the visual field is
represented in the lower right quadrant of
the visual cortex .
Pupillary light reflex
The small number of fibers leave the
optic tract, before reaching the lateral
geniculate nucleus ,they pass to
superior colliculus to terminate in the
pretectal nucleus in the midbrain.
The axons of the cells of the pretectal
nucleus synapse around the cells of
Edinger Westephal portion of the
oculomotor nucleus of the same side
and opposite side . The axons of these
cells ( parasympathetic ) pass in both
oculomotor nerves to relay in the ciliary
ganglion in the orbit .
Through the Short ciliary nerves
constriction of the pupil of both eyes
occur . The constriction of the pupil of
the non- illuminated eye is called the
Consensual light reflex. The constriction
of the pupil of the illuminated eye is
called Direct light reflex .
Accommodation Reflex
Fixation upon a nearby object from a distance lead to convergence of the optic
axes ( eyes ); contraction of the ciliary muscles and relaxation of the suspensory
ligaments to increase the covexity of the lens, thus focusing the image. The pupil
is constricted to increase the depth of the focus.
The pathways of it comprise the optic nerve; optic tract; lateral geniculate body;
optic radiation and visual area of the cerebral cortex which is connected by the
superior longitudinal fasciculus to the eye field of the frontal cortex. The fibers
descend through the internal capsule to the nuclei of oculomotor nerve.
From the accessory oculomotor nuclei fibers pass to ciliaris and sphincter
pupillae ( relaying in the ciliary ganglion) and from the ventral part of the
oculomotor nucleus fibers supply the medial recti for the action of convergence of
the eyes.
Visual field deficits
Disease of the eyeball , cataract , intraocular
haemorrhage ,retinal detachment .
Disease of the optic nerve , multiple sclerosis
and tumours lead to loss of vision in the
affected eye Monocular blindness .
Compression of the optic chiasma by
pituitary tumour lead to bitemporal
hemianopia .
Vascular and neoplastic lesions of the optic
tract , optic radiation or occipital cortex ,
produce a contralateral homonymous
hemianopia.
Retinitis pigmentosa is an inherited
metabolic disorder of the photoreceptor and
retinal pigment epithelial cells. There is
progressive night blindness , peripheral
visual field constriction and pigmentation of
the retina .