2. Introduction
Vision is such an integral part of neural function that even simple tasks,
such as standing and walking, are not easy with the eyes closed.
Two eyes are necessary for wider peripheral vision and depth perception.
Animals have wider peripheral vision than humans because the visual fields
of each eye do not completely overlap.
In the dog, there is about 50% overlap of the visual fields so that both eyes
perceive the middle half of the field of vision.
This area of visual overlap provides binocular vision for judgment of
distances.
The field of vision outside the binocular zone is the monocular zone.
Binocular vision varies greatly in different animals,reflecting the position
of the eyes in the front of the head.
3. Introduction
To maintain clear binocular vision, both eyes move as a unit.
The eye is equiped with a pupil that adjusts aperture diameter and a lens
that focuses light on the retina, where photoreceptor cells receive images.
However, the retina is not just converting the image into nerve impulses, it
also facilitates feature analysis of the captured image.
Feature analysis and visual information processing take place
progressively as visual signals are passed to the thalamus, rostral colliculus
of the midbrain, and visual cortex.
4. Anatomy of the eye
The wall of the eye consists of three concentric layers.
These layers, proceeding from the outer surface of the eye inward, are the
fibrous tunic, vascular tunic, and neuroepithelial (or inner) tunic.
The fibrous tunic consists of the white outer protective layer encasing most
of the eyeball called the sclera which is modified anteriorly to form the
transparent, stratified squamous epithelial layer called the cornea, through
which light rays enter the eye.
It provides mechanical support and protection of the eye.
The vascular tunic consists of three major structures: the iris, ciliary body
anteriorly, and choroid posteriorly collectively known as the uvea.
These structures are highly pigmented and vascularized(contains many of
the blood vessels that nourish the structures in the eyeball).
Lining the posterior two thirds of the choroid(neuroepithelial or inner tunic)
is the retina,the neural tissue containing the receptor cells.
5. Anatomy of the eye
The pigmented and opaque iris, the colored portion of the eye contains
dilator and sphincter muscles.
The dilator muscle opposes the action of the sphincter muscle.
The pupillary sphincter muscle is circularly arranged in the iris near the
pupillary margin.
It is innervated by the ciliary nerve (postganglionic parasympathetic fibers)
from the ciliary ganglion.
Contraction of the sphincter muscle results in a decrease in pupillary size
(miosis).
The pupillary dilator muscle is part of the pigmented anterior epithelial
cells of the iris.
6. Anatomy of the eye
The anterior portion of these cells have cellular extensions that have
structural characteristics of smooth muscle cells.
Thus, most pigmented anterior epithelial cells are myoepithelial in nature
and their cellular extensions represent the dilator muscle.
They are radially arranged.
This muscle is innervated by the sympathetic postganglionic neurons
located in the cranial cervical ganglion.
The postganglionic fibers run with the ciliary branch of the ophthalmic
nerve to reach the dilator muscle.
Contraction of the dilator muscle results in pupillary dilation (mydriasis).
Pupillary dilation reflects the general state of sympathetic tone, and certain
emotions such as pain, fear, and anger will induce pupillary dilation.
The ciliary muscle is the smooth muscle located in the ciliary body.
7. Anatomy of the eye
This muscle is innervated by the ciliary nerve (parasympathetic fibers),
which contracts the muscle.
The ciliary muscle is primarily oriented meridionally in the dog.
The meridional fibers originate in the inner surface of the sclera posterior to
the iridocorneal angle.
They insert in the stroma of the ciliary body.
The crystalline lens is a transparent structure held in place by a circular lens
suspensary ligament (zonule).
The zonule is attached to the thickened anterior part of the choroid, the
ciliary body.
The ciliary body contains circular muscle fibers and longitudinal muscle
fibers that attach near the corneoscleral junction
8. Anatomy of the eye
The ciliary muscle contracts in response to parasympathetic stimulation and
decreases tension on the zonular fibers supporting the lens.
As a result, the lens becomes more spherical due to its intrinsic viscoelastic
properties.
Focusing on a near object requires a more convex lens that has a shorter
focal distance.
Consequently, when the gaze is directed at a near object, the ciliary muscle
contracts, decreasing the distance between the edges of the ciliary body and
relaxing the zonular fibers supporting the lens.
This results in a more convex shape of the lens (i.e., more spherical and
increased focal power) because of the inherent elasticity of the lens.
This process, by which the curvature of the lens changes to focus on a near
or far object, is called accommodation.
9. Anatomy of the eye
When looking at a distant object, the ciliary muscle relaxes and pulls the
zonular fibers away from the lens, making the lens less convex with a longer
focal distance.
This change is necessary for focusing on far objects.
The choroid consists of loose connective tissue with numerous vasculature and
melanocytes.
It serves a nutritive function for ocular tissue.
Melanocytes prevent light that escapes past the retina from being reflected back
into the retina where it would blur the image.
The “eye‐shine” that occurs at night when light enters the eye is caused by the
tapetum lucidum (Latin tapetum, carpet; lucidum, bright) in the choroid.
Although this light‐reflective surface enhances dark‐adapted vision under dim
light, it scatters light in the retina, affecting sharpness of an image.
The retina is the innermost tunic of the eye and is responsible for detection of
light.
10. Anatomy of the eye
The space between the lens and the retina is filled primarily with a clear
gelatinous material called the vitreous (vitreous humor).
Aqueous humor,a clear liquid that nourishes the cornea and lens, is
produced in the ciliary body by diffusion and active transport from plasma.
It flows through the pupil and fills the anterior chamber of the eye. It is
normally reabsorbed through a network of trabeculae into the canal of
Schlemm
A venous channel at the junction between the iris and the cornea (anterior
chamber angle).
Obstruction of this outlet leads to increased intraocular pressure.
Both vitreous humor and aqueous humor maintain pressure within the
eyeball, thus preventing the eyeball from collapsing.
13. RETINA
The retina extends anteriorly almost to the ciliary body.
It is organized in 10 layers and contains the rods and cones, which are the
visual receptors, plus four types of neurons:bipolar,ganglion,horizontal and
amacrine cells.
There are many different synaptic transmitters.
The rods and cones, which are next to the choroid, synapse with bipolar cells,
and the bipolar cells synapse with ganglion cells.
About 12 different types of bipolar cells occur, based on morphology and
function.
The axons of the ganglion cells converge and leave the eye as the optic nerve.
H and A cells are interneurons that modify flow of information at the synapses
among the photoreceptors, bipolar and ganglion cells.
H-cells connect receptor cells to each other in the outer plexiform layer.
Horizontal(H) cells mediate lateral interactions among bipolar cells and
photoreceptor cells
14. Retina
Amacrine(A) cells connect ganglion cells to one another in the inner
plexiform layer via processes of varying length and patterns.
Amacrine cells mediate lateral interactions among bipolar cells and the
ganglion cells in the inner plexiform layer.
At least 29 types of amacrine cells have been described on the basis of their
connections.
Gap junctions also connect retinal neurons to one another, and the
permeability of these gap junctions is regulated.
Because the receptor layer of the retina rests on the pigment epithelium
next to the choroid, light rays must pass through the ganglion cell and
bipolar cell layers to reach the rods and cones.
The pigment epithelium absorbs light rays, preventing the reflection of rays
back through the retina.
Such reflection would produce blurring of the visual images.
15. Retina
The neural elements of the retina are bound together by glial cells called
Müller cells.
The processes of these cells form an internal limiting membrane on the
inner surface of the retina and an external limiting membrane in the
receptor layer.
The optic nerve leaves the eye and the retinal blood vessels enter it at a
point 3 mm medial to and slightly above the posterior pole of the globe.
This region is visible through the ophthalmoscope as the optic disk
There are no visual receptors over the disk, and consequently this spot is
blind (the blind spot).
Near the posterior pole of the eye is a yellowish pigmented spot, the macula
lutea.
16. Retina
This marks the location of the fovea centralis,a thinned-out, rod-free
portion of the retina that is present in humans and other primates.
In it, the cones are densely packed, and each synapses to a single bipolar
cell, which,in turn, synapses on a single ganglion cell, providing a direct
pathway to the brain.
There are very few overlying cells and no blood vessels.
Consequently, the fovea is the point where visual acuity is greatest (see
Clinical Box on slide 21).
When attention is attracted to or fixed on an object, the eyes are normally
moved so that light rays coming from the object fall on the fovea.
The arteries, arterioles, and veins in the superficial layers of the retina near
its vitreous surface can be seen through the ophthalmoscope.
17. Retina
Because this is the one place in the body where arterioles are readily
visible, ophthalmoscopic examination is of great value in the diagnosis and
evaluation of diabetes mellitus, hypertension, and other diseases that affect
blood vessels.
The retinal vessels supply the bipolar and ganglion cells, but the receptors
are nourished, for the most part,by the capillary plexus in the choroid.
This is why retinal detachment is so damaging to the receptor cells.
19. Retina
C - Cone
R -Rod
MB - Midget bipolar
RB - Rod bipolar
FB - Flat bipolar
DG - Diffuse ganglion cell
MG - Midget ganglion cell
H - Horizontal cell
A - Amacrine cell
20.
21.
22. The fovea
The retinal ganglion cells are located in the inner retina(close to the
vitreous humor) whereas the photoreceptors(rods and cones) are located in
the outer retina(closer to the choroid)
Therefore throughout most of the retina light travels through the
ganglion,bipolar,amacrine and horizontal cells before reaching the
photoreceptors
Although these neurones are unmyelinated(ganglion cell axons become
myelinated as they leave the eye) and therefore relatively transparent,they
still cause some distortion of light rays.
The fovea is an area of the central retina designed especially to minimise
this distortion
In the central fovea in an area called the foveola,the inner ganglion and
bipolar cells are pushed aside,allowing allowing light rays an unobstructed
access to the photoreceptors.
23. The fovea
This is functionally significant because it allows light to have a less
distorted pathway to the region of the retina associated with the highest
visual acuity.
The optic disc is nasal to the fovea.
25. Pigment behind the retina either absorbs or
reflects light depending on the animal’s habits
In animals that rely heavily on acute,day light vision there is a pigment in
the epithelial layer between the photoreceptors and the choroid
This pigment absorbs light that has passed by the photoreceptors without
stimulating them
If such light were reflected back into the retina the sharpness of the visual
image would be blurred
In nocturnal animals however these pigmented layers contain a reflecting
pigment and are called the tapetum.
This allows the retinal to make optimal use of what light it receives,but at
the expense of the visual acuity.
Reflection of light off the tapetum causes the familiar ‘night shine’ from
nocturnal animals.
Tapetum lucidum – reflects the remaining light that has passed through the
photoreceptor layer back towards the photoreceptor layer
26. Pigment behind the retina either absorbs or
reflects light depending on the animal’s habits
The back of the eye of cats,
raccoons and many nocturnal
animals has a reflective coating,
which is why their eyes shine at
night.
The reflective coating is useful for
detecting as much light as
possible in low-light conditions.
27. Horner’s Syndrome
Miosis (i.e., small pupillary size), ptosis (i.e., drooping of the eyelid),
enophthalmos (i.e., slight retraction of the eyeball), and partial prolapse of
the third eyelid.
Miosis results from loss of sympathetic control of the pupillary dilator
muscle.
Ptosis is caused by loss of tone in the smooth muscle of the eyelid.
Ptosis leads to slight retraction of the eyeball (i.e., enophthalmos) and a
partially protruded third eyelid
Enophthalmos reflects loss of tone in the periorbital smooth muscle that
normally pulls the eyeball rostrally.
The periorbital smooth muscle also attaches to the base of the third eyelid,
maintaining its normal retracted position.
28. Horner’s Syndrome
Thus, partial prolapse results from loss of retraction of the third eyelid.
In addition, slight retraction of the eye into the orbit (i.e., enophthalmos)
displaces the third eyelid cartilage, also contributing to the partial protrusion of
the third eyelid.
This reflects the fact that the dog has no specific muscle that sweeps the third
eyelid across the cornea, and the displacement of the third eyelid is passive.
In addition to these clinical signs, a dog may display pink‐colored and warmer
skin (best seen in the ear) due to vasodilation.
Horner’s syndrome may be present when the animal suffers from (i) a middle
ear infection, as sympathetic postganglionic fibers pass through the middle ear
in proximity to the petrosal bone; (ii) severe avulsion of the brachial plexus
(C7–T2) that damages the sympathetic preganglionic fibers to the cranial
cervical ganglion; or (iii) spinal cord lesions that disrupt the reticulospinal tract
from the medullary reticular formation that regulates the visceral motor
neurons of the spinal cord.
29. Horner’s Syndrome
Horner's syndrome is associated with damage to the sympathetic
innervation to the eye.
The damage may have numerous causes, and may occur anywhere along
the course of the nerve's route from the brain to the eye.
Horner's syndrome may be associated with brain tumors, spinal cord injury
in the neck, thoracic tumors, injuries to the neck, choke collar injury,
middle ear infections, and viral, immune mediated or idiopathic (of
unknown cause) neuropathies.
35. Overview of visual pathways
The left blue nerve(left temporal fibre) will capture images from the left
nasal visual field and will carry the visual information down the optic nerve
to the lateral geniculate nucleus of the thalamus where it will synapse with
another neuron which will relay the information to the occipital cortex via
the optic tract.
The right blue nerve(right nasal fibre) will capture images from the right
temporal visual field and will crossover or decussate at the optic chiasma
and then carries the visual information to the left geniculate nucleus of the
thalamus where it will synapse with a neuron(the black neuron) that will
relay the information to the occipital lobe via the optic
tract(geniculocalcarine tract).
36. Overview of visual pathways
The orange nerve on the left eyeball(left nasal fibre) will capture images
from the left temporal visual field and it will decussate at the optic chiasm
and then carries the information to the right geniculate nucleus of the
thalamus where it will pass the signal via synapsis with another neuron in
the geniculate body(black neuron in fig) which will relay the visual
information to the occipital lobe via the optic tract.
The orange nerve on the right eyeball(right temporal fibre) will capture
images from the right nasal field and will not crossover but will carry the
visual information to the lateral geniculate nucleus of the thalamus where it
will synapse with another neurone which will relay the information to the
occipital lobe via the optic tract.
38. Lesions that occur in the visual pathway
If a lesion occurs on the left optic nerve (A),the left nasal fibre and the left
temporal fibre(left blue and orange fibres) cannot bring the visual
information down to the occipital lobe resulting in blindness in the left eye
a condition known as Left Anopia and the vice-versa for right eye is true
(called right Anopia).
If a lesion occurs along the optic chiasm(B) the nerves that crossover(left
and right nasal fibres/optic fibres) that normally capture images from
temporal visual fields cannot send the visual information to the occipital
lobe and this results in blindness in the temporal fields on both sides, a
condition called Bitemporal heminopia.
If a lesion occurs after the optic chiasm(C) there is loss of visual
information from left nasal and right temporal field thus there is blindness
in those fields,a condition called right homonymous heminopia.
39. Lesions that occur in the visual pathway
If a lesion occurs along one of the nerves in the optic tract (D) a condition
known as Right homonymous superior quadrant anopia results
40. Optics
Light
Packets of electromagnetic radiation (energy)
Light waves have different wavelengths
Visible light
The (small) range of electromagnetic wavelengths which our eyes can detect:
400-700 nm
Different objects reflect different wavelengths, which we perceive as different
colors
Vision begins when light comes into the eye
Light:
Focused by the cornea and the lens onto the retina (which is a thin layer of
neural tissue at the back of the eye; contains photoreceptors)
Photoreceptors transduce light into neural signals and pass their signals on to
the brain
41. Optics
Location of the eyes
Prey usually have eyes on the sides of their head so they can see behind
them; predators have eyes in front
Refraction
Bending of light rays
Due to change in speed when light passes from one transparent medium to
another
Occurs when light meets surface of different medium at an oblique angle
Image formed at focal point is upside-down and laterally inverted(left-right
reversed)
Accommodation: process by which the lens add extra focusing power by
changing its shape.
42. Optics-Accomodation and the lens
Through accommodation, the lens
changes shape to focus images
from various distances onto the
retina
Primary ocular structures
responsible for accommodation:
Ciliary muscle
Suspensory ligaments
Lens
43. Focusing Light on The Retina
Pathway of light entering eye: cornea, aqueous humor, lens, vitreous
humor, entire neural layer of retina, photoreceptors
Light refracted at boundaries along pathway
Air to cornea/aqueous humor
Aqueous humor to lens
Lens to vitreous humor
Most bending happens at air-cornea boundary
Lens curvature is the “fine adjustment”
44. Focusing For Distant Vision
Eyes best adapted for distant vision
Far point of vision
Distance beyond which no change in lens shape needed for focusing
20 feet for emmetropic (normal) eye
Cornea and lens focus light precisely on retina
Ciliary muscles relaxed
Lens stretched flat by tension in ciliary zonule
45. Focusing For Distant Vision
For distant vision.
Sympathetic input relaxes the ciliary muscle,
tightening the ciliary zonule
and flattening the lens.
For near vision
Light from close objects (<6 m) diverges as approaches eye
Requires eye to make active adjustments using three simultaneous
processes
Accommodation of lenses
Constriction of pupils
Convergence of eyeballs
46. Focusing for distant and close vision.
Sympathetic activation
Nearly parallel rays
from distant object
Lens
Ciliary zonule
Ciliary muscle Inverted
image
Lens flattens for distant vision. Sympathetic input
relaxes the ciliary muscle, tightening the ciliary zonule,
and flattening the lens.
47. Focusing for near vision
Accommodation
Changing lens shape to increase refraction
Near point of vision
Closest point on which the eye can focus
Presbyopia—loss of accommodation with age
Constriction
Accommodation pupillary reflex constricts pupils to prevent most divergent
light rays from entering eye
Convergence
Medial rotation of eyeballs toward object being viewed.
48. Figure 15.13b Focusing for distant and close vision.
Parasympathetic activation
Inverted
image
Divergent rays
from close object
Lens bulges for close vision. Parasympathetic input
contracts the ciliary muscle, loosening the ciliary zonule,
allowing the lens to bulge.
49. Focusing for near vision
For close vision.
Parasympathetic input contracts the ciliary muscle
loosening the ciliary zonule
allowing the lens to bulge.
Light rays from a near object diverge on entering the eye.
50. Problems Of Refraction
Myopia (near sightedness)
Focal point in front of retina, e.g., eyeball too long
Corrected with a concave lens
Hyperopia (farsightedness)
Focal point behind retina, e.g., eyeball too short
Corrected with a convex lens
Astigmatism
Unequal curvatures in different parts of cornea or lens
Corrected with cylindrically ground lenses or laser procedures
54. Photoreceptors
Rods & cones are the primary receptors
Both are divided into 3 parts:
synaptic terminal
inner segment
outer segment
Outer segment- specialized for photoreception
Outer segment- has visual photopigments which initiate the
phototransduction of light into neural electrical signals
55. Photoreceptors-Accomodation and
light and Dark adaptation
Accomodation
The process of adjusting the lens
in your eye for different viewing
distances
Light Dark Adaptation eyes
accomplish this feat by switching
off between using rods and using
cones
Switching between rods and cones
can take a while
57. Functional Anatomy Of Photoreceptors
Rods and cones
Modified neurons
Receptive regions called outer segments
Contain visual pigments (photopigments)
Molecules change shape as absorb light
Inner segment of each joins cell body
59. COLOUR VISION SYSTEM
Man-3 sensitive cones-red, blue, green
dog-two- yellow and blue
Colour is a result of the perception three visual pigments with overlapping
wavelengths.
Equal stimulation of cones having a red pigment & green pigment produces
perception of a yellow colour
60. Figure 15.15a Photoreceptors of the retina.
Process of
bipolar cell
Synaptic terminals
Rod cell body
Inner fibers
Nuclei
Cone cell body
Mitochondria
Connecting cilia
Outer fiber
Apical microvillus
Discs containing
visual pigments
Discs being
phagocytized
Melanin
granules
Pigment cell nucleus
Basal lamina (border
with choroid)
Inner
segment
Pigmented
layer
Outer
segment
The outer segments
of rods and cones
are embedded in the
pigmented layer of
the retina.
Rod cell body
61. Rods
Functional characteristics
Very sensitive to light
Best suited for night vision and peripheral vision
Contain single pigment
Pathways converge, causing fuzzy, indistinct images
62. Cones
Functional characteristics
Need bright light for activation (have low sensitivity)
React more quickly
Have one of three pigments for colored view
Result in high visual acuity
Color blindness–lack of one or more cone pigments
64. DIFFERENCES BETWEEN RODS & CONES
RODS CONES
Used for scotopic vision (Dim light
vision)
Used for photopic vision
Very sensitive to light; sensitive to
scattered light
Not very light sensitive; sensitive to only
direct light
Loss causes nyctalopia Loss causes hemeralopia
Low visual acuity High visual acuity
Not present in fovea Concentrated in fovea
Slow response to light Fast response to light
65. DIFFERENCES BETWEEN RODS & CONES (CTD...)
RODS CONES
More pigment than cones, so can detect
low light levels
Less pigment than rods; require more
light to detect images
Stacks of memb. enclosed disks are
unattached to cell membrane directly
Disks are attached to outer membrane
One type of photosensitive pigment 3 types of photosensitive pigments
Confer achromatic vision Confer colour vision
66. Chemistry Of Visual Pigments
Retinal
Light-absorbing molecule that combines with one of four proteins (opsins)
to form visual pigments
Synthesized from vitamin A
Isomers: cis- (bent) and trans- (straight)
Absorbing a photon causes bent-to-straight (cis –to-trans) shape change
Change from bent-to-straight initiates reactions electrical impulses
along optic nerve
Rhodopsin = cis-retinal (bent retinal) + opsin
67. Figure 15.15b Photoreceptors of the retina.
Rod discs
Rhodopsin, the visual pigment in rods,
is embedded in the membrane that forms
discs in the outer segment.
Visual
pigment
consists of
• Retinal
• Opsin
68. Phototransduction: Capturing Light
Pigment synthesis
Rhodopsin forms and accumulates in dark
Pigment bleaching
Light absorption causes retinal to change to trans isomer
Retinal and opsin separate (rhodopsin breakdown)
Pigment regeneration
trans retinal converted to cis
Cis-retinal rejoins opsin to form rhodopsin
69. .
Enzymes slowly convert
all-trans-retinal to its 11-
cis form in cells of the
pigmented layer; requires
ATP.
Pigment regeneration:
Light absorption by
rhodopsin triggers a
rapid series of steps in
which retinal changes
shape (11-cis to all-
trans) and eventually
releases from opsin.
Pigment bleaching:
11-cis-retinal, derived
from vitamin A, is
combined with opsin
to form rhodopsin.
Pigment synthesis:
1
2H+
2H+
All-trans-
retinal
All-trans-retinal
Rhodopsin
Dark
3
2
11-cis-retinal
Vitamin A
Oxidation
Reduction
Opsin and
Light
11-cis-retinal
O
70. Events of phototransduction.
Retinal absorbs light
and changes shape.
Visual pigment activates.
Light
(1st
messenger)
Receptor G protein Enzyme 2nd
messenger
Visual
pigment
1
Light
11-cis-retinal
Transducin
(a G protein)
All-trans-retinal
2 3
Visual pigment
activates
transducin
(G protein).
Transducin
activates
phosphodiesteras
e (PDE).
4 5
PDE converts
cGMP into GMP,
causing cGMP
levels to fall.
As cGMP levels fall,
cGMP-gated cation
channels close, resulting
in hyperpolarization.
cGMP-gated
cation
channel
open in
dark
cGMP-gated
cation
channel
closed in
light
Phosphodiesterase (PDE)
71. Phototransduction In Cones
Similar as process in rods
Cones far less sensitive to light
Takes higher-intensity light to activate cones
72. Light Transduction Reactions
Light-activated rhodopsin activates G protein transducin
Transducin activates PDE, which breaks down cyclic GMP (cGMP)
In dark, cGMP holds channels of outer segment open Na+ and Ca2+
depolarize cell
In light cGMP breaks down, channels close, cell hyperpolarizes
Hyperpolarization is signal!
73.
74. Signal transmission in the retina .
In the dark
cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
1
Voltage-gated Ca2+
channels open in synaptic
terminals.
Neurotransmitter is
released continuously.
Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
Hyperpolarization closes
voltage-gated Ca2+ channels,
inhibiting neurotransmitter
release.
No EPSPs occur in
ganglion cell.
No action potentials occur
along the optic nerve.
Photoreceptor
cell (rod)
Bipolar
Cell
Ganglion
cell
Ca2+
−40 mV
−40 mV
2
3
4
5
6
7
Ca2+
Na+
75. Signal transmission in the retina
−70 mV
No neurotransmitter
is released.
Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
EPSPs occur in ganglion
cell.
Action potentials
propagate along the
optic nerve.
cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
Lack of IPSPs in bipolar
cell results in depolarization.
Voltage-gated Ca2+
channels close in synaptic
terminals.
1
Photoreceptor
cell (rod)
Bipolar
Cell
Ganglion
cell
In the light
Light
Ca2+
−70 mV
2
3
4
5
6
7
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
Light
76. THE OPTIC PATHWAYS
Light from environment
↓
Passes through pupil
↓
ACCOMMODATION & PUPILLARY LIGHT REFLEX
↓
Retina
↓
Optic nerve via blindspot/
↓ ↓ ↓
Retino-geniculo-striate Retino-tectal Retino-hypothalamic pathway
pathway pathway ↓
↓ ↓ Hypothalamus
Lateral geniculate nucleus Pretectal region ↓
↓ (Anterior rostral colliculus) Seasonal change in day
↓ length
Primary visual cortex Pupillary reflexes
in occipital lobe Reflex orientation of the
↓ eye to visual targets
Conscious visual perception
of form; colour; motion;
Orientation & depth
77. Projections of the Optic Nerve
Lateral Geniculate Nucleas (main relay to cortex)
Superior Colliculus (eye movement)
Pretectal Nuclei (pupillary light reflex)
Accessory optic system
Biological clock (suprachiasmatic n.)
78. BINOCULAR VISION AND DEPTH PERCEPTION
Binocular field: the field of vision, simultaneously rviewed by both eyes.
Varies between spp, depending on the placement of eyes in the skull.
Widest in cats- 90⁰; 60-75⁰ (horses) & 15⁰ (poultry)
The visual fields from each eye intersect so that there is some visual input
from the same stimuli from each eye
Overlap is necessary for normal up-close depth perception
Predatory spp have large binocular fields of vision
Prey spp have relatively smaller binocular fields but extensive peripheral
vision
79. Visual Pathway To The Brain
Axons of retinal ganglion cells form optic nerve
Half of the fibers (medial half) of each optic nerve cross over at optic
chiasm; optic tracts exit
Most optic tract fibers go to lateral geniculate nucleus of thalamus
Fibers from thalamic (LGN) neurons form optic radiation and project to
primary visual cortex in occipital lobes
Other optic tract fibers go to superior colliculi in midbrain (initiating visual
reflexes)
A few ganglion cells contain melanopsin and project to other brain areas
Regulate pupil diameter, daily rhythms
83. There is a light (a red star) to
the left of midline. The star is
visible in both eyes, because it
is not extremely far to the left.
Where are the images of the
star on the two retinas? The
red lines show the nerve
axons activated. Where are
they? (Which optic nerve(s),
which optic tract(s), which
LGN(s)…).
We are viewing this transverse
section through the brain “from
below” – as if we are standing at
the foot of the bed of the patient,
who is lying supine. That’s why
the patient’s left eye and left
visual field are on the right side of
the diagram.
84. Visual Processing
Retinal cells
– Color, brightness, edge detection (by amacrine and horizontal cells)
Lateral geniculate nuclei of thalamus
– Process for depth perception, cone input emphasized, contrast
sharpened
Primary visual cortex (striate cortex)
– Neurons detect edges, object orientation, movement
– Provide form, color, motion inputs to visual association areas
(prestriate cortex)
Occipital lobe centers (prestriate cortex) continues processing form, color,
movement
85. Morphological changes
Ophthalmological examination
Lacrymal flow & Corneal ulcers (Fluorescien test)
Fluorescein test (for corneal integrity)
Neurological exam (pupil light reflex, following of a visual object, avoidance of a
menacing gesture, ERG [for retinal integrity prior to cataract surgery]
Diagnosis of conditions altering vision in
domestic species
86. Conditions that affect the eye
Glaucoma: the draining of the aqueous humor is
blocked and pressure is built up inside the eye which
impinges on the blood vessels and the optic nerve. If
caught early, it can be treated by medication or by
surgery
Cataract: a clouded lens which, if serious, can be
removed and replaced surgically
