3. Vision
Dominant sense
70% of all sensory receptors are in the eyes
Converts the energy of light into electrical nerve
impulses that are interpreted by the brain as sight
Visual receptor cells (photoreceptors) sense and
encode patterns of light that enter the eye
Brain uses these signals to give us images
5. The Eye
Each eye has over a million nerve fibers
Protection for the eye
Most of the eye is enclosed in a bony orbit
A cushion of fat surrounds most of the eye
6. The Eye
Eye acts much like a camera
Lens of eye adjusts to bring object into clear
focus
Pupil of eye constricts to allow less light to
enter in bright setting or dilates to allow more
light to enter in darker setting
Through bending of light rays, image reaches
retina
7. Accessory Structures of the Eye
Eyelids and eyelashes
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles
9. Accessory Structures of the Eye
Eyelids
Eyelids (palpebrae)
Tarsal glands lubricate the eye
The tarsal glands (modified sebaceous gland) secrete a fluid
to keep the eye lids from adhering to each other.
The tarsal plate gives form and support to the eyelids.
Eyelashes
Eyelashes double/triple row of hairs.
Eyelashes are innervated by nerve ending – trigger blinking
Eyebrows
Shade
inhibit sweat
help protect the eyeballs from foreign objects, perspiration, and the
direct rays of the sun.
10.
11. Accessory Structures of the Eye
Conjunctiva
Membrane that lines the eyelids
Connects to the surface of the eye
Secretes mucus to lubricate the eye
Functions:
major
produce a lubricating mucus
prevents the eyes from drying out
other
protection
prevents foreign objects from penetrating beyond the confines of its sac
Conjunctivitis is inflammation, produces a pinkish
discoloration (pinkeye)
12. Accessory Structures of the Eye
Lacrimal apparatus
Lacrimal gland—produces lacrimal fluid
Lacrimal canals—drain lacrimal fluid from eyes
Lacrimal sac—provides passage of lacrimal fluid
towards nasal cavity
Nasolacrimal duct—empties lacrimal fluid into the
nasal cavity
13. FLOW OF TEARS
Lacrimal gland
Lacrimal ducts
Sup. or inf. lacrimal canal
Lacrimal sac
Nasolacrimal duct
Nasal cavity
14. Accessory Structures of the Eye
Function of the lacrimal apparatus
Protects, moistens, wash and lubricates the eye
Empties into the nasal cavity
Properties of lacrimal fluid
Dilute salt solution (tears)
Contains antibodies and lysozyme
15. Accessory Structures of the Eye
Muscles
Two types:
Extrinsic
Are skeletal muscles
Move the eye voluntarily
Intrinsic
Are smooth involuntary muscles
Located within the eye
Iris muscle
Ciliary muscle
16. Accessory Structures of the Eye
Extrinsic eye muscles
Six muscles attach to the outer surface of the eye
Produce eye movements
18. Structure of the Eye
Three Layers or coats or tunics forming the wall
of the eyeball
Fibrous layer
Outside layer
Vascular layer
Middle layer
Sensory layer
Inside layer
20. Structure of the Eye: The Fibrous Layer
Sclera
White connective tissue layer
Seen anteriorly as the “white of the eye”
Cornea
Transparent, central anterior portion
Allows for light to pass through
The only human tissue that can be transplanted
without fear of rejection
Both Composed of dense avascular connective tissue
21. Structure of the Eye: VascularLayer
Choroid
Is a blood-rich nutritive layer in the posterior of the eye
Pigment prevents light from scattering
Modified anteriorly into two structures
Ciliary body—smooth muscle attached to lens that change the
lense shape
Iris—regulates amount of light entering eye
• Pigmented layer that gives eye color
• Composed of circular and radial muscles
• Reflex contraction of circular muscle in bright light (small dia of
pupil)
• Reflex contraction of radial muscle in dim light (large dia of pupil)
Pupil—rounded opening in the iris
24. Structure of the Eye: Sensory Layer
Retina contains two
layers
Outer pigmented
layer
Inner neural layer
Contains receptor
cells
(photoreceptors)
Rods
Cones
25. Structure of the Eye: Sensory Layer
Three layers of neurons make up a major portion of the
retina. Named in the order in which the conduct impulses:
Photoreceptor neurons
Bipolar neurons
Ganglion neurons
Signals pass from photoreceptors via a two-neuron chain
Bipolar neurons
Ganglion cells
Signals leave the retina toward the brain through the optic
nerve
Optic disc (blind spot) is where the optic nerve leaves the
eyeball
Cannot see images focused on the optic disc
27. Structure of the Eye: Sensory Layer
Rods
Most are found towards the edges of the retina
Allow dim light vision and peripheral vision
Objects are seen in shades of gray not color
Contains pigment Rhodopsin
Rhodopsin is highly sensitive to light, light causes a
rapid breakdown of the pigment into its scotopsin and
retinal components
Cones
Allow for detailed color vision
Densest in the center of the retina
Fovea centralis—area of the retina with only cones
No photoreceptor cells are at the optic disc, or blind spot
28. Cone sensitivity
Three types of
cones red, green,
and blue
Different cones are
sensitive to different
wavelengths
Color blindness is
the result of the lack
of one cone type
29. Lens
Biconvex crystal-like structure, flexible
Held in place by a suspensory ligament attached to the
ciliary body
Focuses image onto retina
Changes lens thickness to allow light to be properly
focused onto retina
Cataracts result when the lens becomes hard and
opaque with age
Vision becomes hazy and distorted
Eventually causes blindness in affected eye
31. Watery fluid found between lens and cornea
Similar to blood plasma
Helps maintain intraocular pressure
Provides nutrients for the lens and cornea
Helps to maintain the shape of the eye
Prevent the eyeball from collapsing
Aqueous humor
32. Vitreous humor
Posterior to the lens
Gel-like substance with fine fibrils
Prevents the eye from collapsing
Helps maintain intraocular pressure
Transmits light
33. How do we see?
For vision to occur:
An image must be formed on the retina to stimulate
the rods and cones
Nerve impulses must be conducted to the visual
areas of the cerebral cortex
Interpretation occurs
34. Formation of a retinal image
Four processes focus light rays so that they form
clear images on the retina
Refraction of light rays
Accommodation of the lens
Constriction of the pupils
Convergence of the eyes
35. Process of image formation on the retina
1) Refraction:
bending of light as medium changes to focus light
2) Accommodation of lens for near/distance vision:
shape of lens changed by ciliary muscle to make light focus on retina
3) Dilation /Constriction of pupil:
ANS reflex to prevent scattering of light through edges of lens
4) Convergence of eyes:
to focus both eyes on same object and provide vision
Images are focused on the retina upside‑down and mirror‑image, and the
brain then translates this information.
36. Function of the Lens: Light Refraction
Bending of light
Occurs when light passes
at an angle from a
medium of one optical
density into a medium of
a different density
Convex surfaces cause
light to converge
Concave surfaces cause
light to diverge
38. Pathway of Light Through the Eye
Light must be focused to a point on the retina for
optimal vision
The eye is set for distance vision
Accommodation—the lens must change shape
to focus on closer objects
44. Visual Nerve Pathways
Ganglion cell axons leave the eye to form the
optic sensory nerve
The nerve fibers cross anterior to pituitary gland,
the optic chiasma.
Nerve fibers travel to the thalamus and to reflex
centers.
Optic radiations are nerve pathways that lead to
the visual association area in the occipital lobe.
48. Emmetropia—eye focuses images correctly
on the retina
Myopia (nearsighted)
Distant objects appear blurry
Light from those objects fails to reach the retina
and are focused in front of it
Results from an eyeball that is too long
Errors of Refraction
49. Errors of Refraction
Hyperopia (farsighted)
Near objects are blurry while distant objects are
clear
Distant objects are focused behind the retina
Results from an eyeball that is too short or from a
“lazy lens”
50.
51. Errors of Refraction
Astigmatism
Images are blurry
Results from light focusing as lines, not points, on
the retina due to unequal curvatures of the cornea
or lens
Correction
special lenses with uneven curvature
compensates for eye’s asymmetry
allows the image to focus evenly on retina
laser
52.
53.
54. Errors of Refraction
Cataracts
When lens becomes hard
and opaque, our vision
becomes hazy and distorted
Clouding of lens
Inadequate nutrient delivery
to deeper lens fibers
Causes: diabetes mellitus,
smoking, UV damage,
congenital, age-related
hardening, thickening of lens
55. Night blindness
Inhibited rod function that hinders the ability to see at
night
Hampers one’s ability to drive safely at night
most common cause-----prolonged vitamin A deficiency
leads to rod degeneration
treatment
vitamin A supplements
restore function
if administered before degenerative changes occur
56. Colorblindness
Genetic conditions that result in the inability to see certain
colors
Due to the lack of one type of cone (partial color blindness)
sex-linked
more common in males
most common type
red-green
deficit or absence of red or green cones
seen as same color
either red or green
57. Glaucoma
Can cause blindness due to increasing pressure
within the eye
Normally 15 (12-20) mm Hg
Above 25mm Hg an individual has glaucoma which
if untreated can lead to retinal damage and
blindness
Pressure can increase due to decreased drainage
or excess drainage of the aqueous humor which is
normally re-circulated via vessels into vitreous
humor