Special sense of vision

SPECIAL SPECIAL SENSES

EXPERT VISION ACADEMY

The Senses

Special senses

Smell

Taste

Sight

Hearing

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

sclera

irispupil

teardrainage canal

cornea

Medial commisure

lateral commisure

palpabre

palpabre

Lacrimal caruncle

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

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

Accessory Structures of the Eye

Eyelids and eyelashes

Conjunctiva

Lacrimal apparatus

Extrinsic eye muscles



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.


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)


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

FLOW OF TEARS

Lacrimal gland

Lacrimal ducts

Sup. or inf. lacrimal canal

Lacrimal sac

Nasolacrimal duct

Nasal cavity


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


Muscles Two types:

Extrinsic Are skeletal muscles Move the eye voluntarily

Intrinsic Are smooth involuntary muscles Located within the eye

Iris muscle Ciliary muscle


Extrinsic eye muscles Six muscles attach to the outer surface of

the eye Produce eye movements


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

Structure of the Eye

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

Structure of the Eye: Vascular Layer

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

bright light normal light dim light

Layers of Eye

Structure of the Eye: Sensory Layer

Retina contains two layers Outer

pigmented layer Inner neural

layer Contains

receptor cells (photoreceptors)

Rods Cones


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

Layers of Retina


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

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

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

Segments, or Chambers, of the Eye

Anterior chamber Posterior chamber Vitreous segment

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

Vitreous humor

Posterior to the lens Gel-like substance with fine fibrils Prevents the eye from collapsing Helps maintain intraocular pressure Transmits light

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

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

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.

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

• Convex lenses cause light waves to converge

• Concave lenses cause light waves to diverge

Air

Glass

(a) (b)

Diverginglight waves

Convexsurface

Lightwave

Converginglight waves

Concavesurface

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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


Focusing on a Near Object

Accomodation

Focusing on a Far Object


Image formed on the retina is

Reversed from left to right Upside down Smaller than the object

Images Formed on the Retina

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.

Principal visual pathways from the eyes to the visual cortex.

Visual Fields and Visual Pathways

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


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”


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


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

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

Color blindness 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

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

Science

Special sense of vision