Eyelashes

Help prevent foreign matter (insects) from reaching surface of eye

Conjunctiva

Epithelium covering inner surfaces of eyelids and outer surface of eye 

Group of structures that produces and drains lacrimal fluid or tears

Spreads over anterior surface of eyeball by blinking

Cleans, lubricates, washes away irritating substances, and kills microbes on eyeball

After passing over eyeball, the tears pass through several structures and eventually drain into the nasal cavity (just inferior to inferior nasal concha) 

Superior, inferior, medial, and lateral rectus muscles

Superior and inferior oblique muscles

6 extrinsic eye muscles attached to each eyeball:

1)  Fibrous tunic: outer layer

2)  Vascular tunic or uvea: middle layer

3)  Retina: inner layer

“White of the eye”: tough outer layer of the eyeball (except for the cornea in front)

Composed of dense fibrous connective tissue

Gives shape to eyeball, protects its inner parts, and serves as site of attachment for extrinsic eye muscles

Cornea

Outer layer of eyeball anteriorly

Transparent coat that covers colored iris

Avascular

Curved surface that helps focus light onto the retina

Most sensitive portion of eye due to lots of free nerve endings

Corneal damage may cause blindness

Cornea has restricted ability to repair itself so injuries to it must be promptly treated to prevent scarring and serious vision loss

Once corneal scarring has occurred, vision can be restored by corneal transplant

Most common form of transplant

Transplants can come from unrelated donors (no blood vessels to carry WBCs to attack foreign cells)

Pigmented layer that contains choroid, ciliary body, and iris

Contains numerous blood and lymphatic vessels, and smooth muscles of eye

Functions:

1)  Provide route for blood and lymphatic vessels to supply tissues of eye

2)  Regulate amount of light that enters eye

3)  Secrete and reabsorb aqueous humor that circulates within chambers of eye

4)  Control shape of lens

Functions of Vascular Tunic/Uvea: Middle Layer

Choroid

Posterior portion of vascular tunic

Lines inner surface of sclera

Highly vascularized – provides nutrients to the retina

Contains melanocytes that produce melanin

Causes this layer to be dark brown

Absorbs stray light rays and thus prevents scattering of light in eyeball – so image sent to retina is clearer and sharper

Ciliary body

Anterior portion of vascular tunic – basically where choroid becomes ciliary body

Forms a circular collar around lens

Appears dark brown: contains melanocytes

Contains ciliary processes: secretes aqueous humor

Suspensory ligaments of the lens attaches to the ciliary processes

These ligaments hold the lens behind the iris and centered on the pupil

Contains ciliary muscle: smooth muscle connected to lens

Alters shape of lens adapting it for near or far vision

lens attaches to the ciliary processes

These ligaments hold the lens behind the iris and centered on the pupil

smooth muscle connected to lens

Alters shape of lens adapting it for near or far vision

Iris (“rainbow”)

Colored portion of eyeball

Flattened circular structure between cornea & lens

Contains melanocytes: amount of melanin determines eye color which is genetically determined

Low melanin = blue eyes

Iris (“rainbow”)

Main function is to regulate the amount of light entering the eyeball through the pupil (hole in center of iris)

Contains two layers of smooth muscles called pupillary muscles; when they contract, they change the diameter of the pupil

“Red eye” in photographs

If bright light is directed into pupil – the reflected light is red due to blood vessels over surface of retina

Lines the posterior ¾ of the eyeball and is the beginning of the visual pathway

Contains optic disc: site where optic (II) nerve exits the eyeball

Central retinal artery and vein are bundled with optic nerve

Blind spot: lacks photoreceptors (rods or cones), so you cannot see an image that strikes this spot; do exercise on page 561 of textbook

Pigmented layer

Melanin-containing epithelial layer between choroid and neural layer

Absorbs stray light rays; prevents light scattering

Neural layer

Multilayered outgrowth of brain that contains retinal neurons that process visual data before sending nerve impulses into axons that form optic nerve

Two Types: Rods and cones

Rods

Most abundant photoreceptor

Stimulated by low, dim light

No color: can only see black, white and shades of gray

Basis of night vision

Loss of rod vision = difficulty seeing in dim light

Cones

Stimulated by brighter light

Produces color vision (blue, red, and green cones)

Gives us sharper, clearer images

Loss of cone vision = legal blindness

exact center of posterior portion of retina, at the visual axis of the eye

Contains no rods

small depression in center of macula

Highest concentration of cones!!!

Area of highest visual acuity (sharpness of vision)

Provides ability to see fine details

Surface of retina is only place in body where blood vessels can be viewed directly and examined for pathological changes that occur in diseases such as:

Hypertension, diabetes mellitus, cataracts

Instrument that shines light into eye and allows observer to see through pupil; provides a magnified image of retina, its blood vessels and optic (II) nerve

Neural & pigmented layers of retina separate

Due to trauma (blow to head) or various eye disorders 

Fluid accumulates between layers and causes retina to push outward 

Causes distorted vision and blindness

Photoreceptor layer containing rods and cones is dependent on diffusion of oxygen and nutrients from the choroid

If two layers are not reattached, photoreceptors will degenerate and die

Treated by laser surgery which “welds” the two layers together

Found behind pupil and iris

Held in place by suspensory ligaments that originate on ciliary body of choroid

Avascular and transparent

Consists of a capsule with proteins (crystallins) in layers

Responsible for the clarity and focusing power of lens

Along with cornea focuses images on retina

Loss of transparency of lens

Lens becomes cloudy and turns yellowish due to changes in structure of lens proteins

Over time will need brighter light for reading and visual clarity will fade

Common cause of blindness (if lens becomes completely opaque)

Caused by aging, injury, excessive exposure to UV rays, diabetes, smoking, some medications 

Can remove old lens and replace with artificial lens

Anterior cavity

Composed of anterior chamber ( between cornea and iris) and posterior chamber (between iris and lens)

Contains aqueous humor: watery fluid that nourishes cornea and lens and maintains eye’s shape

Continuously secreted by ciliary processes and drains out of cavity by scleral venous sinus (canal of Schlemm) into the blood; replaced every 90 minutes

Vitreous Chamber (Posterior Cavity)

Much larger than anterior cavity

Lies between lens and retina

Maintains shape of eye

Transparent jelly-like substance

Contains collagen fibers and proteoglycans

Holds retina in place against choroid

Does not undergo continuous replacement like aqueous humor

Pressure in the eye

Produced mainly by aqueous humor and partly by vitreous body

Maintains shape of eyeball and prevents it from collapsing

Stabilizes position of retina by pressing neural part against pigmented part

Pressure can be measured in anterior chamber (fluid pushing against cornea)

Applanation tonography: small, flat disk is placed on anesthetized cornea to measure pressure

Most common cause of blindness in U.S.

Affects 2% of individuals over age 35

Caused by abnormally high intraocular pressure due to buildup of aqueous humor within anterior cavity

Lens compresses into vitreous body which then puts pressure on retinal neurons (painless)

Leads to visual impairment (tunnel vision), irreversible destruction of retinal neurons, damage to optic nerve, and eventual blindness

Risk factors: race (*African-Americans), age, family history, past eye injuries and disorders

Treatment: topical application of drugs or laser surgery (poke hole in anterior chamber)

Three processes of how the eye forms clear images of objects on the retina:

Refraction or bending of light by the cornea and lens

Accommodation: the change in shape of the lens

Constriction or narrowing of the pupil (diameter of the “hole” gets smaller through which light can enter the eye)

Autonomic reflex that occurs simultaneously with accommodation

is the bending of light rays through transparent substances of different densities

The cornea and lens “bend” or refract light rays entering eye

Most refraction occurs at cornea

The lens then focuses light rays from an object toward a specific point of intersection on retina (focal point)

Increase in the curvature of the lens for close or near vision (less than 20 ft.)

Light rays from objects less than 20 ft are more divergent so they must be refracted more by lens to be focused on retina

When you view a close object, the ciliary muscle contracts and the lens becomes more spherical or rounded

Increases its focusing power and causes greater convergence of the light rays 

With aging, lens loses elasticity and its ability to accommodate e.g. its ability to focus on objects that are close

Older people cannot read print at same close range as younger people – need “reading glasses”

Usually begins in mid-forties

Near point of vision = minimum distance from eye an object can be clearly focused with accommodation

3 - 4 inches in child

6 - 8 inches in young adult

33 inches in 60 year old

Images focused on the retina are inverted (upside down) and are left-right reversed

Brain “compensates” early in life for this image reversal, and we are unaware of any difference between orientation of image on retina  and that of object.

eyeball is too long or lens is thicker than normal

Image is focused in front of the retina 

Close objects seen clearly; distant objects are blurred

Distant objects seen clearly; close objects are blurred

: cornea or lens has abnormal curvature

Parts of image are out of focus and vision is blurred or distorted

Correction of vision

Glasses, contact lenses or LASIK

Nearsightedness: use concave lens

Farsightedness: use convex lens

LASIK: laser-assisted in-situ keratomileusis

Doctor uses laser and aid of computer to reshape the curvature of cornea

70% of LASIK patients achieve normal vision

Outer segment:

Contains discs which contain visual pigments: colored proteins that undergo structural changes when it absorbs light

Transduction of light energy into receptor potential occurs here!

Contains nucleus, Golgi complex, mitochondria

Releases neurotransmitters

Synapses with a bipolar cell 

 Inherited inability to distinguish between certain colors

Results from absence or deficiency of one (or two) of the three types of cones

More likely to occur in males (10% of all males)

Most common type is red-green color blindness, in which red cones are missing

Inability to see well at low light levels

Retinal is a derivative of vitamin A and is the light-absorbing portion of rhodopsin

Results from prolonged vitamin A deficiency

Body has vitamin A reserves for several months, and much is stored in pigmented part of retina

Both day and night vision are affected but the problem first becomes apparent at night

1)  External (outer) ear

Collects sound waves and channels them inward

2)  Middle ear

Conveys sound vibrations to oval window

3)  Internal (inner) ear

Contains receptors for hearing and equilibrium (balance)

Auricle

Flap of elastic cartilage shaped like flared end of trumpet and covered with skin

Collects sound waves, channels them into canal

External auditory canal

Curved tube about 1 inch long that lies in temporal bone and leads to eardrum

Channels sound waves to eardrum

Contains hairs & ceruminous glands that secrete cerumen (earwax)

Prevents dust and foreign objects from entering ear

Eardrum (tympanic membrane)

Thin, semitransparent membrane between external auditory canal and middle ear

Vibrates in response to sound waves

Perforated eardrum: tearing of tympanic membrane from trauma, pressure from cotton swab, or middle ear infection

Takes about a month to heal

Otoscope: an instrument that illuminates and magnifies external auditory canal and tympanic membrane

Small air-filled cavity within temporal bone that is lined by epithelium

Contains auditory ossicles

Three small bones which bridge the middle ear cavity from eardrum to oval window (entrance to inner ear)

Malleus 

hammer

vibrate with the eardrum, amplify (make it stronger), and transfer the vibration to the membrane of the oval window (which creates a pressure wave in the fluid of the inner ear)

Middle ear muscles

Tiny skeletal muscles 

Tensor tympani (inserts on malleus)

Stapedius (inserts on stapes)

Contract reflexively to reduce vibrations of the ossicles to very loud sounds to protect inner ear

Auditory (Eustachian tube)

Connects middle ear cavity with nasopharynx (superior portion of throat)

Allows equalization of air pressure between middle ear and atmosphere (when you yawn or swallow); “ear popping”

Route for pathogens to travel from nose and throat to middle ear to cause infection – otitis media

superficial contours of internal ear formed by layer of dense bone

Continuous with surrounding temporal bone

delicate interconnected network of fluid-filled tubes

Receptors for internal ear are found here

1)  Vestibule

1)  Vestibule

Oval-shaped; found in central portion 

Contains receptors for equilibrium (balance)

Three tubes that project from vestibule and are oriented at right angles to each other

Contain receptors for equilibrium

Bony spiral-shaped canal; looks like snails shell

Contains cochlear duct of membranous labyrinth

Contains auditory (hearing) receptors

Contains three channels that are filled with fluid (cochlear duct, scala vestibuli, scala tympani) that are involved in transmission of sound waves

are interconnected and form one long and continuous chamber: 

Begin at oval window; go through scala vestibuli; around top of cochlea; go along scala tympani; end at round window (membrane between fluid-filled cochlear chambers and air-filled middle ear)

is an elongated tubelike structure that lies between scala vestibuli and scala tympani

Contains spiral organ or organ of Corti

Contains hair cells which are the receptors for hearing

Hair cells are sandwiched between basilar and tectorial membranes of spiral organ

Hair cells change mechanical vibrations (sound waves) into electrical signals!!!!!!

Hair cells synapse with sensory and motor neurons with cochlear branch of vestibulocochlear (VIII) nerve

is the perception of sound, which consists of waves of pressure conducted through air or water

Sound travels in waves from a vibrating object

Think of the ripples that are created when you toss a stone in a pond

The frequency (or pitch) of a sound vibration is measured in hertz (Hz); 1 Hz = 1 cycle per second

Entire human audible range from 20 to 20,000 Hz

Typically most sounds heard by human ears vibrate at frequencies from 500 – 5000 Hz

The larger the intensity (size or amplitude) of the vibration, the louder the sound

Sound intensity (loudness) measured in decibels (dB)

Normal conversation = 60 dB 

Hair cells in cochlea are damaged to continued exposure to sounds over 90 dB – can lead eventually to deafness

Sound waves

External ear

Auricle→ external auditory canal→ tympanic membrane 

Middle Ear

Auditory ossicles (malleus→ incus→ stapes)→ oval window 

Inner Ear

Cochlea: →spiral organ (organ of Corti) → hair cell → receptor potential→ nerve impulse

Frequency or pitch of a sound will distort the basilar membrane in the spiral organ of cochlea at specific locations.

High pitch sounds (shorter wavelengths) vibrate basilar membrane nearer oval window

Low pitch sounds (longer wavelengths) vibrate basilar membrane nearer round window

Frequency of a sound is translated into information about its position along basilar membrane

Loudness or intensity of a sound will affect amount of force applied to stapes and ultimately how much the basilar membrane moves at a given location

Louder the sound the more forceful the stapes moves and the more the basilar membrane moves

Hair cells of cochlea → cochlear branch of vestibulocochlear (VIII) nerve → brain stem (medulla oblongata and midbrain) → thalamus → primary auditory areas of cerebral cortex in temporal lobe.

Right and left auditory areas in cerebrum receive nerve impulses from both ears. 

Damage to auditory areas: person can respond to sounds and have normal auditory reflexes but difficult or impossible to interpret sounds and recognize a pattern in them

Caused by damage of hair cells in cochlea or damage to cochlear branch of vestibulocochlear (VIII) nerve

Caused by atherosclerosis (reduces blood supply to ears), repeated exposure to loud noise (destroys hair cells) and/or by certain drugs (aspirin, streptomycin)

Caused by impairment of external or middle ear mechanisms for transmitting sounds to the cochlea

Caused by aging (thickening of eardrum and stiffening of joints in auditory ossicles) and impacted earwax