CH 15: Special Senses – The Eye

CH 15: Special Senses – The Eye

Section 1: Intro to the Eye (p. 548)

Vision is our dominant sense- 70% of all sensory receptors in the body found in eye- nearly half of cerebral cortex involved in visual processing

Basic description - sphere w/ diameter of 2.5cm (1”) - mostly protected by cushion of

fat & walls of bony orbit

The Eye and Vision


Section 2: Accessory Structures of the Eye (pp. 548-

551)

Help to protect eye & aid in function- eyebrows- eyelids- conjunctiva- lacrimal apparatus- extrinsic eye muscles

Accessory Structures of the Eye

Eyebrows- help shade eyes from excessive sunlight- prevent forehead perspiration from reaching eyes


And everyone’s favorite…Eh…it’s only a phase…Some find them pointless…YIKES!!

Eyelids- protect anterior surface of eyelids meet on sides at medial & lateral commissures

“Blinking reflex” - eyelashes in follicles lined w/ very sensitive nerve endings

initiate blinking reflex - occurs every 3-7 seconds - protects the eye from small

particles & drying out


Eyelids- protect anterior surface of eyelids meet on sides at medial & lateral commissures

Eyelid glands - help clean/lubricate the eye & prevent irritation 1) Meibomian (tarsal) glands

- open at edge of eyelid just behind eyelashes- secrete oily substance that prevents eyelid from sticking to eye

& eyelashes from sticking together

2) Ciliary glands- found in eyelash follicles…modified sweat glands


Eyelid infections1) Chalazion

- infected Meibomian gland- causes painful swelling or cyst on eyelid

2) Sty- inflammation of ciliary glands- looks like small pimple around eyelash follicles


Conjunctiva- transparent membrane lining eyelids & surface of eye- only covers white part of eyes…not cornea (clear)- major function = production of lubricating mucus to help

prevent dry eyes

Common problems1) Conjunctivitis

- red, irritation caused by inflammation of conjunctiva2) Pinkeye

- highly contagious bacterial or viral infection of conjunctiva


Lacrimal (“tear”) apparatus- includes the lacrimal glands & ducts that drain excess tears

into nasal cavity

Main structures - lacrimal gland - lacrimal puncta - lacrimal sac - nasolacrimal duct


Tears- dilute saline solution containing mucus, antibodies & lysozymes

to kill bacteria- tears formed by lacrimal gland located above lateral side of eye

- blinking spreads tears across eye toward medial commissure- two small openings (called “lacrimal puncta”) located on medial

commissure collect tears & drain into nasolacrimal duct- nasolacrimal duct drains into nasal cavity

Function of tears- wash away or dilute irritating substances- importance of emotionally induced tears is poorly understood


Extrinsic eye muscles - six strap-like muscles that control eye movements - also help maintain shape of eye - four are rectangular shaped & named for movements they allow - two are wrapped around the eye…one top & one bottom



Extrinsic eye muscles are among the most precisely controlled muscles in the

body!


Section 3: Structure of the Eyeball (pp. 551-556)

Wall of the eyeball contains three layers- Fibrous layer- Vascular layer- Sensory layer

Internal cavity filled w/ fluids called “humors”

Lens separates internal cavity into anterior & posterior segments

Structure of the Eyeball

Fibrous layer- outermost layer of eye- composed of dense, avascular connective tissue

Fibrous Layer

Two regions:1) Sclera - white & opaque posterior region

- protects & shapes eye - anchors extrinsic eye muscles

2) Cornea - transparent anterior 1/6 of eye - bends light as it enters eye - well supplied w/ pain receptors that promote blinking & tear

formation when touched

Fibrous Layer

Vascular layer- middle, pigmented layer also known as the “uvea”

Composed of three regions:1) Choroid region - located on back portion of eye

- supplies blood to all layers of eye - contains brown pigment to absorb light so it can’t be

reflected w/in eye

Vascular Layer

Vascular layer- middle, pigmented layer also known as the “uvea”

Composed of three regions:2) Ciliary body - ring of C.T. & muscles surrounding lens - suspends lens in position & controls its shape

3) Iris - colored part of eye - surrounds pupil (central opening that regulates entering light)

Vascular Layer

More on the pupil… 1) Pupils constrict - to limit amount of light entering - in response to boredom - when looking at something repulsive

2) Pupils dilate (open) - to increase light allowed in - when looking at something appealing - in response to fear - when problem-solving

Vascular Layer

More on the iris… - brown is the only pigment color found in an iris - large amounts of brown pigments create brown/black eyes - small amounts of brown pigment cause light waves to scatter resulting in blue, green, or gray eyes - newborns have blue or gray eyes…pigment develops later

Vascular Layer

Sensory layer- known as the “retina”- contains millions of photoreceptors that convert light

energy into a signal that can be sent to the brain

Sensory Layer

Types of photoreceptors: 1) Rods

- dim-light & peripheral vision receptors- very sensitive to light helping you see in the dark- do not provide sharp images (this is why objects are

fuzzy when in dim lighting)

2) Cones- operate in bright light- provide color vision

Sensory Layer

Other important features of retina:1) Optic disc - known as the “blind spot” b/c it lacks photoreceptors - site where optic nerve leaves the eye

2) Macula lutea - oval region on the back of the eye - located right in the area where the lens focuses light

3) Fovea centralis - small spot in center of macula that contains only cones

Sensory Layer

More on Fovea centralis:- due to density of cones in this spot, anything needing to be

viewed critically must be focused here

- size of the head of a pin, so only a very small portion of the field

of view can be focused on at a given moment

- explains why rapidly changing scenes (watching a train pass by, etc.) requires eyes to flick rapidly to keep image focused here

Sensory Layer

Eye divided into two segments:- chambers divided by lens & ciliary body

Posterior segment - filled with clear gel called vitreous humor - gel transmits light, supports lens, provides intraocular pressure - gel forms during development & lasts lifetime

Anterior segment - filled clear fluid called aqueous humor - fluid forms & drains continually & is in constant motion - supplies nutrients & oxygen to the lens & cornea - drains from eye through Canal of Schlemm

Internal Chambers & Fluids

Lens:- biconvex ( ), transparent, flexible, elastic, & avascular- allows precise focusing of light on retina- becomes dense, more convex, & less elastic w/ age

Lens


Section 4: Physiology of Vision

(pp. 556-559)

Electromagnetic radiation:- all energy waves (radio waves, gamma rays, X rays, etc.)

Visible light- the very small portion of the electromagnetic spectrum that can

stimulate our photoreceptors- can be thought of as energy packets called photons

- rods & cones react to different wavelengths in visible spectrum- color seen is color being reflected off of objects- “white” objects reflect all color wavelengths- “black” objects absorb all color wavelengths

Light

Refraction:- bending of light caused by curvature of lens

Pathway of light entering the eye:- cornea, aqueous humor, lens, vitreous humor, photoreceptors- changing shape of lens ultimately affects where light is focused- if lens is healthy, light will be directed right at fovea centralis

Refraction & Lenses

“Far point of vision”:- the distance beyond which no change in lens shape is needed

for focusing

- 20 feet away for emmetropic (“normal”) eye

- means that the lens is completely relaxed when looking at something 20ft away

Focusing for Distance Vision

Requirements for focusing on things closer than 20ft:- lens shape must be changed to bend light accordingly

1) Accommodation - lens has to bulge to force light to bend more

“Near point of vision” - closest point on which we can focus

- determined on how much lens can bulge

- in young adults = 4 inches from eye

- increases w/ age…can be arms length in elderly

Focusing for Close-up Vision


2) Constriction of pupils - reducing size of pupil limits extra light from entering eye

- excess light would scatter inside eye causing blurriness



3) Convergence of eyes - medial rotation of eyes so that each is directed at object - closer the object, the greater degree of convergence needed

*Long periods of reading or other close work require continuous accommodation, constriction, and convergence. This leads to tired

eye muscles & can result in eyestrain. Periodic staring into the distance helps reduce both.


Myopia: - “near-sighted”

- image is focused in front of retina

- able to see objects up close w/o problem

- distant object appear blurred

- usually results from an eyeball that is too long

Problems with Refraction

Hyperopia: - “far-sighted”

- image is focused behind retina

- able to see distant object w/o problem

- close up objects appear blurred

- usually results from an eyeball that is too tall


Astigmatism:- caused by unequal curvatures in different parts of cornea/lens

- require specially ground lenses, corneal implants, or laser procedures



Section 5: Homeostatic Imbalances

• Vision is not fully functional at birth

• Babies… - born with hyperopia - see only gray tones - uncoordinated eye movements

• Age 5, depth perception & color vision well-developed

• Age 6, emmetropic eyes completely developed

Developmental Aspects

• With age… - lens loses clarity

- dilator muscles of pupil become less efficient

- ability to see clearly drastically decreased by age 70

Developmental Aspects

Glaucoma:- compression of the retina & optic nerve

- caused when drainage of aqueous humor is blocked

- leads to build-up of fluid in the eye & increased pressure

Episcleritis:- inflammation of episclera (tissue between sclera & conjunctiva)

- often associated w/ other diseases in the body

- often affects people who spend a lot of time outdoors in very harsh environments

Homeostatic Imbalances

Cataracts:- clouding of lens

- can be caused by aging, diabetes, heavy smoking, frequent exposure to sunlight


Blepharitis:- inflammation of margins

of the eyelids

Enucleation:- surgical removal of an

eyeball

Exophthalmos:- anteriorly bulging eyes- often a sign of a hyperactive

thyroid gland


Scotoma:- a blind spot other than the normal blind spot

- often indicates the presence of a brain tumor or stroke

Trachoma:- highly contagious bacterial infection of cornea & conjunctiva

- caused by chlamydia

- ultimately causes blindness if left untreated


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CH 15: Special Senses – The Eye