ch 15: special senses – the eye
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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
CH 15: Special Senses – The Eye
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
Accessory Structures of the Eye
And everyone’s favorite…Eh…it’s only a phase…Some find them pointless…YIKES!!
Eyelids- protect anterior surface of eye- lids 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
Accessory Structures of the Eye
Eyelids- protect anterior surface of eye- lids 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
Accessory Structures of the Eye
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
Accessory Structures of the Eye
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
Accessory Structures of the Eye
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
Accessory Structures of the Eye
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
Accessory Structures of the Eye
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
Accessory Structures of the Eye
Accessory Structures of the Eye
Extrinsic eye muscles are among the most precisely controlled muscles in the
body!
CH 15: Special Senses – The Eye
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
CH 15: Special Senses – The Eye
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
Requirements for focusing on things closer than 20ft:- lens shape must be changed to bend light accordingly
2) Constriction of pupils - reducing size of pupil limits extra light from entering eye
- excess light would scatter inside eye causing blurriness
Focusing for Close-up Vision
Requirements for focusing on things closer than 20ft:- lens shape must be changed to bend light accordingly
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.
Focusing for Close-up Vision
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
Problems with Refraction
Astigmatism:- caused by unequal curvatures in different parts of cornea/lens
- require specially ground lenses, corneal implants, or laser procedures
Problems with Refraction
CH 15: Special Senses – The Eye
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
Homeostatic Imbalances
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
Homeostatic Imbalances
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
Homeostatic Imbalances
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