Transcript CH 15: Special Senses – The Eye

CH 15: Special Senses – The Eye
Section 1: Intro to the Eye (p. 548)
The Eye and Vision
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
CH 15: Special Senses – The Eye
Section 2: Accessory Structures of
the Eye (pp. 548-551)
Accessory Structures of the Eye
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
Some
And
Eh…it’s
everyone’s
find YIKES!!
them
only apointless…
favorite…
phase…
Accessory Structures of the Eye
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 infections
1) 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 problems
1) Conjunctivitis
- red, irritation caused by inflammation of conjunctiva
2) 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
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)
Structure of the Eyeball
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
Fibrous Layer
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
Vascular 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
Sensory 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
Internal Chambers & Fluids
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
Lens
Lens:
- biconvex ( ), transparent, flexible, elastic, & avascular
- allows precise focusing of light on retina
- becomes dense, more convex, & less elastic w/ age
CH 15: Special Senses – The Eye
Section 4: Physiology of Vision
(pp. 556-559)
Light
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
Refraction & Lenses
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
Focusing for Distance Vision
“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 Close-up 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.
Problems with Refraction
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
CH 15: Special Senses – The Eye
Section 5: Homeostatic Imbalances
Developmental Aspects
• 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
Homeostatic Imbalances
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