Transcript Special Senses - cloudfront.net

Chapter 8
SPECIAL SENSES
Special senses
 Touch (discussed in ch. 7) , taste, smell, sight
and hearing
 5th sense is equilibrium (receptors in the ear)
 Special sense receptors
 Can be large, complex sensory organs (eyes and
ears)
 Can be localized clusters of receptors (taste buds
and olfactory equilibrium)
Anatomy of the eye
 adult eye is a sphere approximately 1 inch in
diameter
 Only 1/6 of the anterior surface is seen
 The other 5/6 of the eye is enclosed and
protected by fat and the bony orbit
External and accessory
structures
 Eyelids protect the eye anteriorly; eyelids meet
at the medial and lateral canthus
 Eyelashes project from the border of each eyelid
 Meibomian glands (sebaceous glands) produce
an oily secreti0n to lubricate the eye
 Ciliary glands (modified sweat glands) lie
between the eyelashes
 Conjuctiva (membrane) lines the eyelids and
covers part of the outer surface of the eyeball;
secretes mucus for lubrication
External and accessory
structures
 Lacrimal apparatus consists of the lacrimal
glands and ducts (drain lacrimal secretions
into the nasal cavity)
 Lacrimal glands (above lateral side of each
eye) release dilute salt solution (tears) onto
the anterior surface of the eye
 Tears flush across the eye into lacrimal canals
medially, then into the lacrimal sac, and then
to the nasolacrimal duct to the nasal cavity
External and accessory
structures
 Lacrimal secretions have antibodies and
lysozyme – an enzyme that kills bacteria
 Secretions cleanse and protect the eye
surface as well as moistens and lubricates
 Lacrimal secretions increase, they spill over
the eyelids and fill the nasal cavities
External and accessory
structures
 Extrinsic eye muscles are attached to the
outer eye surface
 Produce gross eye movements and make it
possible to “follow” an object
Internal structures
 Eyeball is a hollow sphere
 Wall is composed of three tunics (layers)
 Interior is filled with fluid (humors) that
maintain shape
 Lens is the focusing apparatus
Tunics of the eyeball
 Sclera – outermost tunic
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Thick, white connective tissue
Also called the fibrous tunic
“white of the eye”
Central anterior portion is modified to be clear
(this is the cornea) so that light can enter
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Well supplied with nerves (mostly pain fibers)
Most exposed part
Vulnerable to damage
Can be transplanted without rejection by the
recipient’s immune system
Tunics of the eyeball
 Middle tunic is the choroid
 Blood-rich and nutritive
 Contains dark pigment (which prevents light from
scattering inside the eye)
 Anteriorly it’s modified into ciliary bodies that are
attached to the lens and the iris
 Pigmented iris has a round opening, pupil, for
light to pass
 Iris is made of circularly and radially arranged
smooth muscle fibers (to control size of pupil)
Tunics of the eyeball
 Retina is the innermost (sensory) tunic
 Contains millions of receptor cells (rods and cones)
 Rods / cones are distributed over the entire retina
except where the optic nerve leaves the eyeball (optic
disc or blind spot)
 Rods and cones are photoreceptors since they respond
to light
 Electrical signals go from rods / cones to bipolar cells
then to ganglion cells before entering the optic nerve
to go to the optic cortex
 Result is vision
Rods and cones
 Rods are most dense at the periphery of the
retina
 Cones are most dense at the center of the
retina
 Fovea centralis is lateral to each blind spot
(this contains only cones) is the point of
sharpest vision
Cones
 Three varieties
 Each type is sensitive to a particular
wavelength (one responds to blue light, one
responds to green light, and red cones
respond to red and green lights)
 If more than one type is stimulated,
intermediate colors are “seen”
 If all are stimulated at once, white is “seen”
Lens
 Focuses the light onto the retina
 A flexible, biconvex structure
 Held upright by a suspensory ligament
attached to the ciliary body
Lens
 Divides the eye into 2 parts
 Anterior (aqueous) segment is anterior to the
lens
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Contains clear, watery aqueous humor
Continuously secreted by the choroid
Maintains pressure inside the eye
Provides nutrients for lens and cornea (they lack
blood supply)
 Reabsorbed into venous blood through scleral
venous sinus
Lens
 Posterior (vitreous) segment is posterior to
the lens
 Maintains pressure inside the eye
 Prevents the eyeball from collapsing
Vision Disorders
 Myopia (nearsightedness)
 Elongated eyeball shape
 Lens focuses on objects in front of the retina
(instead of upon it)
 Distant objects are blurry
 Snellen chart is used to diagnose myopia
 Corrective lenses or laser surgery corrects vision
Vision Disorders
 Hyperopia (farsightedness)
 Distance from lens to retina is shortened
 Eyeball is more flattened
 Light focuses behind the retina (instead of upon it)
 Objects in the distance are clear, close objects are
blurry
 Corrective lenses “fix” vision
Vision Disorders
 Presbyopia (age-related farsightedness)
 Onset occurs between ages 40-45
 Lens becomes stiff and discolored
 Blurring of up-close objects
 Impedes ability to read
Vision Disorders
 Astigmatism
 Irregular curvature of cornea or lens
 Blurred vision
 Corrective lenses can partially or completely
correct it
Vision Disorders
 Amblyopia (lazy eye)
 Appears during childhood
 One eye is more dominant; the other eye has poor
vision
 Treatment includes covering the good and
strengthening the muscles of the “lazy eye”
Vision Disorders
 Diplopia (double vision)
 One eye is misaligned and produces 2 images
 Treatment includes a patch, corrective lenses, or
surgery
Vision Disorders
 Strabismus (crossed eyes)
 One eye drifts in different directions
 Malfunctioning extrinsic eye muscles
 Corrected with exercise, corrective lenses, or
surgery
Vision Disorders
 Night blindness
 Rods in retina do NOT function properly
 Usually associated with aging
Cataracts
 Lens becomes hard and opaque
 Due to age
 Causes vision to become hazy
 Eventually causes blindness
 Treatment includes surgical removal of the
“old” lens and transplanting in a new lens
Glaucoma
 Drainage of aqueous humor is blocked
 Fluid puts pressure on the retina and optic
nerve
 Causes pain and blindness
 Symptoms include “halos” around lights,
headaches, blurred vision
 Treatment includes eyedrops to aid aqueous
humor drainage or surgical enlargement of
the drainage canal
Macular degeneration
 Progressive loss of central vision
 Peripheral vision is unaffected
 Dry macular degeneration
 Thinning of retina
 Do not completely lose vision
 Wet macular degeneration
 Leakage of small blood vessels
 Can be corrected with meds / surgery
Diabetic retinopathy
 Damage to the retina due to diabetes (high
blood sugar)
 Swelling and leaking of blood vessels that
supply the retina
 See red spots
 Corrected with surgery
Light refraction
 Light passes from one substance to another
substance with a different density, rays are
bent or refracted
 Light rays are refracted as they encounter the
cornea, aqueous humor, lens, and vitreous
humor
 Refractive powers of the cornea and humors
are constant
Light refraction
 Refractive powers of the lens can change
depending on its shape
 Greater lens convexity the greater the refraction
 Less lens convexity (more flat) less the refraction
Pathway of light
 Human eye is “set” for distance vision
 Light from distant sources approaches the eye as
parallel rays so NO change in lens shape is
needed to focus light on the retina
 At close range, light from close objects scatters,
so the lens must bulge (be more convex) for light
to be focused on the retina
 Bulging of lens occurs when the ciliary bodies contract
 Ability to “focus” on close objects is called
accommodation
Pathway to the brain
 Axons from retina are bundled together and
leave the posterior eye as the optic nerve
 Nerve fibers from each eye cross over each
other at the optic chiasma
 Resulting fiber tracts are called optic tracts
 Contain fibers from the lateral side of the eye on
the same side and the medial side of the opposite
eye
 Synapse with neurons in the thalamus to form the
optic radiation which runs to the occipital lobe
Visual fields
 Each eye has a different view
 Visual fields overlap
 Humans have binocular vision
 “two-eyed” vision
 Gives depth perception or “3-D” vision
 Two slightly different views are fused into one
image
Eye reflexes
 Internal eye muscles controlled by the
Autonomic nervous system
 Can alter lens curvature
 Controls pupil size
 Protects from bright light (photopupillary reflex)
by constricting pupils so photoreceptors are not
damaged
 Accommodation pupillary reflex constricts pupils
to all0w more acute vision of close objects
Eye reflexes
 External eye muscles also controlled by ANS
 Extrinsic muscles control eye movements and
allow the “following” of objects
 Cause convergence (move eyes medially to view
close objects) which is controlled by cranial nerves
III, IV, and VI
Reading
 Requires both sets of eye muscles
 Lens must bulge and pupils must constrict for
focusing close
 Extrinsic muscles must converge the eyes and
move them to follow printed lines
 So long periods of reading cause tiring of the
eyes and may result in eyestrain
The ear
 Sound vibrations move fluid to stimulate
hearing receptors
 Movements of the head disturb fluids around
the balance organs
 Mechanoreceptors allow us to hear a wide
range of sounds but also keep the nervous
system up-to-date on position and
movement of the head
 Receptors for hearing and balance react
independently of each other
Anatomy of the ear
 Three parts
 Outer ear
 Hearing only
 Middle ear
 Hearing only
 Inner ear
 Hearing
 Balance
Anatomy of the ear: Outer
 Pinna
 “ear”
 Collects and directs sound waves in most animals (lost
in humans)
 External auditory canal
 Short tube carved into temporal bone
 1 inch long x ¼ inch wide
 Lined with ceruminous glands that secrete cerumin
(wax)
 Ends at the eardrum, which separates the outer ear
from middle ear
Anatomy of the ear: middle
 Tympanic cavity- air filled chamber in the
temporal bone
 Flanked laterally by eardrum and medially by a
bony wall with the oval window and the round
window
 Auditory tube runs obliquely downward to link
middle ear with throat
 Usually flattened
 Yawning or swallowing can open it to equalize
pressure
 Equalizing pressure allows eardrum to vibrate freely
 In infants, auditory tube is more horizontal
Anatomy of the ear: middle
 Contains 3 ossciles (bones) that transmit vibrations
of the eardrum to fluids in the inner ear
 Bones are named for shape
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Hammer (malleus)
Anvil (incus)
Stirrup (stapes)
Eardrum moves, hammer moves with it and transfers
vibration to anvil
 Anvil passes the movement to the stirrup which presses on
the oval window
 Oval window sets fluids of inner ear into motion which
excites hearing receptors
Anatomy of the ear: inner
 Maze of bony chambers: osseous labyrinth
 Deep in the temporal bone, behind the eye
socket
 Filled with fluid (perilymph); suspends the
membranous labyrinth which contains the
endolymph
 Three subdivisions
 Cochlea
 Vestibule
 Semicircular canals
Mechanism of hearing
 Hearing receptors (hair cells) are located inside
the organ of Corti (inside the cochlea)
 Vibrations reaching the oval window activates
the fluids in the inner ear
 Receptor cells on the basilar membrane in the
organ of Corti are stimulated when the “hairs”
are moved by the tectorial membrane that lies
over them
 Hairs cells transmit impulses along the cochlear
nerve (cranial nerve VIII) to the auditory cortex in
the temporal lobe where the sound is interpreted
Mechanisms of equilibrium
 This is a response to the movement of the
head
 Vestibular apparatus (equilibrium receptors)
are divided into 2 arms
 Static equilibrium
 Dynamic equilibrium
Static equilibrium
 Vestibule contains sacs of receptors called maculae
 Each macula is a patch of receptor cells with “hairs”
embedded in the otolithic membrane (a jelly-like
material) containing otoliths (small pieces of calcium
salts)
 Head movements cause otoliths to “roll” in response
to gravity, this pulls the gel, slides a plate over the
“hairs” to bend them which sends impulses down
vestibular nerve to the cerebellum which receives
info about the position of the head
 This helps keep the head erect and indicates “up and
down”
Dynamic equilibrium
 Receptors found in semicircular canals
 Respond to angular or rotatory movements of
the head
 Each semicircular canal contains a receptor
region: crista ampullaris (hair cells covered with a
gel cap or cupula)
 When the head moves in angular direction,
endolymph in the canal lags and moves in the
opposite direction, pushing cupula opposite the
body’s movement
 Hair cells are stimulated and send impulses up
the vestibular nerve to the cerebellum
Dynamic equilibrium
 When angular motion stops, endolymph
flows in opposite direction and reverses
cupula’s movement; hair cells reduce the rate
of firing
 Causes a reverse movement sensation
Deafness
 Hearing loss or impairment
 Two kinds of deafness
 Conduction deafness
 Something interferes with the conduction of sound
vibrations to the inner ear
 Caused by mechanical problems such as earwax, fusion
of ossicles, ruptured eardrum otitis media
 Sensorineural deafness
 Degeneration or damage to receptor cells in organ of
Corti, cochlear nerve, or neurons of auditory cortex
 Caused by listening to very loud sounds
 Due to problems with nervous system structures
Equilibrium problems
 Cause nausea, dizziness, problems with balance
 Impulses from vestibular apparatus “disagree”
with the visual input
 May have jerky or rolling eye movements
 Meniere’s syndrome is serious problem with
inner ear
 May be caused by heart disease, degeneration of
cranial nerve VIII, pressure on inner ear
 Results in progressive deafness and vertigo (sensation
of spinning)
Chemical senses
 Chemoreceptors are used for taste and
olfaction
 Respond to chemicals in solution
 Olfactory receptors respond to a wider range
of chemicals
 Olfaction and taste compliment each other
Olfactory receptors
 Thousands located in a very tiny area in the
roof of each nasal cavity
 Olfactory receptor cells are neurons having
olfactory hairs (cilia) that protrude from the
epithelium and are bathed by mucus
 When stimulated, receptors send impulses
along the olfactory nerve to the olfactory
cortex of the brain
Olfactory receptors
 Since olfaction is closely related to the limbic
system (emotions), “smells” are long-lasting
and are part of our emotions and memories
 They adapt quickly when exposed to an
unchanging stimulus (ex: woman will stop
smelling her own perfume minutes after it is
applied)
Olfactory disorders
 Anosmias result from head injuries,
aftereffects of nasal cavity inflammation, or
aging
 Most cases caused by zinc deficiency; can be
corrected with supplements
Taste buds
 Specific receptors for sense of taste
 Widely scattered in the oral cavity
 10,000 taste buds, most of which are located
on the tongue (some on the soft palate, some
on inner surface of cheeks)
Tongue
 Dorsal surface covered with papillae of 3 types
 Filiform
 Fungiform
 Circumvallate
 Taste buds located on the sides of the fungiform
and circumvallate papillae
 Gustatory cells respond to chemicals dissolved in
saliva (epithelial cells surrounded by supporting
cells in the taste bud)
Tongue
 Gustatory hairs protrude through the taste
pore; when stimulated, impulses are
transmitted to the brain
 Cranial nerves VII, IX, and X carry taste impulses to
the gustatory cortex
 Facial nerve VII serves the anterior tongue
 Glossopharyngeal and vagus nerves serve the
other taste bud areas
Taste sensations
 4 basic taste sensations that correspond to a
specific type of taste bud
 Sweet receptors – respond to sugar, saccharine,
some amino acids
 Sour receptors – respond to hydrogen ions or
acidity
 Bitter receptors – respond to alkaloids
 Salty receptors – respond to metal ions in solution
Taste sensations
 Tip of tongue – sensitive to salty and sweet
 Sides of tongue – sensitive to sour
 Back of tongue – sensitive to bitter
Taste sensations
 Homeostatic values to tastes
 “sweets” satisfy a need for carbohydrates and
minerals (and some amino acids)
 Sour (citrus) have high levels of vitamin C
 Dislike for bitter is protective – many natural
poisons or spoiled foods are bitter to the
taste
 Taste depends heavily on olfaction; if sense of
smell is inhibited, taste will be altered
Development
 Eyes are formed by the 4th week of embryo
development
 All senses are functional at birth
 Eyes are not fully functional at birth
 Eyeballs continue to enlarge until age 8-9
 Lens grows throughout life
 Infants have foreshortened eyeballs are are hyperopic
 Infants only see gray tones and eye movements are
uncoordinated
 Tear ducts are inoperable until 2 weeks of age
Development
 5 months of age
 Infant can focus on close objects and follow
movement
 Visual acuity is poor
 5 years of age
 Color vision is developed
 Visual acuity is improved to 20/30
 School age to middle age
 Hyperopia is replaced with emmetropia until age 40
 Presbyopia sets in with middle age resulting from
decreased lens elasticity; decreases close vision
 Lacrimal glands are less active
Development
 After middle age
 Lens loses clarity
 Dilator muscles of iris are less efficient keeping
pupils constricted
 Decreased light reaching retina; visual acuity is
decreased by age 70
 Elderly are susceptible to glaucoma and cataracts
 Diabetes and heart disease can also lead to poor
vision and blindness
Development
 Newborns
 Can hear after first cry
 Responses to sound are strictly reflexive
 Can turn toward noises by 3-4 months of age
 Critical listening begins with toddlers as they start
to vocalize
Development
 Infants - adults
 Few problems affect hearing except otitis (ear
infections)
 Elderly adults
 Deterioration of the organ of Corti
 Cannot hear high-pitched sounds (presbycusis);
sensorineural deafness
 Sometimes the ossicles fuse which inhibits sound
conduction
 Hearing aides help to alleviate the problems