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1 8 Special Senses Yong Jeong, MD, PhD Department of Bio and Brain Engineering © 2012 Pearson Education, Inc. The Senses General senses Touch Pressure Pain Temperature Proprioception Special senses Smell Taste Sight Hearing Equilibrium

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    8Special Senses

    Yong Jeong, MD, PhD

    Department of Bio and Brain Engineering

    © 2012 Pearson Education, Inc.

    The Senses

    •General senses• Touch• Pressure• Pain• Temperature• Proprioception

    •Special senses• Smell• Taste• Sight• Hearing• Equilibrium

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    © 2012 Pearson Education, Inc.

    The Eye and Vision

    •70 percent of all sensory receptors are in the eyes

    •Each eye has over a million nerve fibers•Protection for the eye

    •Most of the eye is enclosed in a bony orbit•A cushion of fat surrounds most of the eye

    © 2012 Pearson Education, Inc.

    Accessory Structures of the Eye

    •Eyelids and eyelashes•Conjunctiva•Lacrimal apparatus •Extrinsic eye muscles

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    © 2012 Pearson Education, Inc. Figure 8.1

    Site whereconjunctivamerges withcornea

    Palpebralfissure

    Lateralcommissure(canthus)

    IrisEyelid

    Eyebrow

    EyelidEyelashes

    PupilLacrimalcaruncleMedialcommissure(canthus)Sclera(covered byconjunctiva)

    © 2012 Pearson Education, Inc.

    Accessory Structures of the Eye

    •Eyelids •Meet at the medial and lateral commissure (canthus)

    •Eyelashes•Tarsal glands produce an oily secretion that lubricates the eye

    •Ciliary glands are located between the eyelashes

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    © 2012 Pearson Education, Inc.

    Accessory Structures of the Eye

    •Conjunctiva•Membrane that lines the eyelids•Connects to the outer surface of the eye•Secretes mucus to lubricate the eye and keep it moist

    © 2012 Pearson Education, Inc.

    Accessory Structures of the Eye

    •Lacrimal apparatus = lacrimal gland + ducts•Lacrimal gland—produces lacrimal fluid; situated on lateral aspect of each eye

    •Lacrimal canaliculi—drain lacrimal fluid from eyes medially

    •Lacrimal sac—provides passage of lacrimal fluid towards nasal cavity

    •Nasolacrimal duct—empties lacrimal fluid into the nasal cavity

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    © 2012 Pearson Education, Inc. Figure 8.2a

    Lacrimalgland

    (a)

    Conjunctiva

    Anterioraspect

    EyelidEyelashesTarsalglandsEyelid

    Excretory ductof lacrimal gland

    © 2012 Pearson Education, Inc. Figure 8.2b

    LacrimalglandExcretory ductsof lacrimal gland

    Lacrimal canaliculusNasolacrimal ductInferior meatusof nasal cavity

    Nostril

    Lacrimal sac

    (b)

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    © 2012 Pearson Education, Inc.

    Accessory Structures of the Eye

    •Function of the lacrimal apparatus •Protects, moistens, and lubricates the eye•Empties into the nasal cavity

    •Lacrimal secretions (tears) contain:•Dilute salt solution•Mucus•Antibodies•Lysozyme (enzyme that destroys bacteria)

    © 2012 Pearson Education, Inc.

    Accessory Structures of the Eye

    •Extrinsic eye muscles •Six muscles attach to the outer surface of the eye

    •Produce eye movements

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    © 2012 Pearson Education, Inc. Figure 8.3a

    Superioroblique muscle

    Superioroblique tendon

    Superiorrectus muscle

    Conjunctiva

    Lateral rectusmuscle

    Opticnerve

    Inferiorrectusmuscle

    Inferiorobliquemuscle

    (a)

    © 2012 Pearson Education, Inc. Figure 8.3b

    Trochlea

    Superioroblique muscle

    Medialrectus muscle

    Lateralrectus muscle

    Superioroblique tendon

    Superiorrectus muscle

    (b)

    Inferiorrectus muscle

    Axis atcenter ofeye

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    © 2012 Pearson Education, Inc. Figure 8.3c

    © 2012 Pearson Education, Inc.

    Structure of the Eye

    •Layers forming the wall of the eyeball•Fibrous layer

    •Outside layer •Vascular layer

    •Middle layer •Sensory layer

    • Inside layer

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    © 2012 Pearson Education, Inc. Figure 8.4a

    Ciliary bodyCiliary zonule

    CorneaIrisPupil

    Lens

    ScleraChoroidRetina

    Fovea centralis

    Optic nerve

    (a)

    Aqueoushumor (inanterior segment)

    Scleral venous sinus(canal of Schlemm)

    Optic disc(blind spot)

    Central artery andvein of the retina

    Vitreous humor(in posterior segment)

    © 2012 Pearson Education, Inc. Figure 8.4b

    Ciliary body

    Iris

    LensCorneaCiliary zonule

    (b)

    RetinaChoroidScleraFovea centralisOptic discOptic nerve

    Marginof pupilAqueous humor(in anteriorsegment)

    Vitreous humorin posterior segment

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    © 2012 Pearson Education, Inc.

    Structure of the Eye: The Fibrous Layer

    •Sclera•White connective tissue layer•Seen anteriorly as the “white of the eye”

    •Cornea•Transparent, central anterior portion•Allows for light to pass through•Repairs itself easily•The only human tissue that can be transplanted without fear of rejection

    © 2012 Pearson Education, Inc.

    Structure of the Eye: Vascular Layer

    •Choroid is a blood-rich nutritive layer in the posterior of the eye•Pigment prevents light from scattering

    •Modified anteriorly into two structures•Ciliary body—smooth muscle attached to lens

    • Iris—regulates amount of light entering eye•Pigmented layer that gives eye color•Pupil—rounded opening in the iris

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    © 2012 Pearson Education, Inc.

    Structure of the Eye: Sensory Layer

    •Retina contains two layers•Outer pigmented layer• Inner neural layer

    •Contains receptor cells (photoreceptors)•Rods•Cones

    © 2012 Pearson Education, Inc.

    Structure of the Eye: Sensory Layer

    •Signals pass from photoreceptors via a two-neuron chain•Bipolar neurons•Ganglion cells

    •Signals leave the retina toward the brain through the optic nerve

    •Optic disc (blind spot) is where the optic nerve leaves the eyeball•Cannot see images focused on the optic disc

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    © 2012 Pearson Education, Inc. Figure 8.5a

    RodCone

    (a)

    Pathwayof light

    Pigmentedlayer of retina

    Bipolarcells

    Ganglioncells

    © 2012 Pearson Education, Inc. Figure 8.5b

    Pigmentedlayer of retina

    Neural layerof retina

    Centralarteryand veinof retina Optic

    disc

    Opticnerve(b)

    ScleraChoroid

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    © 2012 Pearson Education, Inc.

    Structure of the Eye: Sensory Layer

    •Neurons of the retina and vision •Rods

    •Most are found towards the edges of the retina

    •Allow dim light vision and peripheral vision•All perception is in gray tones

    © 2012 Pearson Education, Inc.

    Structure of the Eye: Sensory Layer

    •Neurons of the retina and vision •Cones

    •Allow for detailed color vision•Densest in the center of the retina•Fovea centralis–lateral to blind spot

    •Area of the retina with only cones•Visual acuity (sharpest vision) is here

    •No photoreceptor cells are at the optic disc, or blind spot

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    © 2012 Pearson Education, Inc.

    Structure of the Eye: Sensory Layer

    •Cone sensitivity •Three types of cones•Different cones are sensitive to different wavelengths

    •Color blindness is the result of the lack of one cone type

    © 2012 Pearson Education, Inc. Figure 8.6

    560 nm(red cones)530 nm

    (green cones)

    420 nm(blue cones)

    Ligh

    t abs

    orpt

    ion

    by c

    one

    popu

    latio

    ns

    380 450 500 550 600 650 700 750Wavelengths (nanometers)

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    © 2012 Pearson Education, Inc.

    Lens

    •Biconvex crystal-like structure•Held in place by a suspensory ligament attached to the ciliary body

    © 2012 Pearson Education, Inc. Figure 8.4a

    Ciliary bodyCiliary zonule

    CorneaIrisPupil

    Lens

    ScleraChoroidRetina

    Fovea centralis

    Optic nerve

    (a)

    Aqueoushumor (inanterior segment)

    Scleral venous sinus(canal of Schlemm)

    Optic disc(blind spot)

    Central artery andvein of the retina

    Vitreous humor(in posterior segment)

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    © 2012 Pearson Education, Inc.

    Lens

    •Cataracts result when the lens becomes hard and opaque with age•Vision becomes hazy and distorted•Eventually causes blindness in affected eye

    •Risk factors include:•Diabetes mellitus•Frequent exposure to intense sunlight•Heavy smoking

    © 2012 Pearson Education, Inc. Figure 8.7

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    © 2012 Pearson Education, Inc.

    Two Segments, or Chambers, of the Eye

    •Anterior (aqueous) segment•Anterior to the lens•Contains aqueous humor

    •Posterior (vitreous) segment•Posterior to the lens•Contains vitreous humor

    © 2012 Pearson Education, Inc. Figure 8.4a

    Ciliary bodyCiliary zonule

    CorneaIrisPupil

    Lens

    ScleraChoroidRetina

    Fovea centralis

    Optic nerve

    (a)

    Aqueoushumor (inanterior segment)

    Scleral venous sinus(canal of Schlemm)

    Optic disc(blind spot)

    Central artery andvein of the retina

    Vitreous humor(in posterior segment)

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    © 2012 Pearson Education, Inc.

    Anterior Segment

    •Aqueous humor•Watery fluid found between lens and cornea•Similar to blood plasma•Helps maintain intraocular pressure•Provides nutrients for the lens and cornea•Reabsorbed into venous blood through the scleral venous sinus, or canal of Schlemm

    © 2012 Pearson Education, Inc.

    Posterior Segment

    •Vitreous humor•Gel-like substance posterior to the lens•Prevents the eye from collapsing•Helps maintain intraocular pressure

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    © 2012 Pearson Education, Inc.

    Ophthalmoscope

    • Instrument used to illuminate the interior of the eyeball

    •Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina

    © 2012 Pearson Education, Inc. Figure 8.8

    Foveacentralis

    Macula Bloodvessels

    Optic disc Retina

    Lateral Medial

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    © 2012 Pearson Education, Inc.

    Pathway of Light Through the Eye

    •Light must be focused to a point on the retina for optimal vision

    •The eye is set for distance vision (over 20 feet away)

    •Accommodation—the lens must change shape to focus on closer objects (less than 20 feet away)

    © 2012 Pearson Education, Inc. Figure 8.9

    Retina

    Focal pointLight from distant source

    (a)

    Light from near source Focal pointRetina

    (b)

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    © 2012 Pearson Education, Inc.

    Pathway of Light Through the Eye

    • Image formed on the retina is a real image•Real images are:

    •Reversed from left to right •Upside down•Smaller than the object

    © 2012 Pearson Education, Inc. Figure 8.10

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    © 2012 Pearson Education, Inc.

    Visual Fields and Visual Pathways

    •Optic chiasma •Location where the optic nerves cross•Fibers from the medial side of each eye cross over to the opposite side of the brain

    •Optic tracts •Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye

    © 2012 Pearson Education, Inc. Figure 8.11

    Fixation point

    Right eye Left eye

    Opticchiasma

    OpticnerveOptictract

    Opticradiation

    Occipital lobe(visual cortex)

    Thalamus

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    © 2012 Pearson Education, Inc.

    Eye Reflexes

    • Internal muscles are controlled by the autonomic nervous system•Bright light causes pupils to constrict through action of radial, circular, and ciliary muscles

    •Viewing close objects causes accommodation•External muscles control eye movement to follow objects

    •Viewing close objects causes convergence (eyes moving medially)

    © 2012 Pearson Education, Inc.

    A Closer Look

    •Emmetropia—eye focuses images correctly on the retina

    •Myopia (nearsighted)•Distant objects appear blurry •Light from those objects fails to reach the retina and are focused in front of it

    •Results from an eyeball that is too long

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    © 2012 Pearson Education, Inc.

    A Closer Look

    •Hyperopia (farsighted)•Near objects are blurry while distant objects are clear

    •Distant objects are focused behind the retina•Results from an eyeball that is too short or from a “lazy lens”

    © 2012 Pearson Education, Inc.

    Focalplane

    (a) Emmetropic eye

    Correction

    None required

    Concave lens

    Convex lens(b) Myopic eye

    (nearsighted)

    (c) Hyperopic eye(farsighted)

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    © 2012 Pearson Education, Inc.

    A Closer Look

    •Astigmatism• Images are blurry•Results from light focusing as lines, not points, on the retina due to unequal curvatures of the cornea or lens

    © 2012 Pearson Education, Inc.

    Homeostatic Imbalances of the Eyes

    •Night blindness—inhibited rod function that hinders the ability to see at night

    •Color blindness—genetic conditions that result in the inability to see certain colors •Due to the lack of one type of cone (partial color blindness)

    •Cataracts—when lens becomes hard and opaque, our vision becomes hazy and distorted

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    © 2012 Pearson Education, Inc.

    Homeostatic Imbalances of the Eyes

    •Glaucoma—can cause blindness due to increasing pressure within the eye

    •Hemianopia—loss of the same side of the visual field of both eyes; results from damage to the visual cortex on one side only

    © 2012 Pearson Education, Inc.

    The Ear

    •Houses two senses•Hearing •Equilibrium (balance)

    •Receptors are mechanoreceptors•Different organs house receptors for each sense

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    © 2012 Pearson Education, Inc.

    Anatomy of the Ear

    •The ear is divided into three areas•External (outer) ear•Middle ear (tympanic cavity)• Inner ear (bony labyrinth)

    © 2012 Pearson Education, Inc. Figure 8.12

    Auricle(pinna)

    External (outer) ear Middle ear

    Internal (inner) ear

    Oval window

    Vestibule

    Round window

    Hammer(malleus)

    Anvil(incus)

    Stirrup(stapes)

    Auditory ossicles

    Semicircularcanals

    Pharyngotympanic(auditory) tube

    Cochlea

    External acousticmeatus(auditory canal)

    Tympanicmembrane(eardrum)

    Vestibulocochlearnerve

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    © 2012 Pearson Education, Inc.

    The External Ear

    • Involved in hearing only•Structures of the external ear

    •Auricle (pinna)•External acoustic meatus (auditory canal)

    •Narrow chamber in the temporal bone•Lined with skin and ceruminous (wax) glands

    •Ends at the tympanic membrane

    © 2012 Pearson Education, Inc.

    The Middle Ear (Tympanic Cavity)

    •Air-filled cavity within the temporal bone•Only involved in the sense of hearing

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    © 2012 Pearson Education, Inc.

    The Middle Ear (Tympanic Cavity)

    •Two tubes are associated with the inner ear•The opening from the auditory canal is covered by the tympanic membrane

    •The auditory tube connecting the middle ear with the throat•Allows for equalizing pressure during yawning or swallowing

    •This tube is otherwise collapsed

    © 2012 Pearson Education, Inc.

    Bones of the Middle Ear (Tympanic Cavity)

    •Three bones (ossicles) span the cavity•Malleus (hammer)• Incus (anvil)•Stapes (stirrup)

    •Function•Vibrations from eardrum move the hammer anvil stirrup inner ear

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    © 2012 Pearson Education, Inc. Figure 8.12

    Auricle(pinna)

    External (outer) ear Middle ear

    Internal (inner) ear

    Oval window

    Vestibule

    Round window

    Hammer(malleus)

    Anvil(incus)

    Stirrup(stapes)

    Auditory ossicles

    Semicircularcanals

    Pharyngotympanic(auditory) tube

    Cochlea

    External acousticmeatus(auditory canal)

    Tympanicmembrane(eardrum)

    Vestibulocochlearnerve

    © 2012 Pearson Education, Inc.

    Inner Ear or Bony Labyrinth

    • Includes sense organs for hearing and balance

    •Filled with perilymph•Contains a maze of bony chambers within the temporal bone•Cochlea•Vestibule•Semicircular canals

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    © 2012 Pearson Education, Inc. Figure 8.12

    Auricle(pinna)

    External (outer) ear Middle ear

    Internal (inner) ear

    Oval window

    Vestibule

    Round window

    Hammer(malleus)

    Anvil(incus)

    Stirrup(stapes)

    Auditory ossicles

    Semicircularcanals

    Pharyngotympanic(auditory) tube

    Cochlea

    External acousticmeatus(auditory canal)

    Tympanicmembrane(eardrum)

    Vestibulocochlearnerve

    © 2012 Pearson Education, Inc.

    Organs of Equilibrium

    •Equilibrium receptors of the inner ear are called the vestibular apparatus

    •Vestibular apparatus has two functional parts•Static equilibrium•Dynamic equilibrium

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    © 2012 Pearson Education, Inc. Figure 8.14a

    Semicircularcanals

    AmpullaVestibularnerve

    Vestibule

    (a)

    © 2012 Pearson Education, Inc.

    Static Equilibrium

    •Maculae—receptors in the vestibule•Report on the position of the head•Send information via the vestibular nerve

    •Anatomy of the maculae•Hair cells are embedded in the otolithic membrane

    •Otoliths (tiny stones) float in a gel around the hair cells

    •Movements cause otoliths to bend the hair cells

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    © 2012 Pearson Education, Inc. Figure 8.13a

    Membranes in vestibule

    Otoliths

    Hair tuft

    (a)

    Hair cellSupporting cell

    Otolithicmembrane

    Nerve fibers ofvestibular divisionof cranial nerve VIII

    © 2012 Pearson Education, Inc. Figure 8.13b

    Force ofgravityOtoliths

    Hair cell

    Head upright(b)

    Head tilted

    Otolithicmembrane

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    © 2012 Pearson Education, Inc.

    Dynamic Equilibrium

    •These receptors respond to angular or rotary movements

    •Crista ampullaris (in the ampulla of each semicircular canal)—dynamic equilibrium receptors are located in the semicircular canals•Tuft of hair cells covered with cupula (gelatinous cap)

    • If the head moves, the cupula drags against the endolymph

    © 2012 Pearson Education, Inc. Figure 8.14a

    Semicircularcanals

    AmpullaVestibularnerve

    Vestibule

    (a)

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    © 2012 Pearson Education, Inc. Figure 8.14b-c(b)

    Cupula of cristaampullaris

    EndolymphAmpulla

    Flow ofendolymph

    Cupula

    Direction of bodymovement

    (c)

    Nervefibers

    © 2012 Pearson Education, Inc.

    Dynamic Equilibrium

    •Action of angular head movements•The movement of the cupula stimulates the hair cells

    •An impulse is sent via the vestibular nerve to the cerebellum

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    © 2012 Pearson Education, Inc.

    Organs of Hearing

    •Organ of Corti•Located within the cochlea•Receptors = hair cells on the basilar membrane

    •Gel-like tectorial membrane is capable of bending hair cells

    •Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe

    © 2012 Pearson Education, Inc. Figure 8.15a

    Spiralorgan ofCorti

    Temporalbone

    Perilymph in scala vestibuli

    Perilymph inscala tympani

    (a)

    Vestibularmembrane

    Afferent fibersof the cochlearnerve

    Cochlearduct (containsendolymph)

    Temporalbone

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    © 2012 Pearson Education, Inc. Figure 8.15b(b)

    Hair (receptor)cells of spiralorgan of Corti

    Tectorialmembrane

    Vestibularmembrane

    Basilarmembrane

    Supportingcells

    Fibers ofthe cochlearnerve

    © 2012 Pearson Education, Inc.

    Mechanism of Hearing

    •Vibrations from sound waves move tectorial membrane

    •Hair cells are bent by the membrane•An action potential starts in the cochlear nerve

    • Impulse travels to the temporal lobe•Continued stimulation can lead to adaptation

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    © 2012 Pearson Education, Inc. Figure 8.16

    EXTERNAL EAR

    Pinna AuditorycanalEar-drum

    Hammer,anvil, stirrup

    MIDDLE EAR

    Ovalwindow

    INTERNAL EAR

    Fluids in cochlear canalsUpper and middle lower

    TimeSpiral organof Cortistimulated

    Amplificationin middle ear

    AmplitudeOnevibration

    Pres

    sure

    © 2012 Pearson Education, Inc.

    Mechanism of Hearing

    •High-pitched sounds disturb the short, stiff fibers of the basilar membrane•Receptor cells close to the oval window are stimulated

    •Low-pitched sounds disturb the long, floppy fibers of the basilar membrane•Specific hair cells further along the cochlea are affected

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    © 2012 Pearson Education, Inc. Figure 8.17

    StapesScalavestibuli

    Fibers ofsensoryneurons

    PerilymphOvalwindow

    Roundwindow

    Scalatympani

    Basilarmembrane

    Cochlearduct

    Fibers of basilar membrane

    Base (short,stiff fibers)

    Apex(long,floppyfibers)

    20,000(High notes)

    2,000 200Frequency (Hz)

    (a)

    (b)

    20(Low notes)

    © 2012 Pearson Education, Inc.

    Olfaction—The Sense of Smell

    •Olfactory receptors are in the roof of the nasal cavity•Neurons with long cilia•Chemicals must be dissolved in mucus for detection

    • Impulses are transmitted via the olfactory nerve

    • Interpretation of smells is made in the cortex

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    © 2012 Pearson Education, Inc. Figure 8.18

    Olfactory bulb

    Olfactory tract

    Mucus layer(a)

    (b)

    Supporting cellOlfactory receptorcell

    Olfactory hairs(cilia)Route of inhaled air

    containing odor molecules

    Cribriform plateof ethmoid bone

    Olfactory filamentsof the olfactorynerve

    Olfactorymucosa

    © 2012 Pearson Education, Inc.

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    © 2012 Pearson Education, Inc.

    The Sense of Taste

    •Taste buds house the receptor organs•Location of taste buds

    •Most are on the tongue•Soft palate•Cheeks

    © 2012 Pearson Education, Inc. Figure 8.19a

    Epiglottis

    Palatine tonsil

    Lingual tonsil

    Fungiformpapillae

    (a)

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    © 2012 Pearson Education, Inc. Figure 8.19b

    Circumvallate papilla

    Taste buds

    (b)

    © 2012 Pearson Education, Inc.

    The Tongue and Taste

    •The tongue is covered with projections called papillae•Filiform papillae—sharp with no taste buds•Fungiform papillae—rounded with taste buds

    •Circumvallate papillae—large papillae with taste buds

    •Taste buds are found on the sides of papillae

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    © 2012 Pearson Education, Inc.

    Structure of Taste Buds

    •Gustatory cells are the receptors•Have gustatory hairs (long microvilli)•Hairs are stimulated by chemicals dissolved in saliva

    © 2012 Pearson Education, Inc.

    Structure of Taste Buds

    • Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas•Facial nerve•Glossopharyngeal nerve•Vagus nerve

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    Taste Sensations

    •Sweet receptors (sugars)•Saccharine•Some amino acids

    •Sour receptors•Acids

    •Bitter receptors•Alkaloids

    •Salty receptors•Metal ions

    © 2012 Pearson Education, Inc.

    Developmental Aspects of the Special Senses•Formed early in embryonic development•Eyes are outgrowths of the brain•All special senses are functional at birth

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    Chemical Senses: Taste and Smell

    •Both senses use chemoreceptors•Stimulated by chemicals in solution•Taste has four types of receptors•Smell can differentiate a large range of chemicals

    •Both senses complement each other and respond to many of the same stimuli

    © 2012 Pearson Education, Inc.

    Developmental Aspects of the Special Senses•Eye problems

    •Strabismus—“crossed eyes” results from unequal pulls by the external eye muscles in babies

    •Ophthalmia neonatorum—conjunctivitis resulting from mother having gonorrhea. Baby’s eyelids are swollen and pus is produced

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    © 2012 Pearson Education, Inc.

    Developmental Aspects of the Special Senses•Eye problems

    •Presbyopia—“old vision” results from decreasing lens elasticity that accompanies aging

    © 2012 Pearson Education, Inc.

    Developmental Aspects of the Special Senses•Ear problems

    •Presbycusis—type of sensorineural deafness•Otosclerosis—ear ossicles fuse

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    © 2012 Pearson Education, Inc.

    Electric nose, tongue

    © 2012 Pearson Education, Inc.

    Sensory substitution

    • Braille

    • Brain port

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    © 2012 Pearson Education, Inc.

    Retinal implant

    Weiland et al., 2005

    © 2012 Pearson Education, Inc.

    Intracortical Visual Prosthesis

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    Cochlear Implant