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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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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


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)

Iris

Eyelid

Eyebrow

Eyelid

Eyelashes

Pupil

Lacrimalcaruncle

Medialcommissure(canthus)

Sclera(covered byconjunctiva)



•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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•Conjunctiva

•Membrane that lines the eyelids

•Connects to the outer surface of the eye

•Secretes mucus to lubricate the eye and keep it moist



•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

Eyelid

Eyelashes

TarsalglandsEyelid

Excretory ductof lacrimal gland

© 2012 Pearson Education, Inc. Figure 8.2b

LacrimalglandExcretory ductsof lacrimal gland

Lacrimal canaliculus

Nasolacrimal ductInferior meatusof nasal cavity

Nostril

Lacrimal sac

(b)

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•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)



•Extrinsic eye muscles

•Six muscles attach to the outer surface of the eye

•Produce eye movements

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Superioroblique muscle

Superioroblique tendon

Superiorrectus muscle

Conjunctiva

Lateral rectusmuscle

Opticnerve

Inferiorrectusmuscle

Inferiorobliquemuscle

(a)


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


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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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)


Ciliary body

Iris

LensCorneaCiliary zonule

(b)

Retina

ChoroidScleraFovea centralisOptic discOptic nerve

Marginof pupil

Aqueous humor(in anteriorsegment)

Vitreous humorin posterior segment

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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


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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Structure of the Eye: Sensory Layer

•Retina contains two layers

•Outer pigmented layer

• Inner neural layer

•Contains receptor cells (photoreceptors)

•Rods

•Cones



•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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RodCone

(a)

Pathwayof light

Pigmentedlayer of retina

Bipolarcells

Ganglioncells


Pigmentedlayer of retina

Neural layerof retina

Centralarteryand veinof retina

Opticdisc

Opticnerve

(b)

ScleraChoroid

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•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



•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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•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


560 nm(red cones)530 nm

(green cones)

420 nm(blue cones)

Lig

ht

abso

rpti

on

by

con

e p

op

ula

tio

ns

380 450 500 550 600 650 700 750

Wavelengths (nanometers)

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Lens

•Biconvex crystal-like structure

•Held in place by a suspensory ligament attached to the ciliary body



CorneaIrisPupil

Lens

ScleraChoroidRetina

Fovea centralis

Optic nerve

(a)






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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


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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



CorneaIrisPupil

Lens

ScleraChoroidRetina

Fovea centralis

Optic nerve

(a)






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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


Posterior Segment

•Vitreous humor

•Gel-like substance posterior to the lens

•Prevents the eye from collapsing

•Helps maintain intraocular pressure

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Ophthalmoscope

• Instrument used to illuminate the interior of the eyeball

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


Foveacentralis

Macula Bloodvessels

Optic disc Retina

Lateral Medial

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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)


Retina

Focal pointLight from distant source

(a)

Light from near source Focal point

Retina

(b)

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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


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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


Fixation point

Right eye Left eye

Opticchiasma

Opticnerve

Optictract

Opticradiation

Occipital lobe(visual cortex)

Thalamus

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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)


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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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”


Focalplane

(a) Emmetropic eye

Correction

None required

Concave lens

Convex lens(b) Myopic eye

(nearsighted)

(c) Hyperopic eye(farsighted)

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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


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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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


The Ear

•Houses two senses

•Hearing

•Equilibrium (balance)

•Receptors are mechanoreceptors

•Different organs house receptors for each sense

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Anatomy of the Ear

•The ear is divided into three areas

•External (outer) ear

•Middle ear (tympanic cavity)

• Inner ear (bony labyrinth)


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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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


The Middle Ear (Tympanic Cavity)

•Air-filled cavity within the temporal bone

•Only involved in the sense of hearing

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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


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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Auricle(pinna)



Oval window

Vestibule

Round window

Hammer(malleus)

Anvil(incus)

Stirrup(stapes)

Auditory ossicles

Semicircularcanals


Cochlea





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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Auricle(pinna)



Oval window

Vestibule

Round window

Hammer(malleus)

Anvil(incus)

Stirrup(stapes)

Auditory ossicles

Semicircularcanals


Cochlea





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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Semicircularcanals

Ampulla

Vestibularnerve

Vestibule

(a)


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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Membranes in vestibule

Otoliths

Hair tuft

(a)

Hair cellSupporting cell

Otolithicmembrane

Nerve fibers ofvestibular divisionof cranial nerve VIII


Force ofgravityOtoliths

Hair cell

Head upright

(b)

Head tilted

Otolithicmembrane

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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


Semicircularcanals

Ampulla

Vestibularnerve

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


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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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


Spiralorgan ofCorti

Temporalbone

Perilymph in scala vestibuli

Perilymph inscala tympani

(a)

Vestibularmembrane

Afferent fibersof the cochlearnerve

Cochlearduct (containsendolymph)

Temporalbone

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(b)

Hair (receptor)cells of spiralorgan of Corti

Tectorialmembrane

Vestibularmembrane

Basilarmembrane

Supportingcells

Fibers ofthe cochlearnerve


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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EXTERNAL EAR

Pinna Auditorycanal

Ear-drum

Hammer,anvil, stirrup

MIDDLE EAR

Ovalwindow

INTERNAL EAR

Fluids in cochlear canalsUpper and middle lower

TimeSpiral organof Cortistimulated

Amplificationin middle ear

AmplitudeOnevibration

Pre

ssu

re


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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StapesScalavestibuli

Fibers ofsensoryneurons

PerilymphOvalwindow

Roundwindow

Scalatympani

Basilarmembrane

Cochlearduct

Fibers of basilar membrane

Base (short,stiff fibers)

Apex(long,floppyfibers)

20,000(High notes)

2,000 200

Frequency (Hz)

(a)

(b)

20(Low notes)


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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Olfactory bulb

Olfactory tract

Mucus layer(a)

(b)

Supporting cell

Olfactory receptorcell

Olfactory hairs(cilia)Route of inhaled air

containing odor molecules

Cribriform plateof ethmoid bone

Olfactory filamentsof the olfactorynerve

Olfactorymucosa


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The Sense of Taste

•Taste buds house the receptor organs

•Location of taste buds

•Most are on the tongue

•Soft palate

•Cheeks


Epiglottis

Palatine tonsil

Lingual tonsil

Fungiformpapillae

(a)

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Circumvallate papilla

Taste buds

(b)


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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Structure of Taste Buds

•Gustatory cells are the receptors

•Have gustatory hairs (long microvilli)

•Hairs are stimulated by chemicals dissolved in saliva


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


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


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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Developmental Aspects of the Special Senses•Eye problems

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


Developmental Aspects of the Special Senses•Ear problems

•Presbycusis—type of sensorineural deafness

•Otosclerosis—ear ossicles fuse

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Electric nose, tongue


Sensory substitution

• Braille

• Brain port

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Retinal implant

Weiland et al., 2005


Intracortical Visual Prosthesis

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Cochlear Implant