• Five special senses:

• Olfaction

• Taste

• Visual system

• Hearing and balance

• Epithelium of nasal cavity is called olfactory epithelium, located in the roof of nasal cavity

• Olfactory receptor cells present in olfactory epithelium are bipolar neurons with enlarged ends (olfactory vesicles) and contains cilia called olfactory hairs

• Airborne molecules enter nasal cavity and stimulate cilia of olfactory receptors bathed by a layer of mucus

• Odorants dissolve in mucus attach to Odorant receptors of Olfactory cilia

• Associated G protein is activated

• Alpha subunit of G protein activates adenylate cyclase

• And convert ATP to cAMP

• cAMP act as second

messenger opens Na+ and

Ca2+ channels

• Causes depolarization of cilia

• And initiate action potentials in

olfactory neurons

• One receptor may respond to more than one type of odor

• Olfactory epithelium is replaced as it wears down

• Olfactory neurons are replaced by basal cells every two months

• Unique: most neurons are permanent cells (aren't replaced if they die)

• Axons of olfactory neurons (bipolar) in the olfactory epithelium pass through cribiform plate to olfactory bulbs

• Then project through the olfactory tract to olfactory cortex of frontal lobe

• 3 regions in frontal lobe affect conscious perception of smell & interact with limbic system

– Lateral olfactory area: Involved in conscious perception of smell

– Medial olfactory area: Responsible for visceral & emotional reactions to odors

– Intermediate olfactory area: Receives

input from L & M olfactory area

– Axons from L olfactory area project to olfactory tract to olfactory bulb

– In olfactory bulb Sensory information is modulated

•

•

• 10,000 or more taste buds are found on the tongue

• Taste buds are found in papillae of the tongue mucosa

• Types of papillae

– Vallate: Largest, least numerous

– 8-12 in V along border between anterior and posterior parts of the tongue

– Have taste buds

– Fungiform: Mushroom-shaped

– Scattered irregularly over the superior surface of tongue

– Have taste buds

– Foliate: Leaf-shaped

– In folds on the sides of the tongue

– Contain most sensitive taste buds

– Decrease in number with age

– Filiform: Filament-shaped

– Most numerous

– No taste buds

• Taste bud consists of three major cell types

– Supporting cells – insulate the receptor

– Basal cells – dynamic stem cells

– Gustatory cells – taste cells

– Taste cells have microvilli

– (gustatory hairs) extending into taste pores

– Replaced about every 10 days

• There are five basic taste sensations

– Sweet – sugars, saccharin, alcohol, and

some amino acids

– Salt – metal ions

– Sour – hydrogen ions

– Bitter – alkaloids such as quinine and

nicotine

– Umami – (savory) elicited by the amino acid

glutamate

• Activation of Taste Receptors:

• In order to be tasted, a chemical:

– Must be dissolved in saliva

– Must contact gustatory hairs of gustatory cells

– Which contain synaptic vesicles

• Binding of the food chemical:

– To the receptors in the taste cell membrane depolarizes,

releases neurotransmitter from synaptic vesicles

– Binding of neurotransmitter to associated sensory dendrites

initiates an action potential

• Mechanism of Taste

Transduction:

• The stimulus energy of taste is

converted into a nerve impulse

by:

– Na+ influx in salty tastes

– H+ in sour tastes (by directly

entering the cell, by opening

cation channels, or by

blockade of K+ channels)

• Sweet and bitter:

– Sweet and bitter tastants

bind to receptors and

cause depolarization by G

protein mechanism

• Umami : Glutamate bind to

receptors and cause

depolarization by G protein

mechanism

• Taste is 80% smell

• Thermoreceptors, mechanoreceptors,

nociceptors also influence tastes

• Temperature and texture enhance or

detract from taste

• Cranial Nerves VII and IX

carry impulses from taste buds

to the nucleus of tractus

solitarius of the medulla

• Where decussation takes

place and these impulses then

travel to the thalamus

• Then thalamus to taste area of

cortex ( inferior end of

postcentral gyrus)

• The three parts of the ear

are the inner, external and

middle ear

• The external and middle

ear are involved with

hearing

• The inner ear functions in

both hearing and

equilibrium

• The auricle (pinna) is composed of:

– The helix (rim)

– The lobule (earlobe)

• External auditory canal

– Short, curved tube lined with hairs & ceruminous glands

– Produce cerumen wax

– Prevent foreign objects

• Tympanic membrane (eardrum)

– Thin connective tissue membrane that vibrates in response to sound

– Transfers sound energy to the middle ear ossicles

– Boundary between external and middle ears

• Medial to tympanic membrane

• Separated from the inner ear by the oval and round windows

– Two passages for air

• Auditory or eustachian tube: opens into pharynx, equalizes pressure

• Passage to mastoid air cells in mastoid process of temporal bone

• Ossicles:

– malleus, incus, stapes:

transmit vibrations from

eardrum to oval window

• Oval window:

– Connection between middle

and inner ear

– Foot of the stapes rests here

and is held in place by

annular ligament

• Consists of :

• Bony labyrinth

– Tunnels and chambers in the temporal bone

– Divided into three regions:

• Cochlea: hearing

• Vestibule: balance

• Semicircular canals: balance

• Membranous labyrinth

– Series of membranous sacs within the bony labyrinth

• Lymphs

– Endolymph: in membranous labyrinth, high conc. of K+

– Perilymph: similar to CSF, space between membranous labyrinth and bony labyrinth

• Is the central egg-shaped cavity of the bony labyrinth

• Suspended in its perilymph are two membranous labyrinth sacs: the saccule and utricle

• The saccule extends into the cochlea

• The utricle extends into the semicircular canals

• These sacs:

– House balance receptors called maculae

– Respond to gravity and changes in the position of the head

• Anterior, posterior and lateral

semicircular canal is present in each

internal ear

• Membranous semicircular ducts line

each canal and communicate with the

utricle

• The ampulla is the swollen end of each

canal and it houses equilibrium

receptors in a region called the crista

ampullaris

• These receptors respond to angular

movements of the head

• A spiral, conical, bony chamber that: – Extends from the anterior

vestibule

– Coils around a bony pillar called the modiolus

– Contains the cochlear duct which ends at the cochlear apex

– And organ of Corti (hearing receptor)

• The cochlea is divided into three chambers: – Scala vestibuli

– Scala media

– Scala tympani

• The scala tympani terminates at the round window

• The scalas tympani and vestibuli:

– Are filled with perilymph

– Are continuous with each other via the helicotrema at cochlear apex

• The scala media is filled with endolymph

• The “floor” of the cochlear duct

is composed of:

– The bony spiral lamina

– The basilar membrane,

which supports the organ of

Corti

• The cochlear branch of nerve

VIII runs from the organ of

Corti to the brain

• Sound vibrations beat against the eardrum

• The eardrum pushes against the ossicles, which presses

fluid in the inner ear against the oval and round windows

– This movement sets up shearing forces that pull on

hair cells

– Moving hair cells stimulates the cochlear nerve that

sends impulses to the brain

• Properties of Sound:

• Sound is: – A pressure disturbance (alternating

areas of high and low pressure) originating from a vibrating object

– Composed of areas of rarefaction and compression

– Represented by a S-shaped curve or sine wave in wavelength, frequency, and amplitude

– Crests – compressed area

– troughs – rarefied area

• Properties of Sound:

• Frequency – the number of waves that pass a given point in a given time

• Pitch – perception of different frequencies (we hear from 20–20,000 Hz)

• Higher frequency – higher pitch

• Amplitude – intensity of a sound or height of sine wave crest measured in decibels (dB)

• Loudness – subjective interpretation of sound intensity

• The route of sound to the inner ear follows this pathway:

– External ear – pinna, auditory canal, eardrum

– Middle ear – malleus, incus, and stapes to the oval window

– Inner ear – scalas vestibuli and tympani to the cochlear duct

• Stimulation of the organ of Corti

• Generation of impulses in the cochlear nerve

• Sound waves of low frequency (inaudible):

– Travel around the helicotrema

– Do not excite hair cells

• Audible sound waves:

– Penetrate through the cochlear duct

– Vibrate the basilar membrane

– Excite specific hair cells according to frequency of the sound

• Rest on basilar membrane

• And composed of supporting cells and outer and inner hair cells

• Afferent fibers of the cochlear nerve attach to the base of hair cells

• Hair cells have numerous stereocilia (hair)

• Bending of sterocilia:

– Opens mechanically gated K+ ion

channels

– Causes a graded potential and

the release of a neurotransmitter

(probably glutamate)

• The neurotransmitter causes

cochlear fibers to transmit impulses

to the brain, where sound is

perceived

• Impulses from the cochlea pass

via the spiral ganglion of cochlear

nerve to the cochlear nuclei of

medulla

• From there, impulses are sent to

the:

– Superior olivary nucleus

– Inferior colliculus (auditory

reflex center)

• From there, impulses pass to the

auditory cortex in temporal lobe

• Receptors for balance is present in the semicircular

canals and vestibule called Vestibular apparatus

– Maintains our orientation and balance in space

• Two kinds of balance Receptors:

– Vestibular receptors monitor static labyrinth

– Semicircular canal receptors monitor kinetic labyrinth

• Maculae are the sensory receptors for static labyrinth

– Contain supporting cells and hair cells

– Each hair cell has stereocilia and kinocilium embedded in the otolithic membrane

• Otolithic membrane – jellylike mass studded with tiny CaCO3 stones called otoliths

• Utricular hairs respond to horizontal movement

• Saccular hairs respond to vertical movement

• Otolithic movement in the direction of the kinocilia:

– Depolarizes vestibular nerve fibers

– Increases the number of action potentials generated

• Movement in the opposite direction:

– Hyperpolarizes vestibular nerve fibers

– Reduces the rate of impulse propagation

• From this information, the brain is informed of the changing position of the head

• Evaluates position of head relative to gravity

• Crista ampullaris (or crista):

– Is the receptor for Kinetic Labyrinth

– Is located in the ampulla of each

semicircular canal

– Responds to angular movements

• Each crista has support cells and hair

cells that extend into a gel-like mass

called the cupula

• Dendrites of vestibular nerve fibers

encircle the base of the hair cells

• Cristae respond to changes in velocity of rotatory movements of the head

• Directional bending of hair cells in the cristae causes:

– Depolarizations, and rapid impulses reach the brain at a faster rate

– Hyperpolarizations, fewer impulses reach the brain

• The result is that the brain is informed of rotational movements of the head

• 70% of all sensory receptors are in the eye

• Most of the eye is protected by a cushion

of fat and the bony orbit

• Accessory structures include eyebrows,

eyelids, conjunctiva, lacrimal apparatus,

and extrinsic eye muscles

• Eyebrows:

– Shading the eye

– Preventing perspiration from reaching the eye

• Eyelids (palpebrae):

• Protect the eye from foreign objects

– Palpebral fissure: space between two eyelids

– Canthi: eyelids join at lateral and medial margins of eye

– Medial canthus has lacrimal caruncle with modified sweat and sebaceous glands

– Tarsal or Meibomian glands in eyelids lubricate the eye

• Eyelashes: double/triple row of hairs

• Ciliary glands (modified sweat glands) empty into hair follicles of eyebrows, keep them lubricated

• Conjunctiva: thin transparent mucous membrane

–Palpebral conjunctiva: covers inner surface of eyelids

–Bulbar conjunctiva: covers anterior white surface of eye

• Lacrimal gland: produces tears to moisten, lubricate, wash

• located above the lateral end of eye

• Tears pass through ducts and then over eye

• Lacrimal canaliculi: collect excess tears through openings called punctum

• Lacrimal sac leads to nasolacrimal duct: opens into nasal cavity beneath the inferior nasal conchae

• Extrinsic eye muscles

– Six muscles attach to

the outer surface of

the eye

– LR6SO4O3

– Produce eye

movements

• The eyeball is composed of three layers:

• Fibrous layer: sclera and cornea

• Vascular layer: choroid, ciliary body, iris

• Nervous layer: retina

• Sclera: white outer layer of eyeball

• Seen anteriorly as the “white of the eye”

• Maintains shape, protects internal structures, provides muscle attachment point

• Sclera is continuous anteriorly with cornea

• Dense collagenous connective tissue with elastic fibers. Collagen fibers are large and opaque.

• Cornea:

• Avascular, transparent, allows light to enter eye; bends and refracts light

• Consists of connective tissue matrix containing collagen, elastic fibers and proteoglycans

• More proteoglycans than sclera, low water content (water would scatter light)

• Has three regions: choroid, ciliary body, and iris

• Choroid region – A dark brown

membrane that forms the posterior portion of the eye

– Light is focused by choroid pigment onto retina thus preventing scattering of the light

– Supplies blood to all eye tunics

• Ciliary body:

• Composed of smooth muscle bundles (ciliary muscles)

• Anchors the suspensory ligament that holds the lens in place

• Iris:

• The colored part of the eye

• Pupil – central opening of the iris

– Regulates the amount of light entering the eye during:

• Close vision and bright light – pupils constrict

• Distant vision and dim light – pupils dilate

• Two layers

– Pigmented retina: outer, pigmented layer

– simple cuboidal epithelium

– Sensory retina: inner layer of rod and cone cells sensitive to light

• Lens focuses light on macula lutea and fovea centralis of retina

– Macula lutea: small yellow spot

– Fovea centralis: area of greatest visual acuity; photoreceptor cells tightly packed

• Optic disc: blind spot Area through which blood vessels enter eye, where nerve processes from sensory retina meet and exit from eye

• Anterior compartment:

anterior to lens; filled with

aqueous humor

– Anterior chamber:

between cornea and iris

– Posterior chamber:

between iris and lens

– Helps maintain

intraocular pressure;

supplies nutrients,

contributes to refraction

of light

– Glaucoma: abnormal

increase in intraocular

pressure

•Posterior compartment: posterior to lens

• Filled with jelly-like vitreous humor

• Helps maintain intraocular pressure, holds lens and retina in place, refracts light

• Held by suspensory ligaments

attached to ciliary muscles

• Transparent, biconvex

• Made of long columnar epithelial

cells – lens fibers

• Lens fibers – contain transparent

protein crystallin

• Electromagnetic radiation –

all energy waves from short

gamma rays to long radio

waves

• Our eyes respond to a small

portion of this spectrum

called the visible light

• Different cones in the retina

respond to different

wavelengths of the visible

spectrum

• When light passes from one

transparent medium to another

its speed changes and it refracts

(bends)

• Light passing through a convex

lens (as in the eye) is bent so

that the rays converge to a focal

point

• When a convex lens forms an

image, the image is upside

down and reversed right to left

• Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, and the neural layer of the retina to the photoreceptors

• Light is refracted:

– At the cornea

– Entering the lens

– Leaving the lens

• The lens curvature and shape allow for fine focusing of an image

• Light from a distance needs

little adjustment for proper

focusing

• Far point of vision – the

distance beyond which the

lens does not need to

change shape to focus (20

ft.)

• Ciliary muscle is relaxed

• Lens is flat

• Close vision requires:

– Accommodation – changing the lens shape by ciliary muscles to increase refractory power

– Lens becomes more spherical, greater refraction of light

– Constriction –

– pupillary reflex constricts the pupils

– Pupil diameter is small, depth of focus is greater

– Convergence –

– as objects move close to the eye, eyes are rotated medially

• Emmetropic eye – normal eye with light focused properly

• Myopic eye (nearsighted) – the focal point is in front of the retina

– Corrected with a concave lens

• Hyperopic eye (farsighted) – the focal point is behind the retina

– Corrected with a convex lens

• Sensory retina: three layers of neurons: photoreceptor, bipolar, and ganglionic

• Pigmented retina: single layer of cells; filled with melanin

• With choroid, enhances visual acuity reducing light scattering

• Bipolar photoreceptor cells; black and white vision

• Found over most of retina

– Protein rhodopsin changes shape when struck by light; and eventually separates into its two components:

– opsin and retinal

– In absence of light, opsin and retinal recombine to form rhodopsin

• Bipolar photoreceptor cells

• Responsible for color vision and visual acuity

– Numerous in fovea and macula lutea; fewer over rest of retina

– As light intensity decreases so does our ability to see color

– Cone cell contain visual pigment - iodopsin: three types that respond to blue, red and green light

– Overlap in response to light, thus interpretations of gradation of color possible: several millions

• Rods and cones synapse with bipolar cells that synapse with ganglion cells

• Axons of retinal ganglion cells form the optic nerve

• Medial fibers of the optic nerve decussate at the optic chiasm

• Most fibers of the optic tracts continue to the lateral geniculate body of the thalamus

• Other optic tract fibers end in superior colliculi (initiating visual reflexes) and pretectal nuclei (involved with pupillary reflexes)

• Optic radiations travel from the thalamus to the visual cortex

• Myopia: Nearsightedness – Focal point too near lens,

image focused in front of retina

• Hyperopia: Farsightedness – Image focused behind retina

• Presbyopia – Degeneration of

accommodation, corrected by reading glasses

• Astigmatism: Cornea or lens not uniformly curved

• Retinal detachment – Can result in complete

blindness

• Glaucoma – Increased intraocular

pressure by aqueous humor buildup

• Cataract – Clouding of lens

• Macular degeneration – Common in older people,

loss in acute vision

• Diabetes – Dysfunction of peripheral

circulation

Effects of Aging on the

Special Senses

• Slight loss in ability to detect odors

• Decreased sense of taste

• Lenses of eyes lose flexibility

• Development of cataracts, macular

degeneration, glaucoma, diabetic

retinopathy

• Decline in visual acuity and color perception