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The Chemical Senses: Smell And Taste

Smell (olfaction) and taste (gustation)
Chemoreceptors respond to chemicals in aqueous solution

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Olfactory Epithelium and the Sense of Smell

Olfactory epithelium in roof of nasal cavity

Covers superior nasal conchae
Contains olfactory sensory neurons

Bipolar neurons with radiating olfactory cilia
Supporting cells surround and cushion olfactory receptor cells

Olfactory stem cells lie at base of epithelium

Bundles of nonmyelinated axons of olfactory receptor cells form olfactory nerve (cranial nerve I)

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Olfactory Sensory Neurons

Unusual bipolar neurons

Thin apical dendrite terminates in knob
Long, largely nonmotile cilia (olfactory cilia) radiate from knob

Covered by mucus (solvent for odorants)

Olfactory stem cells differentiate to replace them

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Specificity of Olfactory Receptors

Humans can distinguish ~10,000 odors
~400 "smell" genes active only in nose

Each encodes unique receptor protein

Protein responds to one or more odors

Each odor binds to several different receptors
Each receptor has one type of receptor protein

Pain and temperature receptors also in nasal cavities

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Physiology of Smell

Gaseous odorant must dissolve in fluid of olfactory epithelium
Activation of olfactory sensory neurons

Dissolved odorants bind to receptor proteins in olfactory cilium membranes

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Smell Transduction

Odorant binds to receptor 🡪 activates G protein
G protein activation 🡪 cAMP (second messenger) synthesis
cAMP 🡪 Na⁺ and Ca²⁺ channels opening
Na⁺ influx 🡪 depolarization and impulse transmission
Ca²⁺ influx 🡪 olfactory adaptation

Decreased response to sustained stimulus

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Figure 15.21 Olfactory transduction process.

Slide 2

Odorant

Odorant binds

to its receptor.

1

Receptor

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Figure 15.21 Olfactory transduction process.

Slide 3

Odorant

G protein (G_olf)

GDP

Receptor

activates G

protein (G_olf).

2

Odorant binds

to its receptor.

1

Receptor

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Figure 15.21 Olfactory transduction process.

Slide 4

Odorant

G protein (G_olf)

GDP

Odorant binds

to its receptor.

1

G protein

activates adenylate

cyclase.

Receptor

activates G

protein (G_olf).

2

3

Receptor

Adenylate cyclase

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Figure 15.21 Olfactory transduction process.

Slide 5

Odorant

G protein (G_olf)

Adenylate cyclase

GDP

Adenylate cyclase converts ATP to cAMP.

G protein

activates adenylate

cyclase.

Receptor

activates G

protein (G_olf).

Odorant binds

to its receptor.

2

1

3

4

Receptor

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Figure 15.21 Olfactory transduction process.

cAMP opens a cation channel, allowing Na⁺ and Ca²⁺ influx and causing depolarization.

Adenylate cyclase converts ATP to cAMP.

G protein

activates adenylate

cyclase.

Receptor

activates G

protein (G_olf).

Odorant

G protein (G_olf)

Adenylate cyclase

Receptor

cAMP

Open cAMP-gated

cation channel

GDP

Odorant binds

to its receptor.

2

Slide 1

1

3

4

5

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Taste Buds and the Sense of Taste

Receptor organs are taste buds

Most of 10,000 taste buds on tongue papillae

On tops of fungiform papillae
On side walls of foliate and circumvallate (vallate) papillae

Few on soft palate, cheeks, pharynx, epiglottis

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Vallate papilla

Enlarged section of a

vallate papilla.

Taste bud

Figure 15.22b Location and structure of taste buds on the tongue.

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Structure of a Taste Bud

50–100 flask-shaped epithelial cells of 2 types

Gustatory epithelial cells—taste cells

Microvilli (gustatory hairs) are receptors
Three types of gustatory cells

One releases serotonin; others lack synaptic vesicles but one releases ATP as neurotransmitter

Basal epithelial cells—dynamic stem cells that divide every 7-10 days

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Figure 15.22c Location and structure of taste buds on the tongue.

Gustatory

hair

Connective

tissue

Taste fibers

of cranial

nerve

Basal

epithelial

cells

Gustatory

epithelial

cells

Taste

pore

Stratified

squamous

epithelium

of tongue

Enlarged view of a taste

bud (210x).

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Basic Taste Sensations

There are five basic taste sensations

Sweet—sugars, saccharin, alcohol, some amino acids, some lead salts
Sour—hydrogen ions in solution
Salty—metal ions (inorganic salts)
Bitter—alkaloids such as quinine and nicotine; aspirin
Umami—amino acids glutamate and aspartate

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Basic Taste Sensations

Possible sixth taste

Growing evidence humans can taste long-chain fatty acids from lipids
Perhaps explain liking of fatty foods

Taste likes/dislikes have homeostatic value

Guide intake of beneficial and potentially harmful substances

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Physiology of Taste

To taste, chemicals must

Be dissolved in saliva
Diffuse into taste pore
Contact gustatory hairs

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Activation of Taste Receptors

Binding of food chemical (tastant) depolarizes taste cell membrane 🡪 neurotransmitter release

Initiates a generator potential that elicits an action potential

Different thresholds for activation

Bitter receptors most sensitive

All adapt in 3-5 seconds; complete adaptation in 1-5 minutes

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Taste Transduction

Gustatory cell depolarization caused by

Salty taste due to Na⁺ influx (directly causes depolarization)
Sour taste due to H⁺(by opening cation channels)
Unique receptors for sweet, bitter, and umami coupled to G protein gustducin

Stored Ca²⁺ release opens cation channels 🡪 depolarization 🡪 neurotransmitter ATP release

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Gustatory Pathway

Cranial nerves VII and IX carry impulses from taste buds to solitary nucleus of medulla
Impulses then travel to thalamus and from there fibers branch to

Gustatory cortex in the insula
Hypothalamus and limbic system (appreciation of taste)

Vagus nerve transmits from epiglottis and lower pharynx

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Role Of Taste

Triggers reflexes involved in digestion
Increase secretion of saliva into mouth
Increase secretion of gastric juice into stomach
May initiate protective reactions

Gagging
Reflexive vomiting

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Influence of other Sensations on Taste

Taste is 80% smell
Thermoreceptors, mechanoreceptors, nociceptors in mouth also influence tastes

Temperature and texture enhance or detract from taste

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Homeostatic Imbalances of the Chemical Senses

Anosmias (olfactory disorders)

Most result of head injuries and neurological disorders (Parkinson's disease)
Uncinate fits – olfactory hallucinations

Olfactory auras prior to epileptic fits

Taste problems less common

Infections, head injuries, chemicals, medications, radiation for CA of head/neck

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The Ear: Hearing and Balance

Three major areas of ear

External (outer) ear – hearing only
Middle ear (tympanic cavity) – hearing only
Internal (inner) ear – hearing and equilibrium

Receptors for hearing and balance respond to separate stimuli
Are activated independently

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External Ear

Auricle (pinna)Composed of

Helix (rim); Lobule (earlobe)
Funnels sound waves into auditory canal

External acoustic meatus (auditory canal)

Short, curved tube lined with skin bearing hairs, sebaceous glands, and ceruminous glands
Transmits sound waves to eardrum

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External Ear

Tympanic membrane (eardrum)

Boundary between external and middle ears
Connective tissue membrane that vibrates in response to sound
Transfers sound energy to bones of middle ear

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Middle Ear (Tympanic Cavity)

A small, air-filled, mucosa-lined cavity in temporal bone

Flanked laterally by eardrum
Flanked medially by bony wall containing oval (vestibular) and round (cochlear) windows

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Otitis Media

Middle ear inflammation

Especially in children

Shorter, more horizontal pharyngotympanic tubes
Most frequent cause of hearing loss in children

Most treated with antibiotics
Myringotomy to relieve pressure if severe

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Ear Ossicles

Three small bones in tympanic cavity: the malleus, incus, and stapes

Suspended by ligaments and joined by synovial joints
Transmit vibratory motion of eardrum to oval window
Tensor tympani and stapedius muscles contract reflexively in response to loud sounds to prevent damage to hearing receptors

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Two Major Divisions of Internal Ear

Bony labyrinth

Tortuous channels in temporal bone
Three regions: vestibule, semicircular canals, and cochlea
Filled with perilymph – similar to CSF

Membranous labyrinth

Series of membranous sacs and ducts
Filled with potassium-rich endolymph

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Figure 15.26 Membranous labyrinth of the internal ear.

Temporal

bone

Facial nerve

Vestibular nerve

Superior vestibular

ganglion

Inferior vestibular

ganglion

Cochlear nerve

Maculae

Spiral organ

Cochlear duct

in cochlea

Round window

Stapes in

oval window

Saccule in

vestibule

Utricle in

vestibule

Cristae ampullares

in the membranous

ampullae

Lateral

Posterior

in semicircular

canals

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Vestibule

Central egg-shaped cavity of bony labyrinth
Contains two membranous sacs

Saccule is continuous with cochlear duct
Utricle is continuous with semicircular canals

These sacs

House equilibrium receptor regions (maculae)
Respond to gravity and changes in position of head

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Semicircular Canals

Three canals (anterior, lateral, and posterior) that each define ⅔ circle

Lie in three planes of space

Membranous semicircular ducts line each canal and communicate with utricle
Ampulla of each canal houses equilibrium receptor region called the crista ampullaris

Receptors respond to angular (rotational) movements of the head

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Figure 15.26 Membranous labyrinth of the internal ear.

Temporal

bone

Facial nerve

Vestibular nerve

Superior vestibular

ganglion

Inferior vestibular

ganglion

Cochlear nerve

Maculae

Spiral organ

Cochlear duct

in cochlea

Round window

Stapes in

oval window

Saccule in

vestibule

Utricle in

vestibule

Cristae ampullares

in the membranous

ampullae

Lateral

Posterior

in semicircular

canals

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The Cochlea

A spiral, conical, bony chamber

Size of split pea
Extends from vestibule
Coils around bony pillar (modiolus)
Contains cochlear duct, which houses spiral organ (organ of Corti) and ends at cochlear apex

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The Cochlea

Cavity of cochlea divided into three chambers

Scala vestibuli—abuts oval window, contains perilymph
Scala media (cochlear duct)—contains endolymph
Scala tympani—terminates at round window; contains perilymph

Scalae tympani and vestibuli are continuous with each other at helicotrema (apex)

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The Cochlea

The "roof" of cochlear duct is vestibular membrane
External wall is stria vascularis – secretes endolymph
"Floor" of cochlear duct composed of

Bony spiral lamina
Basilar membrane, which supports spiral organ

The cochlear branch of nerve VIII runs from spiral organ to brain

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Figure 15.27a Anatomy of the cochlea.

Helicotrema

at apex

Modiolus

Cochlear nerve,

division of the

vestibulocochlear

nerve (VIII)

Spiral ganglion

Osseous spiral lamina

Vestibular membrane

Cochlear duct

(scala media)

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Properties of Sound

Sound is

Pressure disturbance (alternating areas of high and low pressure) produced by vibrating object

Sound wave

Moves outward in all directions
Illustrated as an S-shaped curve or sine wave

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Properties of Sound Waves

Frequency

Number of waves that pass given point in given time
Pure tone has repeating crests and troughs
Wavelength

Distance between two consecutive crests
Shorter wavelength = higher frequency of sound

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Properties of Sound

Pitch

Perception of different frequencies
Normal range 20–20,000 hertz (Hz)
Higher frequency = higher pitch

Quality

Most sounds mixtures of different frequencies
Richness and complexity of sounds (music)

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Properties of Sound

Amplitude

Height of crests

Amplitude perceived as loudness

Subjective interpretation of sound intensity
Normal range is 0–120 decibels (dB)
Severe hearing loss with prolonged exposure above 90 dB

Amplified rock music is 120 dB or more

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Transmission of Sound to the Internal Ear

Sound waves vibrate tympanic membrane
Ossicles vibrate and amplify pressure at oval window
Cochlear fluid set into wave motion
Pressure waves move through perilymph of scala vestibuli

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Transmission of Sound to the Internal Ear

Waves with frequencies below threshold of hearing travel through helicotrema and scali tympani to round window
Sounds in hearing range go through cochlear duct, vibrating basilar membrane at specific location, according to frequency of sound

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Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.

Slide 2

Tympanic

membrane

Round

window

Auditory ossicles

Oval

window

Cochlear nerve

Scala vestibuli

Route of sound waves through the ear

Malleus

Incus

Stapes

Helicotrema

Sound waves vibrate the tympanic membrane.

1

Scala tympani

Cochlear duct

Basilar

membrane

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Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.

Slide 3

Tympanic

membrane

Round

window

Auditory ossicles

Oval

window

Cochlear nerve

Scala vestibuli

Route of sound waves through the ear

Malleus

Incus

Stapes

Helicotrema

Sound waves vibrate the tympanic membrane.

Auditory ossicles vibrate. Pressure is amplified.

2

1

2

Scala tympani

Cochlear duct

Basilar

membrane

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Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.

Slide 4

Tympanic

membrane

Round

window

Auditory ossicles

Oval

window

Cochlear nerve

Scala vestibuli

Route of sound waves through the ear

Malleus

Incus

Stapes

Helicotrema

3

Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli.

Sound waves vibrate the tympanic membrane.

Auditory ossicles vibrate. Pressure is amplified.

3

2

1

2

Scala tympani

Cochlear duct

Basilar

membrane

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Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.

Slide 5

Tympanic

membrane

Round

window

Auditory ossicles

Oval

window

Cochlear nerve

Scala vestibuli

Route of sound waves through the ear

Malleus

Incus

Stapes

Helicotrema

3

4a

Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli.

Sound waves vibrate the tympanic membrane.

Auditory ossicles vibrate. Pressure is amplified.

Sounds with frequencies below hearing travel through the

helicotrema and do not excite hair cells.

4a

3

2

1

2

Scala tympani

Cochlear duct

Basilar

membrane

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Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.

Slide 6

Tympanic

membrane

Round

window

Auditory ossicles

Oval

window

Cochlear nerve

Scala vestibuli

Route of sound waves through the ear

Malleus

Incus

Stapes

Helicotrema

3

4a

4b

Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli.

Sound waves vibrate the tympanic membrane.

Auditory ossicles vibrate. Pressure is amplified.

Sounds with frequencies below hearing travel through the

helicotrema and do not excite hair cells.

4a

4b

3

2

1

2

Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and

deflecting hairs on inner hair cells.

Scala tympani

Cochlear duct

Basilar

membrane

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Resonance of the Basilar Membrane

Fibers near oval window short and stiff

Resonate with high-frequency pressure waves

Fibers near cochlear apex longer, more floppy

Resonate with lower-frequency pressure waves

This mechanically processes sound before signals reach receptors

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Excitation of Hair Cells in the Spiral Organ

Cells of spiral organ

Supporting cells
Cochlear hair cells

One row of inner hair cells
Three rows of outer hair cells
Have many stereocilia and one kinocilium

Afferent fibers of cochlear nerve coil about bases of hair cells

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Figure 15.27c Anatomy of the cochlea.

Tectorial membrane

Hairs (stereocilia)

Outer hair cells

Supporting cells

Inner hair cell

Afferent nerve

fibers

Fibers of

cochlear

nerve

Basilar

membrane

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Excitation of Hair Cells in the Spiral Organ

Stereocilia

Protrude into endolymph
Longest enmeshed in gel-like tectorial membrane

Sound bending these toward kinocilium

Opens mechanically gated ion channels
Inward K⁺ and Ca²⁺ current causes graded potential and release of neurotransmitter glutamate
Cochlear fibers transmit impulses to brain

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Auditory Pathways to the Brain

Impulses from cochlea pass via spiral ganglion to cochlear nuclei of medulla
From there, impulses sent

To superior olivary nucleus
Via lateral lemniscus to Inferior colliculus (auditory reflex center)

From there, impulses pass to medial geniculate nucleus of thalamus, then to primary auditory cortex
Auditory pathways decussate so that both cortices receive input from both ears

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Figure 15.32 The auditory pathway.

Medial geniculate

nucleus of thalamus

Primary auditory

cortex in temporal lobe

Inferior colliculus

Lateral lemniscus

Superior olivary

nucleus (pons-

medulla junction)

Cochlear nuclei

Midbrain

Medulla

Vestibulocochlear

nerve

Spiral ganglion

of cochlear nerve

Bipolar cell

Spiral organ

Vibrations

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Equilibrium and Orientation

Vestibular apparatus

Equilibrium receptors in semicircular canals and vestibule
Vestibular receptors monitor static equilibrium
Semicircular canal receptors monitor dynamic equilibrium

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Maculae

Sensory receptors for static equilibrium
One in each saccule wall and one in each utricle wall
Monitor the position of head in space, necessary for control of posture
Respond to linear acceleration forces, but not rotation
Contain supporting cells and hair cells
Stereocilia and kinocilia are embedded in the otolith membrane studded with otoliths (tiny CaCO₃ stones)

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Figure 15.33 Structure of a macula.

Macula of

utricle

Macula of

saccule

Stereocilia

Kinocilium

Otoliths

Otolith

membrane

Hair bundle

Hair cells

Supporting

cells

Vestibular

nerve fibers

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Maculae

Maculae in utricle respond to horizontal movements and tilting head side to side
Maculae in saccule respond to vertical movements
Hair cells synapse with vestibular nerve fibers

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Activating Maculae Receptors

Hair cells release neurotransmitter continuously

Movement modifies amount they release

Bending of hairs in direction of kinocilia

Depolarizes hair cells
Increases amount of neurotransmitter release
More impulses travel up vestibular nerve to brain

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Activating Maculae Receptors

Bending away from kinocilium

Hyperpolarizes receptors
Less neurotransmitter released
Reduces rate of impulse generation

Thus brain informed of changing position of head

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Figure 15.34 The effect of gravitational pull on a macula receptor cell in the utricle.

Otolith

membrane

Kinocilium

Stereocilia

Receptor potential

Depolarization

Hyperpolarization

Nerve impulses generated

in vestibular fiber

When hairs bend toward

the kinocilium, the hair cell

depolarizes, exciting the

nerve fiber, which generates

more frequent action potentials.

When hairs bend away

from the kinocilium, the hair cell

hyperpolarizes, inhibiting the nerve

fiber, and decreasing the action

potential frequency.

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The Crista Ampullares (Crista)

Sensory receptor for rotational acceleration

One in ampulla of each semicircular canal
Major stimuli are rotational movements

Each crista has supporting cells and hair cells that extend into gel-like mass called ampullary cupula
Dendrites of vestibular nerve fibers encircle base of hair cells

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Activating Crista Ampullaris Receptors

Cristae respond to changes in velocity of rotational movements of the head
Bending of hairs in cristae causes

Depolarizations, and rapid impulses reach brain at faster rate

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Activating Crista Ampullaris Receptors

Bending of hairs in the opposite direction causes

Hyperpolarizations, and fewer impulses reach the brain

Thus brain informed of rotational movements of head

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Figure 15.35c Location, structure, and function of a crista ampullaris in the internal ear.

Section of

ampulla,

filled with

endolymph

Cupula

Fibers of

vestibular

nerve

Flow of endolymph

At rest, the cupula stands upright.

During rotational acceleration, endolymph moves inside the semicircular canals in the direction opposite the rotation (it lags behind due to inertia). Endolymph flow bends the cupula and excites the hair cells.

As rotational movement slows, endolymph keeps moving in the direction of rotation. Endolymph flow bends the cupula in the opposite direction from acceleration and inhibits the hair cells.

Movement of the ampullary cupula during rotational acceleration and deceleration

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Motion Sickness

Sensory input mismatches

Visual input differs from equilibrium input
Conflicting information causes motion sickness

Warning signs are excess salivation, pallor, rapid deep breathing, profuse sweating
Treatment with antimotion drugs that depress vestibular input such as meclizine and scopolamine

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Homeostatic Imbalances of Hearing

Conduction deafness

Blocked sound conduction to fluids of internal ear

Impacted earwax, perforated eardrum, otitis media, otosclerosis of the ossicles

Sensorineural deafness

Damage to neural structures at any point from cochlear hair cells to auditory cortical cells
Typically from gradual hair cell loss

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Treating Deafness

Research trying to prod supporting cell differentiation into hair cells to treat sensorineural deafness
Cochlear implants for congenital or age/noise cochlear damage

Convert sound energy into electrical signals
Inserted into drilled recess in temporal bone
So effective that deaf children can learn to speak