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Special Senses

• Special sensory receptors

– Distinct, localized receptor cells in head

• Vision - 70% of body's sensory receptors

in eye

• Taste

• Smell

• Hearing

• Equilibrium

The Eye and Accessory Structures

The Lacrimal Apparatus

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Sense of Vision

Ora serrata

Ciliary body

Ciliary zonule

(suspensoryligament)

Cornea

Pupil

Anterior

pole

Anterior

segment(contains

aqueous humor)

Lens

Scleral venous sinus

Posterior segment

(contains vitreous humor)

Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.

Sclera

Choroid

Retina

Macula lutea

Fovea centralis

Posterior pole

Optic nerve

Central artery and

vein of the retina

Optic disc

(blind spot)

Iris

Circulation of Aqueous Humor

Inner Layer: Retina

• Delicate two-layered membrane

– Outer Pigmented layer

• Absorbs light and prevents its scattering

• Phagocytize photoreceptor cell fragments

• Stores vitamin A

– Inner Neural layer

• Transparent

• Composed of three main types of neurons

– Photoreceptors, bipolar cells, ganglion cells

• Signals spread from photoreceptors to bipolar cells to

ganglion cells

• Ganglion cell axons exit eye as optic nerve

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Figure 15.6c Microscopic anatomy of the retina.

Photomicrograph of retina

Nuclei ofganglioncells

Outer segmentsof rods and cones

Choroid

Axons ofganglion cells

Nuclei ofbipolarcells

Nuclei ofrods andcones

Pigmentedlayer of retina

Figure 15.15a Photoreceptors of the retina.

Process of bipolar cell

Synaptic terminals

Rod cell body

Inner fibers

NucleiCone cell body

Mitochondria

Connecting cilia

Outer fiber

Apical microvillus

Discs containingvisual pigments

Discs being phagocytized

Melanin granules

Pigment cell nucleusBasal lamina (border with choroid)

Inn

er

se

gm

en

tP

igm

ente

d l

ayer

Oute

r se

gm

ent

The outer segments of rods and cones are embedded in the pigmented layer of the retina.

Rod cell body

Chemistry Of Visual Pigments

• Retinal

– Light-absorbing molecule that combines with

one of four proteins (opsins) to form visual

pigments

– Synthesized from vitamin A

– Retinal isomers: 11-cis-retinal (bent form)

and all-trans-retinal (straight form)

• Bent form straight form when pigment absorbs

light

• Conversion of bent to straight initiates reactions

electrical impulses along optic nerve

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Figure 15.18 Signal transmission in the retina (1 of 2). Slide 1In the dark

cGMP-gated channelsopen, allowing cation influx.Photoreceptor depolarizes.

1

Voltage-gated Ca2+

channels open in synapticterminals.

Neurotransmitter isreleased continuously.

Neurotransmitter causesIPSPs in bipolar cell.Hyperpolarization results.

Hyperpolarization closesvoltage-gated Ca2+ channels,inhibiting neurotransmitterrelease.

No EPSPs occur inganglion cell.

No action potentials occuralong the optic nerve.

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

Ca2+

−40 mV−40 mV

2

3

4

5

6

7

Ca2+

Na+

Figure 15.18 Signal transmission in the retina. (2 of 2). Slide 1

−70 mVNo neurotransmitter

is released.

Depolarization opensvoltage-gated Ca2+ channels;neurotransmitter is released.

EPSPs occur in ganglioncell.

Action potentialspropagate along theoptic nerve.

cGMP-gated channelsclose, so cation influxstops. Photoreceptorhyperpolarizes.

Lack of IPSPs in bipolar

cell results in depolarization.

Voltage-gated Ca2+

channels close in synapticterminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

Ca2+

−70 mV

2

3

4

5

6

7

Below, we look at a tiny column of retina.The outer segment of the rod, closest to theback of the eye and farthest from theincoming light, is at the top.

Light

Figure 15.15b Photoreceptors of the retina.

Rod discs

Rhodopsin, the visual pigment in rods,

is embedded in the membrane that forms

discs in the outer segment.

Visual

pigment

consists of

• Retinal

• Opsin

Rhodopsin

Dark

Light

2H+

2H+

11-cis-retinalVitamin A

Oxidation

Reduction

11-cis-retinal

All-trans-

retinal

All-trans-retinal

Opsin and

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Figure 15.17 Events of phototransduction. Slide 6

Recall from Chapter 3 thatG protein signaling mechanismsare like a molecular relay race.

Retinal absorbs lightand changes shape.Visual pigment activates.

Receptor G protein Enzyme 2ndmessenger

Visualpigment

1

Light

11-cis-retinal

Transducin(a G protein)

All-trans-retinal

2 3Visual pigmentactivatestransducin(G protein).

Transducinactivatesphosphodiesterase (PDE).

4 5PDE convertscGMP into GMP,causing cGMPlevels to fall.

As cGMP levels fall,cGMP-gated cationchannels close, resultingin hyperpolarization.

cGMP-gatedcation channelopen in dark

cGMP-gatedcation channelclosed in light

Phosphodiesterase (PDE)

Light (1st

messenger)

Visual Pathway to the Brain and Visual

Fields, Inferior View

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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Figure 15.20a Olfactory receptors.

Olfactoryepithelium

Olfactory tract

Olfactory bulb

Nasalconchae

Route ofinhaled air

Figure 15.20b Olfactory receptors.

Olfactorytract

Olfactorygland

Olfactoryepithelium

Mucus

Mitral cell(output cell)

Olfactory bulb

Cribriform plateof ethmoid bone

Filaments ofolfactory nerveLamina propriaconnective tissue

Olfactory stem cell

Olfactory sensoryneuron

Dendrite

Olfactory cilia

Route of inhaled aircontaining odor molecules

Glomeruli

Olfactory axon

Supporting cell

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 proteinactivates adenylatecyclase.

Receptoractivates Gprotein (Golf).

Odorant

G protein (Golf)

Adenylate cyclase

Receptor

cAMPcAMP

Open cAMP-gatedcation channel

GDP

Odorant bindsto 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 vallate papillae

– Few on soft palate, cheeks, pharynx,

epiglottis

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

Epiglottis

Palatine tonsil

Lingual tonsil

Foliatepapillae

Fungiformpapillae

Taste buds are associatedwith fungiform, foliate, andvallate papillae.

To taste, chemicals

must

– Be dissolved in

saliva

– Diffuse into taste

pore

– Contact gustatory

hairs

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

Gustatoryhair

Connective tissue

Taste fibersof cranialnerve

Basalepithelial

cells

Gustatory epithelial

cells

Tastepore

Stratifiedsquamousepitheliumof tongue

Enlarged view of a tastebud (210x).

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Location and Structure of Taste Buds on the

Tongue

Basic Taste Sensations

• There are five basic taste sensations

1. Sweet—sugars, saccharin, alcohol, some

amino acids, some lead salts

2. Sour—hydrogen ions in solution

3. Salty—metal ions (inorganic salts)

4. Bitter—alkaloids such as quinine and

nicotine; aspirin

5. Umami—amino acids glutamate and

aspartate

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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The Gustatory Pathway

The Ear: Hearing and Balance

Three major areas of ear

1. External (outer) ear – hearing only

2. Middle ear (tympanic cavity) – hearing only

3. Internal (inner) ear – hearing and

equilibrium

• Receptors for hearing and balance respond to

separate stimuli

• Are activated independently

Structure of the Ear (1 of 2)

Figure 15.24a Structure of the ear.

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Structure of the Ear (2 of 2)

Figure 15.24b Structure of the ear.

The Three Auditory Ossicles and Associated

Skeletal Muscles

Figure 15.25 The three auditory ossicles and associated skeletal muscles.

Membranous Labyrinth of the Internal

Ear

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Anatomy of the Cochlea

Anatomy of the Cochlea

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Anatomy of the Cochlea

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

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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Pathway of Sound Waves

• HIGH frequency sound

detected hereLOW frequency sound

detected here

Excitation of Hair Cells in the Spiral Organ

• Stereocilia

– Protrude into endolymph

– Longest enmeshed in gel-like tectorial

membrane

– Sound bends 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

Anatomy of the Cochlea

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Bending of Stereocilia Opens or Closes

Mechanically Gated Ion Channels in

Hair Cells

Figure 15.32 Pivoting of stereocilia (hairs) opens or closes mechanically gated ion

channels in hair cells.

Generating Signals

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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Vestibule

• Central egg-shaped cavity of bony labyrinth

• Contains two membranous sacs

1. Saccule is continuous with cochlear duct

2. Utricle is continuous with semicircular canals

• These sacs

– House equilibrium receptor regions (maculae)

– Respond to gravity and changes in position of head

Figure 15.33 Structure of a macula.Macula ofutricle

Macula ofsaccule

StereociliaKinocilium

OtolithsOtolithmembrane

Hair bundle

Hair cells

Supportingcells

Vestibular

nerve fibers

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

Structure and Function of a Macula

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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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

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

Crista ampullaris

Membranous labyrinth

Cristaampullaris

Fibers of vestibular nerve

Hair bundle (kinociliumplus stereocilia)

Hair cell

Supporting cell

Endolymph

Ampullary cupula

Anatomy of a crista ampullaris in a semicircular canal Scanning electron micrograph

of a crista ampullaris (200x)

Section ofampulla,filled withendolymph

Cupula Fibers ofvestibular

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

Optional Slides

• The following slides may be used in

lecture or

• May be helpful in your preparation for

testing, for your reference

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Figure 15.6a Microscopic anatomy of the retina.

Neural layer of retina

Pathway oflight

Optic disc

Central arteryand vein of retina

Pigmentedlayer ofretinaChoroid

Sclera

Opticnerve

Posterior aspect of the eyeball

Functional Anatomy Of Photoreceptors

• Rods and cones

– Modified neurons

– Receptive regions called outer segments

• Contain visual pigments (photopigments)

– Molecules change shape as absorb light

– Inner segment of each joins cell body

Rods

• Functional characteristics

– Very sensitive to light

– Best suited for night vision and peripheral

vision (most common in peripheral retina)

– Non-color vision; Contain single pigment

• Perceived input in gray tones only

– Pathways converge, causing fuzzy, indistinct

images

• Low acuity

– 20 rods for every cone in retina

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Cones

• Functional characteristics

– Need bright light for activation (have low

sensitivity)

– React more quickly

– Have one of three pigments for colored view

– Nonconverging pathways result in detailed, high-

resolution vision

– Most common in central retina

– Color blindness–lack of one or more cone

pigments

Phototransduction: Capturing Light

• Deep purple pigment of rods–rhodopsin

– 11-cis-retinal + opsin rhodopsin

• Pigment synthesis

– Rhodopsin forms and accumulates in dark

• Pigment bleaching

– When rhodopsin absorbs light, retinal changes to all-trans isomer

– Retinal and opsin separate (rhodopsin breakdown)

• Pigment regeneration

– All-trans retinal converted to 11-cis isomer

– Rhodopsin regenerated in outer segments

Information Processing In The Retina

• Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs)

• When light hyperpolarizes photoreceptor cells

– Stop releasing inhibitory neurotransmitter glutamate

– Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells

– Ganglion cells generate APs transmitted in optic nerve to brain

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Figure 15.6b Microscopic anatomy of the retina.

Photoreceptors• Rod• Cone

Ganglioncells Bipolar

cellsAxonsofganglioncells

Amacrine cellHorizontal cell

Pathway of signal output

Pathway of light

Cells of the neural layer of the retina

Pigmentedlayer of retina

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

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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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

Taste Transduction

• Gustatory epithelial 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