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Anatomy & Physiology I
Lecture 14
Chapter 15: The Special Senses: A
Brief Overview
The Senses
• Special sensory receptors
– Distinct, localized receptor cells in head
•
•
•
•
•
Vision
Taste
Smell
Hearing
Equilibrium
Figure 15.1a The eye and accessory structures.
Eyebrow
Eyelid
Eyelashes
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Iris
Eyelid
Pupil
Sclera
(covered by
conjunctiva)
Surface anatomy of the right eye
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Figure 15.1b The eye and accessory structures.
Levator palpebrae
superioris muscle
Orbicularis
oculi muscle
Eyebrow
Tarsal plate
Palpebral
conjunctiva
Tarsal glands
Cornea
Palpebral
fissure
Eyelashes
Bulbar conjunctiva
Conjunctival sac
Orbicularis
oculi muscle
Lateral view; some structures shown in sagittal section
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Conjunctiva
• Transparent mucous membrane
– Produces a lubricating mucous secretion
– Palpebral conjunctiva lines eyelids
– Bulbar conjunctiva covers white of eyes
Structure of the Eyeball
• Wall of eyeball contains three layers
– Fibrous
– Vascular
– Inner
• Internal cavity filled with fluids called humors
Figure 15.4a Internal structure of the eye (sagittal section).
Ora serrata
Ciliary body
Sclera
Ciliary zonule
(suspensory
ligament)
Choroid
Cornea
Iris
Pupil
Anterior
pole
Anterior
segment
(contains
aqueous humor)
Lens
Scleral venous sinus
Posterior segment
(contains vitreous humor)
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Central artery and
vein of the retina
Optic disc
(blind spot)
Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
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Fibrous Layer
• Outermost layer; dense avascular connective
tissue
• Two regions:
– Sclera
– Cornea
Fibrous Layer
• Sclera
– Opaque posterior region
– Protects, shapes eyeball; anchors extrinsic eye
muscles
• Cornea
– Bends light as it enters eye
– Sodium pumps of corneal endothelium on inner face
help maintain clarity of cornea
– Numerous pain receptors contribute to blinking and
tearing reflexes
Vascular Layer
• Middle pigmented layer
• Three regions:
– Choroid
– Ciliary body
– Iris
Vascular Layer
• Choroid region
– Supplies blood to all layers of eyeball
– Brown pigment absorbs light to prevent light
scattering and visual confusion
Vascular Layer
• Ciliary body
– Ring of tissue surrounding lens
– Smooth muscle bundles (ciliary muscles) control
lens shape
Vascular Layer
• Iris
– Colored part of eye
– Pupil—central opening that regulates amount of
light entering eye
Figure 15.5 Pupil constriction and dilation, anterior view.
Sympathetic +
Parasympathetic +
Sphincter pupillae
muscle contracts:
Pupil size decreases.
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Iris (two muscles)
• Sphincter pupillae
• Dilator pupillae
Dilator pupillae
muscle contracts:
Pupil size increases.
Inner Layer (Retina)
• Delicate two-layered membrane:
• Outer Pigmented layer
– Absorbs light and prevents its scattering
• Inner Neural layer
– Transparent
– Composed of three main types of neurons:
• Photoreceptors, bipolar cells, ganglion cells
Figure 15.6a Microscopic anatomy of the retina.
Neural layer of retina
Pigmented
layer of
retina
Choroid
Pathway of
light
Sclera
Optic disc
Central artery
and vein of retina
Optic
nerve
Posterior aspect of the eyeball
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Figure 15.6b Microscopic anatomy of the retina.
Ganglion
cells
Axons
of
ganglion
cells
Bipolar
cells
Photoreceptors
• Rod
• Cone
Amacrine cell
Horizontal cell
Pathway of signal output
Pathway of light
Pigmented
layer of retina
Cells of the neural layer of the retina
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Photoreceptors
• Rods
– Dim light, peripheral vision receptors
– More numerous, more sensitive to light than
cones
– No color vision or sharp images
• Cones
– Vision receptors for bright light
– High-resolution color vision
Light And Optics
• Eyes respond to visible light
– Small portion of electromagnetic spectrum
– Wavelengths of 400-700 nm
• Light
– Packets of energy (photons or quanta) that travel
in wavelike fashion at high speeds
– Color of light objects reflect determines color eye
perceives
Figure 15.10 The electromagnetic spectrum and photoreceptor sensitivities.
10–5
nm
10–3
Gamma
rays
103
nm 1 nm
X rays
UV
nm
106
Infrared
(109 nm =)
nm
1m
103 m
Micro- Radio waves
waves
Light absorption (percent of maximum)
Visible light
Blue
cones
(420 nm)
Green Red
Rods
cones cones
(500 nm) (530 nm) (560 nm)
100
50
0
400
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450
500
550
600
Wavelength (nm)
650
700
Refraction
• Bending of light rays
– Due to change in speed when light passes from
one transparent medium to another
– Occurs when light meets surface of different
medium at an oblique angle
• Curved lens can refract light
Figure 15.12 Light is focused by a convex lens.
Point sources
Focal points
Focusing of two points of light.
The image is inverted—upside down and reversed.
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Vision and Distance
• Eyes best adapted for distant vision
• Far point of vision
– Distance beyond which no change in lens shape
needed for focusing
– Cornea and lens focus light precisely on retina
– Ciliary muscles relaxed
Figure 15.13a Focusing for distant and close vision.
Sympathetic activation
Nearly parallel rays
from distant object
Lens
Ciliary zonule
Ciliary muscle
Inverted
image
Lens flattens for distant vision. Sympathetic input
relaxes the ciliary muscle, tightening the ciliary zonule,
and flattening the lens.
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Focusing and Short Distance
• Light from close objects (<6 m) diverges as
approaches eye
– Requires eye to make active adjustments using
three simultaneous processes
– Accommodation of lenses
– Constriction of pupils
– Convergence of eyeballs
Figure 15.13b Focusing for distant and close vision.
Parasympathetic activation
Divergent rays
Inverted
from close object
image
Lens bulges for close vision. Parasympathetic input
contracts the ciliary muscle, loosening the ciliary zonule,
allowing the lens to bulge.
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Functional Anatomy Of Photoreceptors
• Rods and cones
– Modified neurons
– Contain visual pigments (photopigments)
– Molecules change shape as absorb light
– Process light as action potentials to reach the
optic nerve
Rods
• Functional characteristics
– Very sensitive to light
– Best suited for night vision and peripheral vision
– Contain single pigment
– Perceived input in gray tones only
Cones
• Functional characteristics
– Need bright light for activation (have low
sensitivity)
– React more quickly
– Have one of three pigments for colored view
– Detailed, high-resolution vision
• Color blindness–lack of one or more cone
pigments
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)
– All-trans-retinal (straight form)
Rhodopsin Pigment
• Deep purple pigment of rods
• 11-cis-retinal + opsin = rhodopsin
Figure 15.15a Photoreceptors of the retina.
Process of
bipolar cell
Synaptic terminals
Rod cell body
Inner fibers
Rod cell body
Nuclei
Cone cell body
Mitochondria
The outer segments
of rods and cones
are embedded in the
pigmented layer of
the retina.
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Pigmented layer
Inner
Outer segment segment
Outer fiber
Melanin
granules
Connecting cilia
Apical microvillus
Discs containing
visual pigments
Discs being
phagocytized
Pigment cell nucleus
Basal lamina (border
with choroid)
Figure 15.15b Photoreceptors of the retina.
Rod discs
Visual
pigment
consists of
• Retinal
• Opsin
Rhodopsin, the visual pigment in rods,
is embedded in the membrane that forms
© 2013 Pearson Education, Inc. discs in the outer segment.
Phototransduction
• Light converts 11-cis retinal to All-trans-retinal
– Activates a G protein signal transduction pathway
– Activates Phosphodiesterase to convert cGMP to
GMP
– Loss of GMP closes Na/Ca channels from ions
entering cell resulting in hyperpolarization
• Similar process of both rods and cones
Figure 15.16 The formation and breakdown of rhodopsin.
11-cis-retinal
2H+
1 Pigment synthesis:
Oxidation
11-cis-retinal, derived
from vitamin A, is
Vitamin A
11-cis-retinal Rhodopsin
combined with opsin
to form rhodopsin.
Reduction
2H+
3 Pigment regeneration:
Enzymes slowly convert
all-trans-retinal to its 11cis form in cells of the
pigmented layer; requires
ATP.
Dark
Light
2 Pigment bleaching:
Light absorption by
rhodopsin triggers a
rapid series of steps in
which retinal changes
shape (11-cis to alltrans) and eventually
releases from opsin.
Opsin and
All-transretinal
O
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All-trans-retinal
Figure 15.17 Events of phototransduction.
Slide 1
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
2nd
Light Receptor G protein Enzyme
messenger
(1st
messenger)
1 Retinal absorbs light
and changes shape.
Visual pigment activates.
Phosphodiesterase (PDE)
Visual
pigment
All-trans-retinal
Light
cGMP-gated
cation
channel
open in
dark
11-cis-retinal
Transducin
(a G protein)
2 Visual pigment
activates
transducin
(G protein).
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3 Transducin
activates
phosphodiesteras
e (PDE).
4 PDE converts
cGMP into GMP,
causing cGMP
levels to fall.
cGMP-gated
cation
channel
closed in
light
5 As cGMP levels fall,
cGMP-gated cation
channels close, resulting
in hyperpolarization.
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 1
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
3 Neurotransmitter is
released continuously.
Ca2+
4 Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
5 Hyperpolarization closes
voltage-gated Ca2+ channels,
inhibiting neurotransmitter
release.
Bipolar
Cell
6 No EPSPs occur in
ganglion cell.
7 No action potentials occur
along the optic nerve.
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Ganglion
cell
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
Light
Light
Photoreceptor
cell (rod)
−70 mV
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
4 Lack of IPSPs in bipolar
cell results in depolarization.
5 Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
Bipolar
Cell
Ca2+
Ganglion
cell
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6 EPSPs occur in ganglion
cell.
7 Action potentials
propagate along the
optic nerve.
Slide 1
Adapting to Bright Light
• Move from darkness into bright light
– Both rods and cones strongly stimulated
– Pupils constrict to lessen entering light
– Large amounts of pigments broken down
instantaneously, producing glare
Adapting to Darkness
• Move from bright light into darkness
– Cones stop functioning in low-intensity light
– Rod pigments bleached; system turned off
– Rhodopsin accumulates in dark
– Pupils dilate to allow more light in
– Increased light allows for improved vision in dark
settings
Visual Processing
• Thalamus
– Process for depth perception, cone input
emphasized, contrast sharpened
• Primary visual cortex (striate cortex)
– Neurons respond to dark and bright edges, and
object orientation
– Provide form, color, motion inputs to visual
association areas
Depth Perception
• Both eyes view same image from slightly
different angles
• Three-dimensional results from cortical fusion
of slightly different images
• Requires input from both eyes
Figure 15.19 Visual pathway to the brain and visual fields, inferior view.
Both eyes
Fixation point
Right eye
Suprachiasmatic
nucleus
Pretectal
nucleus
Lateral
geniculate
nucleus of
thalamus
Superior
colliculus
Left eye
Optic nerve
Optic chiasma
Optic tract
Lateral
geniculate
nucleus
Superior
colliculus
(sectioned)
Uncrossed
(ipsilateral) fiber
Crossed
(contralateral) fiber
Optic
radiation
Occipital
lobe
(primary visual
cortex)
The visual fields of the two eyes overlap considerably.
Note that fibers from the lateral portion of each retinal field
do not cross at the optic chiasma.
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Corpus callosum
Photograph of human brain, with the right side
dissected to reveal internal structures.
The Sense of Smell
• Olfactory epithelium in roof of nasal cavity
– Contains olfactory sensory neurons
– Bundles of nonmyelinated axons of olfactory
receptor cells form olfactory nerve (CN I)
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
Smell Transduction
•
•
•
•
Odorant binds to receptor activates G protein
G protein activation cAMP synthesis
cAMP activates Na+ and Ca2+ channels
ion influx depolarizes cell
Figure 15.21 Olfactory transduction process.
Slide 1
1 Odorant binds
to its receptor.
Odorant
Adenylate cyclase
G protein (Golf)
cAMP
cAMP
Open cAMP-gated
cation channel
Receptor
GDP
2 Receptor
activates G
protein (Golf).
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3 G protein
activates
adenylate
cyclase.
4 Adenylate
cyclase converts
ATP to cAMP.
5 cAMP opens a
cation channel,
allowing Na+ and
Ca2+ influx and
causing
depolarization.
Taste Senses
• Receptor organs are taste buds
– Most of 10,000 taste buds on tongue papillae
• 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
Activating Taste Receptors
• Binding of food chemical depolarizes taste cell
membrane
– neurotransmitter release
– Initiates a generator potential that elicits an action
potential
• Different thresholds for activation
– Bitter receptors most sensitive
Tasteduction
• 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 activation
• neurotransmitter ATP release
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
Figure 15.23 The gustatory pathway.
Gustatory
cortex
(in insula)
Thalamic
nucleus
(ventral
posteromedial
Pons
nucleus)
Solitary nucleus
in medulla
oblongata
Facial
nerve (VII)
Glossopharyngeal
nerve (IX)
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Vagus nerve (X)
Influences on Taste
• Taste is 80% smell
• Thermoreceptors, mechanoreceptors,
nociceptors in mouth also influence tastes
– Temperature
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
Figure 15.24a Structure of the ear.
Middle Internal ear
External ear
(labyrinth)
ear
Auricle
(pinna)
Helix
Lobule
External
acoustic Tympanic Pharyngotympanic
meatus membrane (auditory) tube
The three regions of the ear
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External Ear
• External ear
– Funnels sound waves into auditory canal
• External acoustic meatus (auditory canal)
– Transmits sound waves to eardrum
• 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
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
Figure 15.24b Structure of the ear.
Oval window
(deep to stapes)
Entrance to mastoid
antrum in the
epitympanic recess
Malleus
(hammer)
Incus
Auditory
(anvil)
ossicles
Stapes
(stirrup)
Tympanic membrane
Semicircular
canals
Vestibule
Vestibular
nerve
Cochlear
nerve
Cochlea
Round window
Middle and internal ear
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Pharyngotympanic
(auditory) tube
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
Internal Ear
• Two primary divisions
– Semicircular canals
– Cochlea
Semicircular Canals
• Three canals (anterior, lateral, and posterior)
that each define ⅔ circle
– Lie in three planes of space (x, y and z)
– Receptors respond to angular (rotational)
movements of the head
– Work in tandem with eyes and muscles for
coordination, balance, positioning, and movement
The Cochlea
• A spiral, conical, bony chamber
• Transmits sound waves via hair cells to the
cochlear branch of CN VIII
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
Figure 15.28 Sound: Source and propagation.
Area of
high pressure
(compressed
molecules)
Air pressure
Wavelength
Area of
low pressure
(rarefaction)
Crest
Trough
Distance Amplitude
A struck tuning fork alternately compresses
and rarefies the air molecules around it, creating
alternate zones of high and low pressure.
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Sound waves radiate
outward in all
directions.
Figure 15.29 Frequency and amplitude of sound waves.
Pressure
High frequency (short wavelength) = high pitch
Low frequency (long wavelength) = low pitch
0.01
0.02
Time (s)
0.03
Frequency is perceived as pitch.
Pressure
High amplitude = loud
Low amplitude = soft
0.01
0.02
Time (s)
0.03
© 2013 Pearson Education,
Inc.
Amplitude
(size or intensity) is perceived as loudness.
Transmission of Sound
• Sound waves vibrate tympanic membrane
• Ossicles vibrate and amplify pressure within
internal ear
• Cochlear fluid set into wave motion
• Wave vibration activates action potential
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Slide 1
Auditory ossicles
Malleus Incus
Stapes
Cochlear nerve
Oval
window
Scala vestibuli
Helicotrema
4a
Scala tympani
Cochlear duct
2
3
4b
Basilar
membrane
1
Tympanic
membrane
Round
window
Route of sound waves through the ear
1 Sound waves
2 Auditory ossicles
3 Pressure waves
created by the stapes
vibrate the tympanic vibrate. Pressure is
pushing on the oval
amplified.
membrane.
window move through
fluid in the scala
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vestibuli.
4a Sounds with
frequencies below hearing
travel through the
helicotrema and do not
excite hair cells.
4b Sounds in the hearing
range go through the
cochlear duct, vibrating the
basilar membrane and
deflecting hairs on inner hair
cells.
Figure 15.32 The auditory pathway.
Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivary
nucleus (ponsmedulla junction)
Midbrain
Cochlear nuclei
Vibrations
Medulla
Vestibulocochlear
nerve
Vibrations
Spiral ganglion
of cochlear nerve
Bipolar cell
Spiral organ
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Auditory Processing
• Pitch
– impulses from specific hair cells in different positions
along membrane
• Loudness
– by increased numbers of action potentials that result
when hair cells experience larger deflections
• Localization of sound
– relative intensity and timing of sound waves reaching
both ears
End
• No lab
• Student Presentations