Chapter15
The Special Senses
Introduction to Sensory Receptors:
Receptor Classification
• Receptor distribution
• Special senses receptors
• located within head
• specialized, complex sense organs
• five special senses
– gustation (taste)
– olfaction (smell)
– vision (sight)
– hearing (audition)
– equilibrium (balance and acceleration)
Introduction to Sensory Receptors:
Receptor Classification
• Stimulus origin
– Exteroceptors
• detect stimuli from external environment
• receptors in the skin, special senses, membranes lining
– Interoceptors
• detect stimuli in internal organs
• primarily stretch receptors in smooth muscle walls
• mostly unaware of these sensations
• also temperature, pressure, chemical changes, perceived pain
– Proprioceptors
• detect body and limb movements
• muscles, tendons, and joints
Receptor Classification
• Modality of stimulus
– Thermoreceptors
• respond to changes in temperature
• present in both skin and hypothalamus
– Photoreceptors
• located in the eye
• detect changes in light intensity, color, movement
– Mechanoreceptors
• respond to touch, pressure, vibration, and stretch
• most cutaneous receptors and ear
– Baroreceptors
• detect changes in stretch or distension
• involved in regulation of blood pressure
– Nocioceptors
• respond to painful stimuli
Tactile Receptors (Figure 16.3)
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Unencapsulated
tactile receptors
Tactile disc
Free nerve ending
Encapsulated
tactile receptors
Tactile corpuscle
End bulb
Bulbous corpuscle
Root hair plexus
Lamellated
corpuscle
The General Senses: Referred Pain
• Referred pain
– Sensory nerve signals from certain viscera
• not perceived as originating from organ
• perceived as originating from dermatomes of skin
– Same ascending tracts within spinal cord
• house cutaneous and visceral sensory neurons
– Sensory cortex unable to differentiate actual and false stimuli
– Stimulus localized incorrectly
Source of Referred Pain (Figure 16.4)
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Posterior root ganglion
Nociceptors in skin
(pain receptors)
Sensory
pathway
to brain
Sympathetic
trunk ganglion
Sensory axons
Spinal cord
Gray ramus
White ramus
Nociceptors
in wall of cecum
and appendix
Somatic sensory
Visceral sensory
The General Senses: Referred Pain
Clinical View: Phantom Pain
–
–
–
–
–
–
Sensation associated with removed body part
Occurs following amputation of a limb
Experience of pain from removed part
Stimulation of sensory neuron pathway on remaining portion
Cell body still alive
Pain sometimes quite severe
Olfaction and Gustation—Olfaction:
The Sense of Smell
• Olfactory organs
– Organs of smell
• Olfactory receptor cells
– Olfactory hairs
• unmyelinated extensions projecting from dendrites
• house receptor proteins for detecting specific odorant molecule
– Olfactory nerve (CN I)
– Olfactory bulbs
• terminal ends of olfactory tracts
• inferior to the frontal lobes of the brain
– Olfactory tracts
• project directly to primary olfactory cortex in temporal lobe
• projects to hypothalamus, amygdala, and other regions
Olfactory Organs (Figure 16.6)
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Olfactory
tract
Olfactory
bulb
Mitral cell
Tufted cell
Olfactory bulb
Olfactory
nerves
(receptor cells)
Nasal
conchae
Cribriform plate
Olfactory glomerulus
Olfactory nerves in
cribriform foramen
Olfactory gland
Lamina propria
Basal cell
Supporting cell
Olfactory receptor cell
Cribriform plate
of ethmoid bone
Olfactory
epithelium
Olfactory
epithelium
in nasal cavity
Mucus layer
Axon
Cell body
Dendrite
Olfactory hairs
Odor molecules
Olfactory Pathway to the Cerebrum
The sensory
neurons within
the olfactory
organ are
stimulated by
chemicals in the
air.
Axons leaving
the olfactory
epithelium
collect into 20 or
more bundles
that penetrate the
cribriform plate
of the ethmoid.
Olfactory organ
The first
synapse occurs
in the olfactory
bulb, which is
located just
superior to the
cribriform plate.
Axons leaving the
olfactory bulb travel
along the olfactory
tract to reach the
olfactory cortex, the
hypothalamus, and
portions of the limbic
system.
The distribution of olfactory
information to the limbic
system and hypothalamus
explains the profound
emotional and behavioral
responses, as well as the
memories, that can be
triggered by certain smells.
Cribriform plate
of ethmoid
Olfactory epithelium
Superior nasal concha
Figure 15.1
1
Olfaction and Gustation—Olfaction:
The Sense of Smell
• Detecting smells
– Deep breathing
• helps facilitate mixing of air in superior nasal cavity
• helps diffusion of odor molecules into mucus layer
Olfactory pathway
action potential propagated through axon of olfactory receptor cells
causes release of neurotransmitter from terminal ends of axon
propagates signals through olfactory pathways
sensory information reaches
cerebral cortex, allowing for conscious smell
hypothalamus, controlling visceral reactions
amygdala, odor recognition and emotional association
Smell adaptation
ion channels altered once receptors stimulated
interferes with subsequent receptor potentials
adaptation to odors occurring rapidly
Olfaction and Gustation—Gustation:
The Sense of Taste
• Gustation
– Sense of taste
– From molecules we eat and drink
– Gustatory cells
• taste receptors housed in specialized organs, taste buds
– Involves mechanoreceptors and thermoreceptors
• provide information about texture and temperature
• Papillae and taste buds of the tongue
– Papillae, epithelial and connective tissue elevations
– On dorsal tongue surface
Taste buds
have appearance
of an onion
contain
numerous taste
receptors,
gustatory cells
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Root of
tongue
have 7 to 10
day life span
enclosed by
supporting cells
basal cells,
constantly
replacing
gustatory cells
declining taste
after age 50
reduction in
gustatory cell
replacement
and number
of taste buds
Body of
tongue
Apex of
tongue
Epithelium
Epithelium
Filiform papilla
Taste bud
Fungiform papilla
(a) Dorsal surface of tongue
Taste bud
Epithelium
Foliate papilla
Vallate Papilla and Taste Bud (Figure 16.7b-c)
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Epithelium
Stratified squamous
epithelium of tongue surface
Gustatory
cell
Gustatory
microvillus
Taste pore
Supporting
cell
(b) Vallate papilla
Sensory
nerve
Basal cell
(c) Taste bud
Olfaction and Gustation—Gustation:
The Sense of Taste
Gustatory discrimination and physiology of taste
• Five basic taste sensations
– Sweet
• produced by organic compounds, e.g., sugar or artificial sweeteners
– Salt
• produced by metal ions, e.g., Na+ and K+
– Sour
• associated with acids, e.g., vinegar
– Bitter
• produced by alkaloids, e.g., unsweetened chocolate
– Umami
• taste related to amino acids to produce meaty flavor
Olfaction and Gustation—Gustation:
The Sense of Taste
• Gustatory discrimination and physiology of taste
– Gustatory pathway (continued)
• stimulates sensory neuron to convey information to brain
– CN VII and CN IX
• reaches medulla
• triggers increased salivation and stomach secretions
• triggers gag or vomiting in response to noxious stimuli
• relayed to thalamus
• then relayed to gustatory cortex for conscious taste
• ability to taste heavily dependent on olfactory sense
– e.g., food tasting bland during a cold
• taste from info from gustatory and olfactory receptors
External Anatomy of the Eye and Surrounding
Accessory Structures (Figure 16.9a)
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Eyebrow
Eyelashes
Superior eyelid
Lacrimal caruncle
Pupil
Medial palpebral
commissure
Palpebral fissure
Iris
Lateral palpebral
commissure
Sclera (covered
by ocular
conjunctiva)
Inferior eyelid
(a)
© The McGraw-Hill Companies, Inc./JW Ramsey, photographer
• External accessory structures of the eye
– Eyebrows
• curved rows of thick, short hairs
• prevent sweat from dripping into open eyes
– Eyelashes
• extend from margins of eyelids
• prevent large objects coming into contact with eye
– Eyelids
• formed by fibrous core (tarsal plate)
• tarsal glands sebaceous glands
– produce secretion to prevent tear overflow
– keep eyelids from adhering
• ciliary glands
– form secretory products
– contribute to gritty material around eyelids after waking
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Sagittal
plane
Levator palpebrae
superioris muscle
External
Anatomy of
the Eye and
Surrounding
Accessory
Structures
(Figure
16.9b)
Eyebrow
Orbicularis oculi muscle
Conjunctival fornix
Ocular conjunctiva
Palpebral conjunctiva
Tarsal plate
Eyelashes
Pupil
Lens
Iris
aCornea
Palpebral fissure
Eyelid
Tarsal glands
Orbital fat
Orbicularis oculi muscle
(b)
Visual Receptors: Accessory
Structures of the Eye
• Conjunctiva
–
–
–
–
Specialized stratified squamous epithelium
Continuous lining over external anterior eye
Continuous lining over internal surface of eyeli
Contains numerous goblet cells
• lubricate and moisten the eye
– Contains many blood vessels
• supply nutrients to avascular sclera
– Contains abundant nerve endings
– Does not cover surface of cornea
Visual Receptors: Accessory
Structures of the Eye
Clinical View: Conjunctivitis
–
–
–
–
Most common nontraumatic eye complaint
Inflammation and reddening of the conjunctiva
Due to viral infection, bacterial infection, allergens, chemicals, irritants
Trachoma
• chronic, contagious form of conjunctivitis
• caused by Chlamydia trachomatis
• common cause of neonatal blindness in developing countries
• newborn affected as passes through birth canal
• inflammatory process causing scarring and thickening of
conjunctiva
Visual Receptors: Accessory
Structures of the Eye
• Lacrimal apparatus
–
–
–
–
Produces, collects, and drains lacrimal fluid from eye
Reduces friction from eyelid movement
Cleanses and moistens eye surface
Helps prevent bacterial infection
• through action of antibacterial enzyme, lysozyme
– Lacrimal gland
• located in superolateral depression of orbit
• continuously produces lacrimal fluid
• washed over eyes by blinking eyelids
– Nasolacrimal duct
• receives fluid from lacrimal sac
• drains fluid into lateral side of nasal cavity
– mixes with mucus
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Lacrimal puncta
Lacrimal gland
(orbital part)
Lacrimal caruncle
1
Lacrimal canaliculi
Lacrimal gland
(palpebral part)
2
Lacrimal sac
3
Lacrimal
Apparatus
(Figure
16.10)
4
Nasolacrimal duct
5
Nasal cavity
Nostril
1 Lacrimal fluid (tears) is produced in the lacrimal gland.
2 Lacrimal fluid is dispersed across eye surface when we blink.
3 Lacrimal fluid enters the lacrimal puncta, drains into the lacrimal canaliculi,
and collects in the lacrimal sac.
4 Lacrimal fluid from the lacrimal sac drains through the nasolacrimal duct.
5 Lacrimal fluid enters the nasal cavity.
Visual Receptors: Eye Structure
• Characteristics
– Almost spherical
organ
– Mostly receding
into skull orbit
– Orbital fat
• cushions
posterior and
lateral eye
• provides
support and
protection
Visual Receptors: Eye Structure
• Fibrous tunic
– External layer of the eye wall
– Composed of posterior sclera and anterior cornea
– Sclera
• the “white” of the eye
• composed of dense irregular connective tissue
• provides eye shape
• protects eye’s internal components
• attachment site for extrinsic eye muscles
– Cornea
• convex transparent structure
• contains no blood vessels
• convex shape refracting light rays coming into the eye
Visual Receptors: Eye Structure
• Vascular tunic
– Middle layer of eye wall
– Also called uvea
– Houses
• extensive blood vessels, lymph vessels
– Three regions
choroid
ciliary body
Choroid
• most extensive posterior region
• houses vast capillaries supporting the retina
• cells filled with pigment from melanocytes
• pigment absorbing extraneous light
iris
Visual Receptors: Eye Structure
• Ciliary body:
Ciliary muscles
• suspensory ligaments extending from muscle to lens
• contraction changing tension on ligaments, altering lens shape
Ciliary processes
• contain capillaries secreting aqueous humor
• Iris
most anterior region
colored portion of the eye
composed of
two layers of pigment-forming cells
two groups smooth muscle fibers
vascular and nervous structures
• Pupil
black opening at center of iris
pupil size controlled by two smooth muscle layers
controls amount of light entering the eye
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Pupillary constriction
Pupillary dilation
Bright light
Low light
Pupil
Diameter
(Figure
16.12)
Sphincter pupillae
contracts
(parasympathetic innervation)
Dilator pupillae
contracts
(sympathetic innervation)
Visual Receptors: Eye Structure
• Retina
– Internal layer of eye wall
– Outer pigmented layer and inner neural layer
– Pigmented layer
• internal to choroid and attached to it
• provides vitamin A for photoreceptors (light-detecting cells)
• light rays passing through inner layer absorbed here
– Inner neural layer
• houses photoreceptors (rods and cones) and associated neurons
• responsible for absorbing light rays
• converts into nerve signals transmitted to brain
Visual Receptors: Eye Structure
• Retina (continued)
– Optic disc
• contains no photoreceptors
• where ganglion axons exit toward brain
• commonly termed the blind spot
– Macula lutea
• rounded, yellowish region lateral to optic disc
• contains fovea centralis, depressed pit
– highest proportion of cones and few rods
– area of sharpest vision
A photograph of the retinal surface, taken through the cornea, pupil, and lens of the right eye
Optic disc
(blind spot)
Fovea
Macula lutea
Central retinal artery and vein
emerging from center of optic disc
Figure 15.14 2
Structure and Organization of the Retina (Figure 16.13a-b)
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Choroid
Rod
Cone
Pigmented
layer
Retina
Sclera
Choroid
Optic disc
Photoreceptor
cells
Horizontal cell
Bipolar cells
Retina
Amacrine cell
Neural
layer
Ganglion cells
Axons of ganglion
cells to optic nerve
Incoming light
Nerve signal
response to light
through retina
Optic
nerve
Fovea centralis
(b)
(a)
Anatomy of the Internal Eye (Figure 16.11a)
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Fibrous tunic
Sclera
Cornea
Vascular tunic
Iris
Ciliary body
Choroid
Retina
Pigmented layer
Neural layer
(a)
Visual Receptors: Eye Structure
Clinical View: Detached Retina
–
–
–
–
–
–
Occurs when outer pigmented and inner neural layers separate
May result from head trauma
Increased risk in diabetics and nearsighted individuals
Results in nutrient deprivation in inner neural layer
Symptoms of “floaters” and “curtain” in affected eye
Symptoms of flashes of light, decreased vision
Visual Receptors: Eye Structure
Clinical View: Macular Degeneration
– Physical deterioration of macula lutea
– Leading cause of blindness in developed countries
– May be associated with
• diabetes, ocular infection, hypertension, eye trauma
– Loss of visual acuity in center of visual field
– Diminished color perception and “floaters”
Visual Receptors: Eye Structure
• Lens
– Strong deformable transparent structure
– Composed of precisely arranged layers of cells
• have lost their organelles
• filled completely by crystallin protein
– Focuses incoming light onto the retina
– Suspensory ligaments
• transmit tension enabling lens to change shape
– Ciliary muscles
• when relaxed, ciliary body moved posteriorly
• tension of suspensory ligaments increases causes lens to flattendefault position for the lens view distant objects
• opposite process initiated for close objects
• contraction of muscle, moves ciliary body anteriorly
• reduces suspensory ligaments’ tension lens more spherical
• process termed accommodation
Visual Receptors: Eye Structure
Clinical View: Cataracts
–
–
–
–
–
Small opacities within the lens
Usually as a result of aging
Difficulty focusing on close objects
Reduced visual clarity and reduced color intensity
Needs to be removed when interferes with normal activities
Normal eye
Eye with
cataract
Figure 15.18 3
Visual Receptors: Eye Structure
• Cavities of the eye
– Posterior cavity
• posterior to lens and anterior to retina
• occupied by vitreous humor
– transparent, gelatinous fluid between lens and retina
– helps maintain eye shape and supports retina
– transmits light from lens to retina
– Anterior cavity
• anterior to lens and posterior to cornea
– Aqueous humor
• fluid in anterior chamber
• filtrate of blood plasma resembling cerebrospinal fluid
• produced by ciliary processes
• provides nutrients and oxygen to lens and cornea
Clinical View: Glaucoma
– Characterized by increased intraocular pressure
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Anterior cavity
Anterior chamber
Posterior chamber
Posterior cavity
Aqueous
Humor:
Secretion and
Reabsorption
(Figure
16.16)
Iris
Cornea
Lens
Pupil
2
Anterior
chamber
Suspensory
ligaments
Posterior
chamber
3
Anterior cavity
(contains aqueous
humor)
1
Scleral venous sinus
Angle
Posterior cavity
(contains
vitreous humor)
Ciliary
processes
1
Aqueous humor is secreted by the ciliary processes into the posterior chamber.
2
Aqueous humor moves from the posterior chamber, through the pupil, to
the anterior chamber.
3
Excess aqueous humor is resorbed via the scleral venous sinus.
Anatomy of the Internal Eye (Figure 16.11b)
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Ora serrata
Central
artery of Central
retina vein of
retina
Ciliary muscle
Ciliary process
Ciliary body
Suspensory ligaments
Limbus
Scleral venous sinus
CN II (optic)
Lens
Iris
Cornea
Optic disc
Pupil
Fovea centralis
Posterior cavity
Retina
Choroid
Sclera
(b)
Anterior chamber
Posterior chamber
Anterior cavity
Visual Receptors: Eye Structure
Clinical View: Functional Visual Impairments
– Emmetropia, normal vision
• parallel rays of light focused exactly on retina
– Hyperopia
• trouble seeing up close (far-sighted)
• only convergent rays from distant points brought to focus
• eyeball too short
• focus posterior to retina
• corrective convex lens
– Astigmatism
• unequal focusing
• due to unequal curvatures in one or more refractive surfaces
Visual Receptors: Eye Structure
Clinical View: Functional Visual Impairments
(continued)
– Myopia
• trouble seeing faraway objects (near-sighted)
• only rays close to eye focus on retina
• eyeball too long
• focus anterior to retina in vitreous body
• corrective concave lens
– Presbyopia
• age-related change
• lens less able to become spherical
• reading close-up words difficult
• corrective convex lens
– Can be treated with various surgical techniques
The shape of the eye and the site at which light is focused for three conditions
Enmetropia, or normal vision
Emmetropia
Myopia, or nearsighted vision
Myopia
Diverging
lens
In the normal healthy eye, when
the ciliary muscle is relaxed and
the lens is flattened, the image of
a distant object will be focused on
the retina’s surface. This condition
is called emmetropia (emmetro-,
proper + opia, vision), or normal
vision.
If the eyeball is too deep or the resting
curvature of the lens is too great, the image
of a distant object is projected in front of the
retina. Such individuals are said to be
nearsighted because vision at close range is
clear but distant objects are blurry and out
of focus. Their condition is more formally
termed myopia (myein, to shut + ops, eye).
Myopia can be treated by
placing a diverging lens in
front of the eye. Diverging
lenses have at least one
concave surface and spread
the light rays apart as if the
object were closer to the
viewer.
Hyperopia, or farsighted vision
Hyperopia
If the eyeball is too shallow or the lens is too
flat, hyperopia results. The ciliary muscle
must contract to focus even a distant object
on the retina, and at close range the lens
cannot provide enough refraction to focus
an image on the retina. Individuals with this
problem are said to be farsighted, because
they can see distant objects most clearly.
Hyperopia can be corrected by
placing a converging lens in
front of the eye. Converging
lenses have at least one
convex surface and provide
the additional refraction
needed to bring nearby
objects into focus.
Figure 15.17
Lens Shape in Far Vision and Near Vision (Figure 16.15)
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Ciliary muscles
contract, moving
ciliary body closer
to the lens.
Ciliary muscles relaxed
Lens flattened
Suspensory
ligaments taut
(a) Lens shape for distant vision
Lens thickened,
more spherical
Suspensory
ligaments relaxed
(b) Lens shape for near vision (accommodation)
Visual Receptors: Physiology of Vision
Photoreceptors
• Rods
–
–
–
–
–
More numerous than cones
Primarily located in peripheral regions of neural layer
Especially important in dim light
Detect movement well but have poor sharpness
Cannot distinguish color
• Cones
–
–
–
–
–
Less numerous than rods
Activated by high-intensity light
Provide precise visual sharpness and color recognition
Primarily located in fovea centralis
Subdivided into three types of cones red blue green
• each best detecting different wavelengths
Absorption Wavelengths (Figure 16.19)
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Blue cones
420 nm
Rods
500 nm
Green cones
531 nm
Percent of cone response
100
Red cones
558 nm
80
60
40
20
400
500
600
Wavelength (nm)
700
Visual Receptors: Physiology of Vision
Clinical View: Color Blindness
– X-linked recessive condition more common in males
– Absence or deficit in one type of cone cell
– Red and green most commonly affected
• results in difficulty distinguishing red and green
– Dark adaptation
• return of sensitivity to low light levels after bright light
• cones initially nonfunctional in low light
• rods still bleached from bright light conditions
• may take 20 to 30 minutes to see well
Visual Receptors: Visual Pathways
• Pathway characteristics
– Optic nerve
• formed from converged ganglionic axons
• project from each eye
• converge at the optic chiasm anterior to pituitary gland
• medial axons crossing to opposite side of brain
• lateral regions remaining on same side
– Optic tracts
• extend laterally from optic chiasm
• composite of axons originating from both eyes
• The ear
Ear Structure
– Detects sound and movements of the head
– Signals transmitted via vestibulocochlear nerve (CN VIII)
– Partitioned into external, middle, and inner ear
The ear’s three anatomical regions: the external ear, the middle ear, and the inner ear
External Ear
Middle Ear
Inner Ear
The visible portion of the ear;
collects and directs sound waves
toward the middle ear
An air-filled chamber; is connected to the
nasopharynx by the auditory tube
Site of sensory organs for hearing and
equilibrium; receives amplified sound
waves from the middle ear
Elastic cartilages
Auditory ossicles
Semicircular canals
Petrous part of
temporal bone
Auricle
Facial nerve (VII)
Vestibulocochlear
nerve (VIII)
Bony labyrinth
Tympanic
cavity
To
nasopharynx
External
acoustic
meatus
Tympanic membrane
(tympanum or eardrum)
Auditory tube
(pharyngotympanic tube
or Eustachian tube)
Figure 15.4
1
Ear Structure
• External ear
– Auricle
• funnel-shaped visible part of ear
• skin-covered, elastic cartilage-supported structure
• protects entry into ear and directs sound waves in
– External acoustic meatus
• bony tube extending slightly superiorly from lateral head
– Tympanic membrane (eardrum)
• partition between external and middle ear
• vibrates when sound waves hit it
• transmits sound wave energy into middle and inner ear
• fine hairs guarding opening
• cerumen, waxlike secretion of ceruminous glands
– combines with dead sloughed cells to form earwax
– may help impede growth of microorganisms
Ear Structure
• Middle ear
– Contains air-filled tympanic cavity
• sound transmitted through here via auditory ossicles
– Bound medially by bony wall
• houses oval window and round window
• separates middle ear from inner ear
– Auditory tube (Eustachian tube)
• passage extending from middle ear into nasopharynx
• has normally closed opening at connection to nasopharynx
• allows pressure to equalize within middle ear
Hearing and Equilibrium Receptors:
Ear Structure
• Auditory ossicles
–
–
–
–
Three smallest bones of the body
Amplify and transmit sound waves into inner ear
Vibrate when sound waves strike tympanic membrane
Malleus
• attached to medial surface of tympanic membrane
• resembles a hammer in shape
– Incus
• middle ossicle resembling an anvil
– Stapes
• resembles a stirrup
• has disclike footplate fitting into oval window
• marks lateral wall of inner ear
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Temporal bone (petrous part)
Middle Ear
(Figure
16.25)
Auditory ossicles
Malleus
Incus
Stapes
Oval window
Stapedius
Tensor tympani
(cut)
Tympanic
membrane
Round window
External acoustic meatus
Auditory tube
Tympanic cavity
Hearing and Equilibrium Receptors:
Ear Structure
Clinical View: Otitis Media
– Infections of the middle ear
– Most often experienced by young children
• horizontal, short auditory tubes
– Causative agent from respiratory infection
• may spread from pharynx through auditory tube
– Fluid accumulation in middle ear
– Pressure, pain, sometimes reduced hearing
– Evaluated with otoscope
– May require myringotomy, surgical procedure
Ear Structure
• Inner ear
– Bony labyrinth
• spaces within the temporal bone
• contains membrane-lined fluid-filled tubes
• contains endolymph, similar to intracellular fluid
– Three partitions of bony labyrinth
• cochlea
– houses cochlear duct
– The organ of Corti, located in the cochlear duct, is the auditory
organ. It contains about 20,000 hearing receptor cells and many
supporting cells.
• vestibule
– contains two saclike, membranous parts
• semicircular canal
– membranous labyrinth here termed the semicircular ducts
Inner Ear (Figure 16.26)
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Membranous labyrinth
(semicircular duct)
Endolymph
Bony labyrinth
(semicircular canals)
Perilymph
Membranous labyrinth
(semicircular ducts)
Bone
Bony labyrinth
(semicircular canal)
Cochlear branch
of CN VIII
Utricle
Bony labyrinth
(cochlea)
Vestibule
Saccule
Apex of cochlea
(contains helicotremma)
Spiral
organ
Membranous labyrinth
(cochlear duct)
Endolymph
Membranous labyrinth
Bony labyrinth
Ampullae
Connection to
cochlear duct
Perilymph
Spiral
ganglion
Membranous labyrinth
(cochlear duct)
Bony labyrinth
(cochlea)
Bone
Hearing and Equilibrium Receptors:
Physiology of Hearing
Structures for hearing
• Cochlea
– Snail-shaped spiral chamber within bone of inner ear
– Houses the spiral organ, responsible for hearing
• Spiral organ
– Within cochlear duct
– Thick sensory epithelium consisting of hair cells and supporting cells
– Hair cells
• sensory receptors of inner ear for hearing
• release neurotransmitter molecules to sensory neurons
• covered on apical surface with long microvilli, stereocilia
Hearing and Equilibrium Receptors:
Physiology of Hearing
• The pathway from sound wave to nerve signal
–
–
–
–
–
–
–
–
Sound waves funneled by auricle of external ear
Enter external acoustic meatus
Make tympanic membranes vibrate
Sound waves amplified- bones in middle ear
Foot of stapes moving in oval window
Transmits sound waves into pressure waves within inner ear
Pressure waves continuing through perilymph
Causes distortion of hair cells
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External ear
Pressure
High
pressure
(loud)
Middle ear
Low
pressure
(less loud)
Amplification
in middle ear
Amplitude
Tectorial
membrane
Direction of sound waves
Sound Waves
Pathways
Through the Ear
(Figure 16.28)
Auditory ossicles
Malleus Incus
Hair cell
Stapes
Cochlear branch
of CN VIII
Basilar
membrane
Oval window
Scala vestibuli
Helicotrema
2
3
Cochlear duct
External
acoustic meatus
5
1
Vestibular membrane
4
Spiral organ
Basilar membrane
Tympanic membrane
Scala tympani
Round window
Auditory tube
2 Tympanic membrane vibration moves auditory ossicles;
sound waves are amplified.
4 Pressure waves cause the vestibular membrane to move,
resulting in pressure wave formation in theendolymphwithin
the cochlear duct and displacement of a specific region of the
basilar membrane. Hair cells in the spiral organ are distorted,
initiating a nerve signal in the cochlear branch of CN VIII.
3 The stapes at the oval window generates pressure waves
in the perilymph within the scala vestibuli.
5 Remaining pressure waves are transferred to the scala tympani
and exit the inner ear via the round window.
1 Sound waves enter ear and cause the tympanic membrane
to vibrate.
Mechanisms of Equilibrium and Head Movement
• Equilibrium
Awareness and monitoring of head position-
– Monitored by Vestibule and semicircular ducts
– Information sent to brain to help keep our balance
– Vestibule: macula and otolithic membrane
Macula composed of layer of hair cells and supporting cells
Otolithic membrane formed of gelatinous layer and otoliths- small
masses of calcium carbonate crystals-positioned influenced by head
position
• Semicircular ducts
– Semicircular canal
• Detection of rotational movement
• with head rotation, lagging of endolymph
• pushes against cupula, causing bending of stereocilia
• results in altered neurotransmitter release from hair cells
• stimulation of sensory neurons
• response primarily to changes in velocity
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Semicircular
ducts
Ampullae
Cupula
Vestibular
branch of CN VIII
Ampulla
(Figure
16.34)
Cupula
Endolymph
Kinocilium
Stereocilia
Hair cell
Crista
ampullaris
Vestibular
branch of
CN VIII
Supporting
cell
Macula Structure (Figure 16.32b-c)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Otoliths
Otolithic
membrane
Gelatin
layer
Kinocilium
Stereocilia
Hair cell
Macula
Supporting
cells
Vestibular
branch of
CN VIII
(b) Macula
Vestibular
nerve
branches
(c) Hair cell
Macula in Upright Head Position
(Figure 16.33a)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Otoliths
Gelatin layer
Otolithic membrane
Kinocilium
Head upright
Stereocilia
Otolithic
membrane
Stereocilia parallel to kinocilium
• Neurotransmitter released at
regular interval
• Steady rate of nerve signals are
transmitted along vestibular
branch of CN VIII
Hair cell
Standard nerve signal frequency
Vestibular branch
of CN VIII
Supporting cell
(a) Macula in upright head position
Macula inAltered Head Position
(Figure 16.33b)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Stereocilla bent toward
kinocilium
• Hair cells depolarize, increasing
neurotransmitter release
Head tilted
upward
• Increased nerve signal
frequency along vestibular
branch of CN VIII
Excitation
Stereocilia of hair cells bend.
Otolithic membrane moves.
Stereocilla bent away
from kinocilium
Gravitational
force
•Hair cells hyperpolarize,
inhibiting neurotransmitter
release
Head tilted
downward
•Decreased nerve signal
frequency along vestibular
branch of CN VIII
Inhibition
(b) Macula in altered head position
Function of the Crista Ampullaris
(Figure 16.35)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Head still
Head rotating
Ampulla
Section of ampulla
filled with
endolymph
Ampulla
Cupula being moved
by the inertia of
the endolymph
Bending
stereocilia
Axons of
vestibular branch
of CN VIII
Nerve signals
sent to brain
Physiology of the Ear
• Sound waves
– frequency
– amplitude
• Outer and middle ear
– reception
– increase & decrease sound
• Cochlea
– organ of hearing
• sound ---> cochlear nerve ---> temporal lobe
• Pitch transmission
– frequency specific hairs
• Loudness transmission
– amplitude
– more hairs stimulated
Homeostasis Imbalances
• Deafness
– conduction deafness - hearing aid
– sensorineural deafness
• loud noise
• damage organ of corti
• Tinnitus
– ringing in ears, clicking
• Meniere’s Syndrome
– arteriosclerosis
– cranial nerve fluid pressure & “howling”
• Motion Sickness
– sensory input
Figure 15.8
3