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PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
Ninth Edition
College
Human Anatomy & Physiology
CHAPTER
15
The Special
Senses: Part A
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Special Senses
• Special sensory receptors
– Distinct, localized receptor cells in head
•
•
•
•
•
Vision
Taste
Smell
Hearing
Equilibrium
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The Eye and Vision
• 70% of body's sensory receptors in eye
• Visual processing by ~ half cerebral cortex
• Most of eye protected by cushion of fat
and bony orbit
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Accessory Structures of the Eye
• Protect the eye and aid eye function
– Eyebrows
– Eyelids (palpebrae)
– Conjunctiva
– Lacrimal apparatus
– Extrinsic eye muscles
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Figure 15.1a The eye and accessory structures.
Eyebrow
Eyelid
Eyelashes
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
Iris
Eyelid
Pupil
Lacrimal Medial
Sclera
(covered by caruncle commissure
conjunctiva)
Surface anatomy of the right eye
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Eyebrows
• Overlie supraorbital margins
• Function
– Shade eye from sunlight
– Prevent perspiration from reaching eye
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Eyelids
•
•
•
•
Protect eye anteriorly
Separated at palpebral fissure
Meet at medial and lateral commissures
Lacrimal caruncle
– At medial commissure
– Contains oil and sweat glands
• Tarsal plates—supporting connective
tissue
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Eyelid Muscles
• Levator palpebrae superioris
– Gives upper eyelid mobility
• Blink reflexively every 3-7 seconds
– Protection
– Spread secretions to moisten eye
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Eyelids
• Eyelashes
– Nerve endings of follicles initiate reflex
blinking
• Lubricating glands associated with eyelids
– Tarsal (Meibomian) glands
• Modified sebaceous glands
• Oily secretion lubricates lid and eye
– Ciliary glands between hair follicles
• Modified sweat glands
© 2013 Pearson Education, Inc.
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
• Conjunctival sac between palpebral and
bulbar conjunctiva
– Where contact lens rests
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Lacrimal Apparatus
• Lacrimal gland and ducts that drain into nasal
cavity
• Lacrimal gland in orbit above lateral end of eye
• Lacrimal secretion (tears)
– Dilute saline solution containing mucus, antibodies,
and lysozyme
– Blinking spreads tears toward medial commissure
– Tears enter paired lacrimal canaliculi via lacrimal
puncta
– Then drain into lacrimal sac and nasolacrimal duct
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Figure 15.2 The lacrimal apparatus.
Lacrimal sac
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
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Extrinsic Eye Muscles
• Six straplike extrinsic eye muscles
– Originate from bony orbit; insert on eyeball
– Enable eye to follow moving objects; maintain shape
of eyeball; hold in orbit
• Four rectus muscles originate from common
tendinous ring; names indicate movements
– Superior, inferior, lateral, medial rectus muscles
• Two oblique muscles move eye in vertical plane
and rotate eyeball
– Superior and inferior oblique muscles
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Figure 15.3a Extrinsic eye muscles.
Superior oblique muscle
Superior oblique tendon
Superior rectus muscle
Lateral rectus muscle
Inferior
rectus
muscle
Inferior
oblique
muscle
Lateral view of the right eye
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Figure 15.3b Extrinsic eye muscles.
Trochlea
Superior oblique muscle
Superior oblique tendon
Superior rectus muscle
Axis of
rotation
of eye
Inferior
rectus muscle
Medial
rectus muscle
Lateral
rectus muscle
Common
tendinous ring
Superior view of the right eye
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Figure 15.3c Extrinsic eye muscles.
Muscle
Action
Controlling cranial nerve
Lateral rectus
Moves eye laterally
VI (abducens)
Medial rectus
Superior rectus
Inferior rectus
Moves eye medially
III (oculomotor)
Elevates eye and turns it medially
III (oculomotor)
Depresses eye and turns it medially
III (oculomotor)
Elevates eye and turns it laterally
III (oculomotor)
Depresses eye and turns it laterally
IV (trochlear)
Inferior oblique
Superior oblique
Summary of muscle actions and innervating cranial nerves
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Structure of the Eyeball
• Wall of eyeball contains three layers
– Fibrous
– Vascular
– Inner
• Internal cavity filled with fluids called
humors
• Lens separates internal cavity into
anterior and posterior segments
(cavities)
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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.
© 2013 Pearson Education, Inc.
Figure 15.4b Internal structure of the eye (sagittal section).
Ciliary body
Ciliary
processes
Vitreous humor
in posterior
segment
Iris
Margin
of pupil
Anterior
segment
Lens
Cornea
Ciliary zonule
(suspensory
ligament)
Retina
Choroid
Sclera
Fovea centralis
Optic disc
Optic nerve
Photograph of the human eye.
© 2013 Pearson Education, Inc.
Fibrous Layer
• Outermost layer; dense avascular
connective tissue
• Two regions: sclera and cornea
1. Sclera
• Opaque posterior region
• Protects, shapes eyeball; anchors extrinsic eye
muscles
• Continuous with dura mater of brain posteriorly
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Fibrous Layer
2. Cornea
• Transparent anterior 1/6 of fibrous layer
• 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
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Vascular Layer (Uvea)
• Middle pigmented layer
• Three regions: choroid, ciliary body, and
iris
1. Choroid region
• Posterior portion of uvea
• Supplies blood to all layers of eyeball
• Brown pigment absorbs light to prevent light
scattering and visual confusion
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Vascular Layer
2. Ciliary body
• Ring of tissue surrounding lens
• Smooth muscle bundles (ciliary muscles) control
lens shape
• Capillaries of ciliary processes secrete fluid
• Ciliary zonule (suspensory ligament) holds lens in
position
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Vascular Layer
3. Iris
• Colored part of eye
• Pupil—central opening that regulates amount of
light entering eye
– Close vision and bright light—sphincter pupillae
(circular muscles) contract; pupils constrict
– Distant vision and dim light—dilator pupillae
(radial muscles) contract; pupils dilate –
sympathetic fibers
– Changes in emotional state—pupils dilate when
subject matter is appealing or requires
problem-solving skills
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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
• Originates as outpocketing of brain
• Delicate two-layered membrane
– Outer Pigmented layer
•
•
•
•
Single-cell-thick lining
Absorbs light and prevents its scattering
Phagocytize photoreceptor cell fragments
Stores vitamin A
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Inner Layer: Retina
– 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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The Retina
• Optic disc (blind spot)
– Site where optic nerve leaves eye
– Lacks photoreceptors
• Quarter-billion photoreceptors of two types
– Rods
– Cones
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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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Figure 15.6c Microscopic anatomy of the retina.
Nuclei of
ganglion
cells
Outer segments
of rods and cones
Nuclei of Nuclei of
bipolar rods and
cells
cones
Photomicrograph of retina
Axons of
ganglion cells
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Choroid
Pigmented
layer of retina
Photoreceptors
• Rods
– Dim light, peripheral vision receptors
– More numerous, more sensitive to light than
cones
– No color vision or sharp images
– Numbers greatest at periphery
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Photoreceptors
• Cones
– Vision receptors for bright light
– High-resolution color vision
– Macula lutea exactly at posterior pole
• Mostly cones
• Fovea centralis
– Tiny pit in center of macula with all cones; best vision
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Blood Supply to the Retina
• Two sources of blood supply
– Choroid supplies outer third (photoreceptors)
– Central artery and vein of retina supply inner
two-thirds
• Enter/exit eye in center of optic nerve
• Vessels visible in living person
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Figure 15.7 Part of the posterior wall (fundus) of the right eye as seen with an ophthalmoscope.
Central
artery
and vein
emerging
from the
optic disc
Optic disc
Macula
lutea
Retina
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Internal Chambers and Fluids
• The lens and ciliary zonule separate eye
into two segments
– Anterior and posterior segments
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Internal Chambers and Fluids
• Posterior segment contains vitreous humor that
– Transmits light
– Supports posterior surface of lens
– Holds neural layer of retina firmly against pigmented
layer
– Contributes to intraocular pressure
– Forms in embryo; lasts lifetime
• Anterior segment composed of two chambers
– Anterior chamber—between cornea and iris
– Posterior chamber—between iris and lens
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Internal Chambers and Fluids
• Anterior segment contains aqueous humor
– Plasma like fluid continuously formed by capillaries of
ciliary processes
– Drains via scleral venous sinus (canal of Schlemm) at
sclera-cornea junction
– Supplies nutrients and oxygen mainly to lens and
cornea but also to retina, and removes wastes
• Glaucoma: blocked drainage of aqueous humor
increases pressure and causes compression of
retina and optic nerve  blindness
© 2013 Pearson Education, Inc.
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.
© 2013 Pearson Education, Inc.
Figure 15.8 Circulation of aqueous humor.
Cornea
Lens
Posterior
segment
(contains
vitreous
humor)
Iris
Lens epithelium
Lens
Cornea
2
Corneal epithelium
Corneal endothelium
Aqueous humor
1 Aqueous humor
forms by filtration
from the capillaries
in the ciliary
processes.
2 Aqueous humor
flows from the
posterior chamber
through the pupil into
the anterior chamber.
Some also flows
through the vitreous
humor (not shown).
3 Aqueous humor is
reabsorbed into the
venous blood by the
scleral venous sinus.
© 2013 Pearson Education, Inc.
Anterior
segment
(contains
aqueous
humor)
Anterior chamber
Ciliary zonule
(suspensory
ligament)
Posterior chamber
Scleral venous
sinus
Corneoscleral
junction
3
1
Ciliary
processes
Ciliary
muscle
Bulbar
conjunctiva
Sclera
Ciliary
body
Lens
• Biconvex, transparent, flexible, and avascular
• Changes shape to precisely focus light on retina
• Two regions
– Lens epithelium anteriorly; Lens fibers form bulk of
lens
– Lens fibers filled with transparent protein crystallin
– Lens becomes more dense, convex, less elastic with
age
• cataracts (clouding of lens) consequence of aging, diabetes
mellitus, heavy smoking, frequent exposure to intense
sunlight
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Cataracts
• Clouding of lens
– Consequence of aging, diabetes mellitus,
heavy smoking, frequent exposure to intense
sunlight
– Some congenital
– Crystallin proteins clump
– Vitamin C increases cataract formation
– Lens can be replaced surgically with artificial
lens
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Figure 15.9 Photograph of a cataract.
© 2013 Pearson Education, Inc.
Light And Optics: Wavelength And Color
• 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
© 2013 Pearson Education, Inc.
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)
100
50
0
400
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Green Red
Rods
cones cones
(500 nm) (530 nm) (560 nm)
450
500
550
600
Wavelength (nm)
650
700
Light And Optics: Refraction And Lenses
• 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
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Figure 15.11 Refraction.
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Refraction and Lenses
• Light passing through convex lens (as in
eye) is bent so that rays converge at focal
point
– Image formed at focal point is upside-down
and reversed right to left
• Concave lenses diverge light
– Prevent light from focusing
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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.
© 2013 Pearson Education, Inc.
Focusing Light on The Retina
• Pathway of light entering eye: cornea, aqueous
humor, lens, vitreous humor, entire neural layer
of retina, photoreceptors
• Light refracted three times along pathway
– Entering cornea
– Entering lens
– Leaving lens
• Majority of refractory power in cornea
• Change in lens curvature allows for fine focusing
© 2013 Pearson Education, Inc.
Focusing For Distant Vision
• Eyes best adapted for distant vision
• Far point of vision
– Distance beyond which no change in lens
shape needed for focusing
• 20 feet for emmetropic (normal) eye
• Cornea and lens focus light precisely on retina
• Ciliary muscles relaxed
• Lens stretched flat by tension in ciliary
zonule
© 2013 Pearson Education, Inc.
Figure 15.13a Focusing for distant and close vision.
Nearly parallel rays
from distant object
Sympathetic activation
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.
© 2013 Pearson Education, Inc.
Focusing For Close Vision
• 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
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Focusing For Close Vision
• Accommodation
– Changing lens shape to increase refraction
– Near point of vision
• Closest point on which the eye can focus
– Presbyopia—loss of accommodation over age 50
• Constriction
– Accommodation pupillary reflex constricts pupils to
prevent most divergent light rays from entering eye
• Convergence
– Medial rotation of eyeballs toward object being
viewed
© 2013 Pearson Education, Inc.
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.
© 2013 Pearson Education, Inc.
Figure 15.13c Focusing for distant and close vision.
View
Ciliary muscle
Lens
Ciliary zonule
(suspensory
ligament)
The ciliary muscle and ciliary zonule are arranged
sphincterlike around the lens. As a result, contraction
loosens the ciliary zonule fibers and relaxation tightens
them.
© 2013 Pearson Education, Inc.
Problems Of Refraction
• Myopia (nearsightedness)
– Focal point in front of retina, e.g., eyeball too long
– Corrected with a concave lens
• Hyperopia (farsightedness)
– Focal point behind retina, e.g., eyeball too short
– Corrected with a convex lens
• Astigmatism
– Unequal curvatures in different parts of cornea or lens
– Corrected with cylindrically ground lenses or laser
procedures
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Figure 15.14 Problems of refraction. (1 of 3)
Emmetropic eye (normal)
Focal
plane
Focal point is
on retina.
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Figure 15.14 Problems of refraction. (2 of 3)
Myopic eye (nearsighted)
Eyeball
too long
Uncorrected
Focal point is in
front of retina.
Corrected
© 2013 Pearson Education, Inc.
Concave lens moves focal
point further back.
Figure 15.14 Problems of refraction. (3 of 3)
Hyperopic eye (farsighted)
Eyeball
too short
Uncorrected
Focal point is
behind retina.
Corrected
© 2013 Pearson Education, Inc.
Convex lens moves focal
point forward.
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
© 2013 Pearson Education, Inc.
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)
Photoreceptor Cells
•
•
•
•
Vulnerable to damage
Degenerate if retina detached
Destroyed by intense light
Outer segment renewed every 24 hours
– Tips fragment off and are phagocytized
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Rods
• Functional characteristics
– Very sensitive to light
– Best suited for night vision and peripheral
vision
– Contain single pigment
• Perceived input in gray tones only
– Pathways converge, causing fuzzy, indistinct
images
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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
– Color blindness–lack of one or more cone
pigments
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Table 15.1 Comparison of Rods and Cones
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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
© 2013 Pearson Education, Inc.
Phototransduction: Capturing Light
• Deep purple pigment of rods–rhodopsin
– 11-cis-retinal + opsin  rhodopsin
– Three steps of rhodopsin formation and
breakdown
• Pigment synthesis
• Pigment bleaching
• Pigment regeneration
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Figure 15.15b Photoreceptors of the retina.
Rod discs
Visual
pigment
consists of
• Retinal
• Opsin
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Rhodopsin, the visual pigment in rods,
is embedded in the membrane that forms
discs in the outer segment.
Phototransduction: Capturing Light
• 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
© 2013 Pearson Education, Inc.
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
All-trans-retinal
© 2013 Pearson Education, Inc.
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).
© 2013 Pearson Education, Inc.
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.
Phototransduction In Cones
• Similar as process in rods
• Cones far less sensitive to light
– Takes higher-intensity light to activate cones
© 2013 Pearson Education, Inc.
Light Transduction Reactions
• Light-activated rhodopsin activates G
protein transducin
• Transducin activates PDE, which breaks
down cyclic GMP (cGMP)
• In dark, cGMP holds channels of outer
segment open  Na+ and Ca2+ depolarize
cell
• In light cGMP breaks down, channels
close, cell hyperpolarizes
– Hyperpolarization is signal!
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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.
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
6 EPSPs occur in ganglion
cell.
7 Action potentials
propagate along the
optic nerve.
Slide 1
Light Adaptation
• Move from darkness into bright light
– Both rods and cones strongly stimulated
• Pupils constrict
– Large amounts of pigments broken down
instantaneously, producing glare
– Visual acuity improves over 5–10 minutes as:
• Rod system turns off
• Retinal sensitivity decreases
• Cones and neurons rapidly adapt
© 2013 Pearson Education, Inc.
Dark Adaptation
• Move from bright light into darkness
– Cones stop functioning in low-intensity light
– Rod pigments bleached; system turned off
– Rhodopsin accumulates in dark
– Transducin returns to outer segments
– Retinal sensitivity increases within 20–30
minutes
– Pupils dilate
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Night Blindness
• Nyctalopia
• Rod degeneration
– Commonly caused by vitamin A deficiency
– If administered early vitamin A supplements
restore function
– Also caused by retinitis pigmentosa
• Degenerative retinal diseases that destroy rods
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Visual Pathway To The Brain
• Axons of retinal ganglion cells form optic
nerve
• Medial fibers of optic nerve decussate at
optic chiasma
• Most fibers of optic tracts continue to
lateral geniculate body of thalamus
• Fibers from thalamic neurons form optic
radiation and project to primary visual
cortex in occipital lobes
© 2013 Pearson Education, Inc.
Visual Pathway
• Fibers from thalamic neurons form optic
radiation
• Optic radiation fibers connect to primary
visual cortex in occipital lobes
• Other optic tract fibers send branches to
midbrain, ending in superior colliculi
(initiating visual reflexes)
© 2013 Pearson Education, Inc.
Visual Pathway
• A small subset of ganglion cells in retina
contain melanopsin (circadian pigment),
which projects to:
– Pretectal nuclei (involved with pupillary
reflexes)
– Suprachiasmatic nucleus of hypothalamus,
timer for daily biorhythms
© 2013 Pearson Education, Inc.
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.
© 2013 Pearson Education, Inc.
Corpus callosum
Photograph of human brain, with the right side
dissected to reveal internal structures.
Depth Perception
• Both eyes view same image from slightly
different angles
• Depth perception (three-dimensional
vision) results from cortical fusion of
slightly different images
• Requires input from both eyes
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Visual Processing
• Retinal cells split input into channels
– Color, brightness, angle, direction, speed of
movement of edges (sudden changes of
brightness or color)
• Lateral inhibition decodes "edge" information
– Job of amacrine and horizontal cells
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Visual Processing
• Lateral geniculate nuclei of 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 (prestriate cortices)
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Cortical Processing
• Occipital lobe centers (anterior prestriate
cortices) continue processing of form, color, and
movement
• Complex visual processing extends to other
regions
– "What" processing identifies objects in visual field
• Ventral temporal lobe
– "Where" processing assesses spatial location of
objects
• Parietal cortex to postcentral gyrus
– Output from both passes to frontal cortex
• Directs movements
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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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Figure 15.20a Olfactory receptors.
Olfactory
epithelium
Olfactory tract
Olfactory bulb
Nasal
conchae
Route of
inhaled air
© 2013 Pearson Education, Inc.
Figure 15.20b Olfactory receptors.
Olfactory
tract
Olfactory
gland
Olfactory
epithelium
Mucus
Mitral cell
(output cell)
Glomeruli
Olfactory bulb
Cribriform plate
of ethmoid bone
Filaments of
olfactory nerve
Lamina propria
connective tissue
Olfactory axon
Olfactory stem cell
Olfactory sensory
neuron
Supporting cell
Dendrite
Olfactory cilia
Route of inhaled air
containing odor molecules
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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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Olfactory Pathway
• Olfactory receptor cells synapse with
mitral cells in glomeruli of olfactory bulbs
• Axons from neurons with same receptor
type converge on given type of glomerulus
• Mitral cells amplify, refine, and relay
signals
• Amacrine granule cells release GABA to
inhibit mitral cells
– Only highly excitatory impulses transmitted
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The Olfactory Pathway
• Impulses from activated mitral cells travel via
olfactory tracts to piriform lobe of olfactory cortex
• Some information to frontal lobe
– Smell consciously interpreted and identified
• Some information to hypothalamus, amygdala,
and other regions of limbic system
– Emotional responses to odor elicited
© 2013 Pearson Education, Inc.
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).
© 2013 Pearson Education, Inc.
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 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
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Figure 15.22a Location and structure of taste buds on the tongue.
Epiglottis
Palatine tonsil
Lingual tonsil
Foliate
papillae
Fungiform
papillae
Taste buds are associated
with fungiform, foliate, and
vallate papillae.
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Figure 15.22b Location and structure of taste buds on the tongue.
Vallate papilla
Taste bud
© 2013 Pearson Education, Inc.
Enlarged section of a
vallate papilla.
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 epithelial 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
© 2013 Pearson Education, Inc.
Figure 15.22c Location and structure of taste buds on the tongue.
Connective
tissue
Gustatory
hair
Taste fibers
of cranial
nerve
Basal Gustatory Taste
epithelial epithelial pore
cells
cells
© 2013 Pearson Education, Inc.
Enlarged view of a taste
bud (210x).
Stratified
squamous
epithelium
of 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
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Basic Taste Sensations
• Possible sixth taste
– Growing evidence humans can taste longchain 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 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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
Vagus nerve (X)
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
•
•
© 2013 Pearson Education, Inc.
Receptors for hearing and balance respond to
separate stimuli
Are activated independently
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Middle Ear
• Epitympanic recess—superior portion of
middle ear
• Mastoid antrum
– Canal for communication with mastoid air
cells
• Pharyngotympanic (auditory) tube—
connects middle ear to nasopharynx
– Equalizes pressure in middle ear cavity with
external air pressure
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Pharyngotympanic
(auditory) tube
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Figure 15.25 The three auditory ossicles and associated skeletal muscles.
View
Superior
Malleus
Incus Epitympanic recess
Lateral
Anterior
© 2013 Pearson Education, Inc.
Pharyngotym- Tensor
tympani
panic tube
muscle
Tympanic Stapes Stapedius
membrane
muscle
(medial view)
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
© 2013 Pearson Education, Inc.
Figure 15.26 Membranous labyrinth of the internal ear.
Temporal
bone
Semicircular ducts
in semicircular
canals
Anterior
Posterior
Lateral
Facial nerve
Vestibular nerve
Cristae ampullares
in the membranous
ampullae
Superior vestibular
ganglion
Inferior vestibular
ganglion
Cochlear nerve
Maculae
Spiral organ
Utricle in
vestibule
Cochlear duct
in cochlea
Saccule in
vestibule
© 2013 Pearson Education, Inc.
Stapes in
oval window
Round window
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
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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
© 2013 Pearson Education, Inc.
Figure 15.26 Membranous labyrinth of the internal ear.
Temporal
bone
Semicircular ducts
in semicircular
canals
Anterior
Posterior
Lateral
Facial nerve
Vestibular nerve
Cristae ampullares
in the membranous
ampullae
Superior vestibular
ganglion
Inferior vestibular
ganglion
Cochlear nerve
Maculae
Spiral organ
Utricle in
vestibule
Cochlear duct
in cochlea
Saccule in
vestibule
© 2013 Pearson Education, Inc.
Stapes in
oval window
Round window
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
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
Figure 15.27b Anatomy of the cochlea.
Vestibular membrane
Tectorial membrane
Cochlear duct
(scala media;
contains
endolymph)
Stria
vascularis
Spiral organ
Basilar
membrane
© 2013 Pearson Education, Inc.
Osseous spiral lamina
Scala
vestibuli
(contains
perilymph)
Scala
tympani
(contains
perilymph)
Spiral
ganglion
Figure 15.27c Anatomy of the cochlea.
Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
© 2013 Pearson Education, Inc.
Figure 15.27d Anatomy of the cochlea.
Inner
hair
cell
Outer
hair
cell
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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.
© 2013 Pearson Education, Inc.
Sound waves radiate
outward in all
directions.
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
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
0.02
Time (s)
0.03
Amplitude (size or intensity) is perceived as loudness.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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.
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
© 2013 Pearson Education, Inc.
Figure 15.30b Pathway of sound waves and resonance of the basilar membrane.
Basilar membrane
High-frequency sounds displace the
basilar membrane near the base.
Medium-frequency sounds displace the
basilar membrane near the middle.
Low-frequency sounds displace the
basilar membrane near the apex.
Fibers of basilar membrane
Apex
(long,
floppy
fibers)
Base (short,
stiff fibers)
20,000
© 2013 Pearson Education, Inc.
2000
200
Frequency (Hz)
20
Different sound frequencies cross the basilar membrane at
different locations.
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
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Auditory Processing
• Pitch perceived by impulses from specific
hair cells in different positions along
basilar membrane
• Loudness detected by increased numbers
of action potentials that result when hair
cells experience larger deflections
• Localization of sound depends on relative
intensity and relative timing of sound
waves reaching both ears
© 2013 Pearson Education, Inc.
Equilibrium and Orientation
• Vestibular apparatus
– Equilibrium receptors in semicircular canals
and vestibule
– Vestibular receptors monitor static equilibrium
– Semicircular canal receptors monitor dynamic
equilibrium
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
Figure 15.33 Structure of a macula.
Kinocilium
Stereocilia
Macula of
utricle
Macula of
saccule
Otolith
Otoliths membrane
Hair bundle
Hair cells
© 2013 Pearson Education, Inc.
Vestibular
nerve fibers
Supporting
cells
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
© 2013 Pearson Education, Inc.
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
Nerve impulses generated
in vestibular fiber
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Depolarization
When hairs bend toward
the kinocilium, the hair cell
depolarizes, exciting the
nerve fiber, which generates
more frequent action potentials.
Hyperpolarization
When hairs bend away
from the kinocilium, the hair cell
hyperpolarizes, inhibiting the nerve
fiber, and decreasing the action
potential frequency.
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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Figure 15.35a–b Location, structure, and function of a crista ampullaris in the internal ear.
Ampullary cupula
Crista
ampullaris
Endolymph
Hair bundle
(kinocilium
plus stereocilia)
Membranous
labyrinth
Crista
ampullaris
Fibers of vestibular nerve
Hair cell
Supporting
cell
Anatomy of a crista ampullaris in a semicircular canal
Section of
ampulla,
filled with
endolymph
Cupula
Fibers of
vestibular
nerve
At rest, the cupula stands upright.
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Scanning electron micrograph
of a crista ampullaris (200x)
Flow of endolymph
During rotational acceleration,
As rotational movement slows,
endolymph moves inside the
endolymph keeps moving in the
semicircular canals in the direction
direction of rotation. Endolymph flow
opposite the rotation (it lags behind due bends the cupula in the opposite
to inertia). Endolymph flow bends the
direction from acceleration and
cupula and excites the hair cells.
inhibits the hair cells.
Movement of the ampullary cupula during rotational acceleration and deceleration
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
At rest, the cupula stands upright.
Flow of endolymph
During rotational acceleration,
As rotational movement slows,
endolymph moves inside the
endolymph keeps moving in the
semicircular canals in the direction
direction of rotation. Endolymph flow
opposite the rotation (it lags behind due
bends the cupula in the opposite
to inertia). Endolymph flow bends the
direction from acceleration and
cupula and excites the hair cells.
inhibits the hair cells.
Movement of the ampullary cupula during rotational acceleration and deceleration
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Vestibular Nystagmus
• Strange eye movements during and
immediately after rotation
– Often accompanied by vertigo
• As rotation begins eyes drift in direction
opposite to rotation, then CNS
compensation causes rapid jump toward
direction of rotation
• As rotation ends eyes continue in
direction of spin then jerk rapidly in
opposite direction
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Equilibrium Pathway to the Brain
• Equilibrium information goes to reflex centers in
brain stem
– Allows fast, reflexive responses to imbalance
• Impulses travel to vestibular nuclei in brain stem
or cerebellum, both of which receive other input
• Three modes of input for balance and
orientation:
– Vestibular receptors
– Visual receptors
– Somatic receptors
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Figure 15.36 Neural pathways of the balance and orientation system.
Input: Information about the body’s position in space comes from
three main sources and is fed into two major processing areas in the
central nervous system.
Somatic receptors
(skin, muscle
and joints)
Vestibular
receptors
Visual
receptors
Cerebellum
Vestibular
nuclei
(brain stem)
Central nervous
system processing
Oculomotor control
(cranial nerve nuclei
III, IV, VI)
Spinal motor control
(cranial nerve XI nuclei
and vestibulospinal tracts)
(eye movements)
(neck, limb, and trunk
movements)
Output: Responses by the central nervous system provide fast
reflexive control of the muscles serving the eyes, neck, limbs, and
trunk.
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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
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Homeostatic Imbalances of Hearing
• Tinnitus
– Ringing or clicking sound in ears in absence
of auditory stimuli
– Due to cochlear nerve degeneration,
inflammation of middle or internal ears, side
effects of aspirin
• Ménière's syndrome: labyrinth disorder
that affects cochlea and semicircular
canals
– Causes vertigo, nausea, and vomiting
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Developmental Aspects
• All special senses are functional at birth
• Chemical senses—few problems occur until
fourth decade, when these senses begin to
decline
– Odor and taste detection poor after 65
• Vision—optic vesicles protrude from
diencephalon during fourth week of development
– Vesicles indent to form optic cups; their stalks form
optic nerves
– Later, lens forms from ectoderm
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Developmental Aspects
• Vision not fully functional at birth
• Babies hyperopic, see only gray tones, eye
movements uncoordinated, tearless for 2 weeks
• Depth perception, color vision well developed by
age three; emmetropic eyes developed by year
six
• With age, lens loses clarity, dilator muscles less
efficient, visual acuity drastically decreased by
age 70 and lacrimal glands less active so eyes
dry, more prone to infection
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Developmental Aspects
• Ear development begins in three-week embryo
• Inner ears develop from otic placodes, which
invaginate into otic pit and otic vesicle
• Otic vesicle becomes membranous labyrinth,
and surrounding mesenchyme becomes bony
labyrinth
• Middle ear structures develop from pharyngeal
pouches
• Branchial groove develops into outer ear
structures
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Developmental Aspects
• Newborns can hear but early responses
reflexive
• Language skills tied to ability to hear well
• Congenital abnormalities common
– Missing pinnae, closed or absent external
acoustic meatuses
– Maternal rubella causes sensorineural
deafness
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Developmental Aspects
• Few ear problems until 60s when
deterioration of spiral organ noticeable
• Hair cell numbers decline with age
– Presbycusis occurs first
• Loss of high pitch perception
• Type of sensorineural deafness
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