Download Document

15
The Special
Senses
PowerPoint® Lecture Presentations prepared by
Alexander G. Cheroske
Mesa Community College at Red Mountain
© 2011 Pearson Education, Inc.
Section 1: Olfaction and Gustation
• Learning Outcomes
• 15.1 Describe the sensory organs of smell,
trace the olfactory pathways to their
destinations in the cerebrum, and explain
how olfactory perception occurs.
• 15.2 Describe the sensory organs of gustation.
• 15.3 Describe gustatory reception, briefly
describe the physiologic processes
involved in taste, and trace the gustatory
pathway.
© 2011 Pearson Education, Inc.
Section 1: Olfaction and Gustation
• Special senses introduction
• Special sense organs provide us with information about
external environment
• Two types of receptors used
1. Dendrites of specialized neurons
•
Bind chemicals producing a depolarization of the cell or
generator potential
•
Example: olfactory (smell) receptors
2. Specialized receptors that synapse with sensory neurons
•
Stimulated receptor releases chemical transmitters that
depolarize sensory neuron (generator potential)
•
•
Small delay due to synapse
Examples: vision, hearing, taste, equilibrium
© 2011 Pearson Education, Inc.
The function of olfactory receptors
Stimulus
Action removed
potentials
Stimulus
Dendrites
Threshold
Generator potential
Stimulus
to CNS
Specialized olfactory neuron
Figure 15 Section 1
© 2011 Pearson Education, Inc.
1
The function of receptors for the senses of
taste, vision, equilibrium, and hearing
Receptor cell
Stimulus
removed
Stimulus
Threshold
Receptor
depolarization
Axon
Action
potentials
Stimulus
Stimulus
to CNS
Synaptic
delay
Receptor cell Synapse Axon of sensory neuron
Generator potential
Figure 15 Section 1
© 2011 Pearson Education, Inc.
2
Module 15.1: Olfaction
• Olfaction
• Provided by olfactory organs
• Located in nasal cavity, either side of nasal
septum
• Cover:
• Inferior surface of cribiform plate
• Superior portion of perpendicular plate
• Superior nasal conchae of ethmoid
© 2011 Pearson Education, Inc.
Module 15.1: Olfaction
• Olfactory pathway
• Sensory neurons in olfactory organ stimulated by
chemicals
• Olfactory epithelium axons collect into 20 or more
bundles penetrating cribiform plate of ethmoid bone
• Synapse with olfactory bulb
• Axons leaving bulb travel along olfactory tract to
olfactory cortex, hypothalamus, and portions of
limbic system
• Explains why smells can produce profound emotional
and behavioral responses
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
1
Module 15.1: Olfaction
• Olfactory organ composition
• Two layers
1. Olfactory epithelium
•
Olfactory receptor cells
•
Each cell produces knob (base of 20 cilia)
•
10–20 million receptors in 5 cm2 area
•
Supporting cells
•
Basal (stem) cells
•
Replace worn-out receptors
•
One of the few examples of neuronal replacement
2. Lamina propria
•
Contains olfactory glands that produce mucus
© 2011 Pearson Education, Inc.
A portion of an olfactory organ, which consists of the
olfactory epithelium and the lamina propria
Olfactory
To
(Bowman) olfactory
gland
bulb
Olfactory nerve fibers
Lamina propria
Basal cell:
divides to replace worn-out
olfactory receptor cells
Developing olfactory
receptor cell
Olfactory epithelium
Olfactory receptor cell
Supporting cell
Mucous layer
Knob
Olfactory cilia: surfaces
contain receptor proteins
Figure 15.1
© 2011 Pearson Education, Inc.
2
Module 15.1: Olfaction
• Steps of olfactory reception
1. Binding of odorant (dissolved chemical) to
receptor protein
•
Activates adenylyl cyclase (enzyme converting
ATP to cAMP)
2. cAMP opens sodium channels, depolarizing
membrane
3. With sufficient depolarization, an action
potential may be generated and relayed to
CNS
© 2011 Pearson Education, Inc.
Module 15.1: Olfaction
• Odorants
• Generally small organic molecules
• Strongest smells associated with molecules
with either high water or lipid solubilities
• As few as four odorant molecules can activate
receptor
© 2011 Pearson Education, Inc.
Step 1: The binding of an odorant
to its receptor protein leads to the
activation of adenylyl cyclase, the
enzyme that converts ATP to
cyclic-AMP (cAMP).
Step 2: The cAMP then opens
sodium channels in the
plasma membrane, which, as a
result, begins to depolarize.
RECEPTOR
CELL
Inactive
enzyme
Step 3: If sufficient
depolarization occurs, an
action potential is triggered in
the axon, and the information
is relayed to the CNS.
Sodium
ions enter
Active
enzyme
Depolarized
membrane
Odorant
molecule
MUCOUS
LAYER
Closed
sodium
channel
The process of olfactory reception on the surface membranes of the olfactory cilia
Figure 15.1
© 2011 Pearson Education, Inc.
3
Module 15.1 Review
a. Describe olfaction.
b. Which neurons associated with olfaction are
capable of regenerating?
c. Trace the olfactory pathway, beginning at the
olfactory epithelium.
© 2011 Pearson Education, Inc.
Module 15.2: Gustation
•
Gustation or taste provides information about
consumed food and liquids
•
Taste (gustatory) receptors
•
Found mainly on superior surface of tongue
within taste buds
•
Also some located in pharynx and larynx but
decrease in importance and abundance with
age
© 2011 Pearson Education, Inc.
Module 15.2: Gustation
•
Taste bud structure
•
Gustatory cells
•
Each has slender microvilli into surrounding fluids
through narrow opening (taste pore) of taste bud
•
Each only survives ~10 days
•
Approximately 40–100 receptor cells/bud
•
Basal cells
•
Stem cells that divide and mature to produce more
gustatory cells
© 2011 Pearson Education, Inc.
The structure of taste buds
Transitional
cell
Gustatory cell
Taste hairs
(microvilli)
Basal cell
Taste
Diagrammatic view pore
of a taste bud
Taste
buds
Taste bud
Taste buds
LM x 650
LM x 280
Figure 15.2
© 2011 Pearson Education, Inc.
3
–
4
Module 15.2: Gustation
•
Taste bud location
•
Recessed along epithelium lining tongue projections
(lingual papillae; papilla, nipple-shaped mound)
•
Papillae types
•
•
•
Circumvallate (circum-, around + vallate, wall) papillae
•
Large with deep folds containing ~100 taste buds
•
Located in V-shape on tongue posterior
Fungiform (fungus, mushroom) papillae
•
Shaped like small buttons with shallow depressions
•
Each contains ~5 taste buds
Filiform (filum, thread) papillae
•
© 2011 Pearson Education, Inc.
Provide friction but contain no taste buds
Circumvallate Papillae
The lingual papillae on the superior surface of the tongue
Are relatively large and are surrounded by
deep epithelial folds; each contains as many
as 100 taste buds
Water receptors
(pharynx)
Taste
buds
Umami
Circumvallate papillae
Fungiform Papillae
Sour
Bitter
Salty
Sweet
Contain about five taste buds each
Filiform Papillae
Provide friction that
helps the tongue
move objects around
in the mouth but do
not contain taste
buds
Figure 15.2
© 2011 Pearson Education, Inc.
1 –
2
Module 15.2: Gustation
•
Taste sensations
•
Four primary sensations: sweet, salty, sour, and bitter
•
•
Found in taste buds all over tongue
Two other sensations
1.
Umami
•
Meaty or savory
•
•
2.
Receptor binds amino acids
Discovered in Japan
Water receptors
•
Demonstrated in human pharynx
•
Information sent to hypothalamus to manage thirst
© 2011 Pearson Education, Inc.
Module 15.2: Gustation
•
Taste receptor sensitivity
•
More sensitive to unpleasant stimuli
•
100,000× more sensitive to bitter, 1000×
more sensitive to sour (acids) compared to
sweet and salty
•
May have survival value
•
•
Toxic compounds are often bitter
•
Acids can create chemical burns
Overall sensitivity declines with age
•
Number of taste receptors declines
•
Number of olfactory receptors declines
© 2011 Pearson Education, Inc.
Module 15.2 Review
a. Define gustation.
b. Describe filiform papillae.
c. Relate the adaptive sensitivity of taste receptors
for bitter and sour sensations, to sweet and
salty sensations.
© 2011 Pearson Education, Inc.
Module 15.3: Gustatory receptors and pathways
•
Mechanism of gustatory reception
•
Two types
1. Chemically gated ion channels whose
stimulation produces depolarization of the cell
and release of neurotransmitters
•
Salt and sour receptors
2. Taste receptor activates G-proteins
(gustducins) that activate 2nd messenger
system to release neurotransmitters
•
Sweet, bitter, and umami receptors
© 2011 Pearson Education, Inc.
The mechanisms involved in gustatory reception
Salt and Sour Receptors
Sweet, Bitter, and Umami Receptors
Salt receptors and sour receptors are
chemically gated ion channels whose
stimulation produces depolarization of the
cell.
Receptors responding to stimuli that
produce sweet, bitter, and umami
sensations are linked to G proteins called
gustducins (GUST-doos-inz)—protein
complexes that use second messengers to
produce their effects.
Receptor
cells
Sour,
salt
Sweet, bitter,
or umami
Membrane
receptor
Gated ion
channel
Resting plasma
membrane
Inactive
G protein
Active
G protein
Channel opens
Plasma membrane
depolarizing
Plasma membrane
depolarizing
Active
G protein
Active
2nd messenger
Depolarization of membrane stimulates
release of chemical neurotransmitters.
Inactive
2nd messenger
Activation of second messengers
stimulates release of chemical
neurotransmitters.
Figure 15.3
© 2011 Pearson Education, Inc.
1
Module 15.3: Gustatory receptors and pathways
•
Gustatory information is relayed to the
cerebral cortex along three different cranial
nerves dependent on the location of the
receptor
1. Facial nerve (VII) – anterior 2/3 of tongue to
line of circumvallate papillae
2. Glossopharyngeal nerve (IX) – circumvallate
papillae and posterior 1/3 of tongue
3. Vagus nerve (X) – surface of epiglottis
© 2011 Pearson Education, Inc.
Module 15.3: Gustatory receptors and pathways
•
Gustatory pathway
•
Receptors respond to stimulation
•
Relay information to appropriate cranial nerve
•
Sensory afferents synapse in solitary
nucleus of medulla oblongata
•
Postsynaptic neuron axons cross over at
medial lemniscus with other somatic sensory
information and relay to thalamus
•
After synapse in thalamus, impulse is routed to
appropriate area of primary sensory cortex
© 2011 Pearson Education, Inc.
The components of the gustatory pathway
After another synapse in the thalamus,
the information is projected to the
appropriate portions of the gustatory
cortex of the insula.
The axons of the postsynaptic neurons
cross over and enter the medial
lemniscus of the medulla oblongata.
Cranial Nerves Carrying
Gustatory Information
The facial nerve (VII)
innervates all the taste buds
located on the anterior
two-thirds of the tongue,
from the tip to the line of
circumvallate papillae.
The sensory afferents carried by
these three cranial nerves
synapse in the solitary
nucleus of the medulla
oblongata.
The glossopharyngeal
nerve (IX) innervates the
circumvallate papillae and
the posterior one-third of
the tongue.
The vagus nerve (X)
innervates taste buds
scattered on the surface of
the epiglottis.
Start
Receptors respond
to stimulation.
Figure 15.3
© 2011 Pearson Education, Inc.
2
Module 15.3: Gustatory receptors and pathways
•
Central processing of gustatory sensations
•
Conscious perception of taste occurs at the
primary sensory cortex
•
Taste sensation is analyzed with taste-related
sensations
•
“Peppery” or “burning hot” from afferents in trigeminal
nerve (V)
•
Olfactory stimulation significantly contributes to
taste perception
•
Central adaptation quickly reduces sensitivity to
new tastes
© 2011 Pearson Education, Inc.
Module 15.3 Review
a. What are gustducins?
b. Identify the cranial nerves that carry gustatory
information.
c. Trace the gustatory pathway from the taste
receptors to the cerebral cortex.
© 2011 Pearson Education, Inc.
Section 2: Equilibrium and Hearing
•
Learning Outcomes
• 15.4 Describe the structures of the external,
middle, and inner ear, and explain how
they function.
• 15.5 Describe the structures and functions of
the bony labyrinth and membranous
labyrinth.
• 15.6 Describe the functions of hair cells in the
semicircular ducts, utricle, and saccule.
© 2011 Pearson Education, Inc.
Section 2: Equilibrium and Hearing
•
Learning Outcomes
• 15.7 Describe the structure and functions of the
organ of Corti.
• 15.8 Explain the anatomical and physiological
basis for pitch and volume sensations for
hearing.
• 15.9 Trace the pathways for the sensations of
equilibrium and hearing to their respective
destinations in the brain.
© 2011 Pearson Education, Inc.
Section 2: Equilibrium and Hearing
•
Equilibrium and Hearing
•
Chemoreceptors compared to mechanoreceptors
•
Olfactory and gustatory receptors are located in
epithelia exposed to the external environment
• Olfactory receptors are modified neurons
• Gustatory receptors communicate with sensory neurons
•
Equilibrium and hearing receptors are isolated and
protected from external environment
• Located in inner ear
• Information is integrated and organized locally before
forwarding to CNS
© 2011 Pearson Education, Inc.
Sensory receptors that are located within epithelia exposed to the
external environment
Gustatory
receptor
Olfactory
receptor
Figure 15 Section 2
© 2011 Pearson Education, Inc.
1
Section 2: Equilibrium and Hearing
•
Hair cell receptors of the inner ear
•
Free surfaces covered with specialized processes
•
80–100 stereocilia (like long microvilli)
•
May contain single large kinocilium
•
Hair cells are mechanoreceptors that are not actively
moved
•
External forces push against processes causing distortion of
cell membrane and neurotransmitter release
•
Provide information about direction and strength of
mechanical stimuli
•
Complex inner ear structure determines what stimuli can
reach different hair cells
© 2011 Pearson Education, Inc.
Inner ear
Location of the receptors for
equilibrium and hearing
Displacement
in this direction
stimulates hair cell
Displacement
in this direction
inhibits hair cell
Stereocilia
Kinocilium
Receptors for equilibrium
and hearing, which are
isolated and protected from
the external environment
Hair cell
Dendrite of
sensory neuron
Supporting cell
A hair cell, the receptor located in the inner ear
Figure 15 Section 2
© 2011 Pearson Education, Inc.
2
-
3
Module 15.4: Ear regions and structures
•
Three anatomical regions of the ear
1. External ear – visible portion that collects and
directs sound waves toward middle ear
•
Auricle
•
External acoustic meatus (passageway in
temporal bone)
•
•
Lined with
•
Ceruminous glands (secrete waxy cerumen)
•
Hairs
Has some protection against entering foreign objects,
insects, and bacteria
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Tympanic membrane
(tympanum or eardrum)
Auditory tube
(pharyngotympanic tube
or Eustachian tube)
Figure 15.4
1
Module 15.4: Ear regions and structures
•
Three anatomical regions of the ear (continued)
2. Middle ear
•
•
Tympanic membrane (also tympanum or eardrum)
•
Border between external and middle ear
•
Thin, transparent sheet
Auditory ossicles (3)
•
Bones that connect tympanic membrane to receptor
complexes of inner ear (smallest bones and synovial joints
of body)
1.
Malleus (attached at three points to tympanum)
2.
Incus (middle ossicle)
3.
Stapes (bound to oval window of cochlea)
© 2011 Pearson Education, Inc.
Module 15.4: Ear regions and structures
•
Three anatomical regions of the ear (continued)
2.
Middle ear (continued)
•
•
Auditory tube (pharyngotympanic or Eustachian tube)
•
Connects middle ear to pharynx to equalize pressure on either
side of tympanic membrane
•
Also can lead to bacterial infection (otitis media)
Middle ear muscles
•
Tensor tympani muscle
•
•
Connects to malleus and can dampen tympanic membrane
vibrations
Stapedius muscle (smallest muscle in body)
•
© 2011 Pearson Education, Inc.
Connects to stapes and reduces its movement at oval
window
The structures of the middle ear
Auditory Ossicles
Malleus
Incus
Stapes
Temporal bone
(petrous part)
Stabilizing
ligament
Oval
window
Branch of
facial nerve
VII (cut)
Muscles of the Middle Ear
Tensor tympani muscle
External
acoustic
meatus
Stapedius muscle
Tympanic cavity
(middle ear)
Auditory tube
Round window
Tympanic membrane
© 2011 Pearson Education, Inc.
Figure 15.4
2
Module 15.4: Ear regions and structures
•
Sound impulse pathway
•
Sound waves vibrate tympanic membrane
•
Tympanic membrane and ossicles amplify and
conduct vibrations to oval window of inner ear
•
Vibrations can be dampened by actions of
middle ear muscles
Animation: The Ear: Balance
Animation: The Ear: Ear Anatomy
© 2011 Pearson Education, Inc.
Module 15.4 Review
a. Name the three tiny bones located in the
middle ear.
b. What is the function of the auditory tube?
c. Why are external ear infections relatively
uncommon?
© 2011 Pearson Education, Inc.
Module 15.5: Labyrinths of the inner ear
•
Bony labyrinth
•
Shell of dense bone
•
Surrounds and protects membranous labyrinth
•
Filled with perilymph (similar to CSF)
•
Consists of three parts
1. Semicircular canals
2. Vestibule
3. Cochlea
© 2011 Pearson Education, Inc.
Module 15.5: Labyrinths of the inner ear
•
Membranous labyrinth
•
Collection of fluid-filled tubes and chambers
•
Houses receptors for hearing and equilibrium
•
Filled with endolymph
•
Receptors only function when exposed to
unique ionic composition
© 2011 Pearson Education, Inc.
Module 15.5: Labyrinths of the inner ear
•
Membranous labyrinth (continued)
•
Consists of three parts
1. Semicircular ducts (within semicircular canals)
•
Receptors stimulated by rotation of head
2. Utricle and saccule (within vestibule)
•
Provide sensations of gravity and linear acceleration
3. Cochlear duct (within cochlea)
•
Sandwiched between pair of perilymph-filled chambers
•
Resembles snail shell
•
Receptors stimulated by sound
© 2011 Pearson Education, Inc.
The structures of the inner ear
Bony Labyrinth
Surrounds and protects the membranous labyrinth and
contains a fluid called perilymph
Semicircular canals
Vestibule
Cochlea
Receptor
areas
Membranous Labyrinth
Houses the receptors for equilibrium
and hearing and contains a fluid
called endolymph
Semicircular duct
Utricle and saccule
Cochlear duct
Bony labyrinth
Perilymph
Membranous labyrinth
Endolymph
A cross section of a semicircular canal
© 2011 Pearson Education, Inc.
KEY
Membranous
labyrinth
Bony labyrinth
Figure 15.5
1
–
2
Module 15.5 Review
a. Identify the components of the bony labyrinth.
b. Describe hair cells.
c. Explain the regional differences among the
receptor complexes in the membranous
labyrinth.
© 2011 Pearson Education, Inc.
Module 15.6: Receptors for equilibrium
•
Receptors for equilibrium
•
Semicircular ducts
•
Structure
•
Three ducts continuous with utricle and filled with
endolymph
1.Anterior
2.Posterior
3.Lateral
•
Each contains an enlarged region (ampulla) with area
(crista) housing receptors
• Hair cells with kinocilia and stereocilia embedded within
gelatinous matrix
© 2011 Pearson Education, Inc.
The location and structure of an ampulla, which
contains receptors that respond to rotation
The location of the ampullae within the inner ear
Semicircular Ducts
Ampulla
Anterior
Posterior
Lateral
Utricle
Cupula
Ampulla filled
with endolymph
Hair cells
Crista
Supporting cells
Sensory nerve
The structure of an ampulla
Figure 15.6
© 2011 Pearson Education, Inc.
1
Module 15.6: Receptors for equilibrium
•
Semicircular ducts
•
Function
•
Head rotating in plane of one duct causes
endolymph movement and cupula bends, causing
distortion of hair cells
•
Movement one way causes stimulation
•
Opposite movement causes inhibition
•
Horizontal rotation (“no”) stimulates lateral duct
receptors
•
Nodding (“yes”) stimulates anterior duct receptors
•
Tilting head stimulates posterior duct receptors
© 2011 Pearson Education, Inc.
Direction of
duct rotation
Direction of relative
endolymph movement
Direction of
duct rotation
Semicircular duct
Ampulla
At rest
The response when the head rotates in the plane of a semicircular duct
Figure 15.6
© 2011 Pearson Education, Inc.
2
Anterior semicircular
duct for “yes”
Posterior semicircular
duct for tilting head to the side
The analysis of complex movements, which involves the response of each
semicircular duct to rotational movements
Figure 15.6
© 2011 Pearson Education, Inc.
3
Module 15.6: Receptors for equilibrium
•
Utricle and saccule
•
Provide equilibrium sensations, whether body
is stationary or moving
•
Connected by slender passageway continuous
with endolymphatic duct ending in
endolymphatic sac
•
After being secreted in cochlear duct,
endolymph returns to general circulation in
endolymphatic sac
© 2011 Pearson Education, Inc.
Module 15.6: Receptors for equilibrium
•
Utricle and saccule (continued)
•
Contain hair cells clustered in maculae
•
Processes embedded in gelatinous mass with
calcium carbonate crystals (statoconia; statos,
standing + conia, dust)
•
Whole complex called otolith (“earstone”)
•
With head in upright position, statoconia sit on
hair cells compressing, but not bending, them
•
With tilted position or with linear movement,
otolith movement bends hair cell processes
stimulating macular receptors
© 2011 Pearson Education, Inc.
The anatomy of equilibrium receptors in the utricle and saccule
Utricle
The locations of the utricle and
saccule in the inner ear
Endolymphatic sac
Endolymphatic
duct
Saccule
Gelatinous material
Otolith
An otolith, which consists of a
gelatinous matrix and statoconia
Statoconia
Maculae
Nerve fibers
Figure 15.6
© 2011 Pearson Education, Inc.
4
–
5
The effects of gravity and linear acceleration on the hair cell processes in the maculae
When your head is in the normal, upright
position, the statoconia sit atop the macula.
Their weight presses on the macular
surface, pushing the hair cell processes
down rather than to one side or another.
When your head is tilted, the pull of gravity on the statoconia shifts
them to the side, thereby distorting the hair cell processes and stimulating the macular receptors. A similar mechanism accounts for your
perception of linear acceleration, as when your car speeds up suddenly. The statoconia lag behind, and the effects on the hair cells is
comparable to tilting your head back.
Gravity
Gravity
Receptor
output
increases
Otolith
moves
“downhill,”
distorting hair
cell processes
Figure 15.6
© 2011 Pearson Education, Inc.
6
Module 15.6 Review
a. Define statoconia.
b. Cite the function of receptors in the saccule
and utricle.
c. Damage to the cupula of the lateral
semicircular duct would interfere with what
perception?
© 2011 Pearson Education, Inc.
Module 15.7: Receptors for hearing
• Receptors for hearing
• Cochlear duct
• Lies between pair of perilymphatic chambers
• Vestibular duct (separated by vestibular membrane)
• Tympanic duct (separated by basilar membrane)
• Both ducts connect at tip of cochlear spiral creating one
long chamber
• Begins at oval window
• Ends at round window
• Hair cells for hearing located in organ of Corti on
basilar membrane
Animation: The Ear: Receptor Complexes
© 2011 Pearson Education, Inc.
The location of the cochlea in the inner ear
Round window
Stapes at
oval window
Vestibular duct
Cochlear duct
Tympanic duct
Cochlear Vestibular
branch
branch
Vestibulocochlear
nerve (VIII)
Semicircular
canals
KEY
From oval window
to tip of spiral
From tip of spiral
to round window
Figure 15.7
© 2011 Pearson Education, Inc.
1
A cross section of the cochlea
From oval
window
Vestibular membrane
Basilar membrane
Vestibular duct
Organ of Corti
Cochlear duct
Tympanic duct
Temporal bone
(petrous part)
Cochlear nerve
To round
window
© 2011 Pearson Education, Inc.
Vestibulocochlear nerve (VIII)
Figure 15.7
2
Sectional view of the
cochlear spiral
LM x 200
Figure 15.7
© 2011 Pearson Education, Inc.
2
Module 15.7: Receptors for hearing
• Organ of Corti
• Hair cells arranged in longitudinal rows
• Lack kinocilia
• In contact with overlying tectorial membrane
• Sound waves cause pressure waves within perilymph,
vibrating the basilar membrane and organ of Corti
• Hair cells press into tectorial membrane and are
distorted/stimulated
• Sensory neurons relay the message through the spiral
ganglion and cochlear branch of vestibulocochlear nerve
(VIII)
© 2011 Pearson Education, Inc.
A sectional view showing a single
turn of the cochlea
Bony cochlear wall
Spiral ganglion
Vestibular duct
Vestibular membrane
Cochlear duct
Basilar membrane
Tympanic duct
Cochlear branch of the
vestibulocochlear
nerve (VIII)
Organ of Corti
Figure 15.7
© 2011 Pearson Education, Inc.
3
The structure of the organ of Corti
Tectorial membrane
Outer
hair cell
Basilar membrane
Inner hair cell
Nerve fibers
Figure 15.7
© 2011 Pearson Education, Inc.
4
Shear
At rest
Pressure wave
in perilymph
The distortion of hair cells in response to pressure changes within the
perilymph triggered by sound waves arriving at the tympanic membrane
Figure 15.7
© 2011 Pearson Education, Inc.
4
Module 15.7 Review
a. Where is the organ of Corti located?
b. Name the fluids found within the vestibular
duct, tympanic duct, and cochlear duct.
c. Identify the features visible in the LM
sectional view of the cochlear spiral.
© 2011 Pearson Education, Inc.
Module 15.8: Sensations of pitch and volume
•
Physical characteristics of sound
•
Consists of waves of pressure conducted
through a medium
•
In air, a pressure wave consists of alternating
regions of compressed and separated
molecules
•
Wavelength of sound
•
Distance between two adjacent wave crests
(peaks) or between two adjacent wave troughs
© 2011 Pearson Education, Inc.
Sounds, which consist of pressure waves
conducted through a medium such as air
Wavelength of
a sound
Air molecules
Tuning fork
Tympanic
membrane
Figure 15.8
© 2011 Pearson Education, Inc.
1
Module 15.8: Sensations of pitch and volume
•
Pitch
•
Sensory response to wave frequency
•
Frequency = number of waves passing a fixed point
in a given time
•
•
Pitch and wave frequency are directly related
•
•
Often measured in waves or cycles/sec called hertz (Hz)
Example: high-pitched sound might be 15,000 Hz
while low-pitched sound might be 100 Hz or less
All sound travels at same speed so as frequency
increases, wavelength must become shorter
© 2011 Pearson Education, Inc.
A graph showing the relationships among the
characteristics of sound waves.
Sound energy arriving at
tympanic membrane
Wavelength, which is
inversely related to
frequency
Amplitude of a sound
1 wavelength
Time (sec)
Figure 15.8
© 2011 Pearson Education, Inc.
2
Module 15.8: Sensations of pitch and volume
•
Volume or intensity (energy in sound waves)
•
Energy variation in sound is represented by
changes in wave height or amplitude
•
Wave amplitude = wave energy = perceived
loudness
•
•
Directly related
Sound energy reported in decibels (dB)
© 2011 Pearson Education, Inc.
Figure 15.8
© 2011 Pearson Education, Inc.
3
Module 15.8: Sensations of pitch and volume
•
Sound wave energy causes movements of
flexible structures in the ear
•
At a particular frequency and amplitude,
object will vibrate at same frequency
•
= Resonance
•
Examples: tympanic membrane, basilar
membrane
© 2011 Pearson Education, Inc.
Module 15.8: Sensations of pitch and volume
•
Basilar membrane flexibility changes along
length
•
Different sound frequencies vibrate different
areas of basilar membrane
•
Location = pitch
•
Number of hair cells stimulated: volume
•
As stapes pushes on oval window, basilar
membrane distorts toward round window
•
Opposite action when stapes retracts
© 2011 Pearson Education, Inc.
The movement of the flexible basilar membrane in response to
sound waves with a frequency of 6000 Hz
Stapes
at oval
window
Round
window
Stapes
moves
inward
Cochlea
16,000 Hz
6000 Hz
1000 Hz
Basilar membrane
Basilar membrane distorts
toward round window
Round
window
pushed
outward
Stapes
moves
outward
Round
window
pulled
inward
Basilar membrane distorts
toward oval window
Figure 15.8
© 2011 Pearson Education, Inc.
4
Events Involved in Hearing
Sound
waves arrive
at the
tympanic
membrane.
Movement of
the tympanic
membrane
causes
displacement of
the auditory
ossicles.
Movement of the
stapes at the oval
window
establishes
pressure waves
in the perilymph
of the vestibular
duct.
The pressure
waves distort the
basilar
membrane on
their way to the
round window of
the tympanic
duct.
Vibration of the
basilar
membrane
causes vibration
of hair cells
against the
tectorial
membrane.
Information about
the region and the
intensity of
stimulation is
relayed to the CNS
over the cochlear
branch of cranial
nerve VIII.
Tympanic duct
Basilar membrane
Cochlear duct
Vestibular membrane
Movement
of sound
waves
Vestibular duct
Tympanic
membrane
Round
window
Figure 15.8
© 2011 Pearson Education, Inc.
5
Module 15.8 Review
a. Define decibel.
b. Beginning at the external acoustic meatus, list,
in order, the structures involved in hearing.
c. How would sound perception be affected if the
round window could not bulge out as a result
of increased perilymph pressure?
© 2011 Pearson Education, Inc.
Module 15.9: Sensory pathways for equilibrium
and hearing
• Sensory pathway for equilibrium
1. Hair cells in vestibule and semicircular canals
become stimulated
2. Sensory neurons take information through
vestibular branch of vestibulocochlear nerve
(VIII)
3. Vestibular nuclei integrate information from
both ears and relay to cerebral cortex,
cerebellum, and motor nuclei of brain stem
and spinal cord
© 2011 Pearson Education, Inc.
Module 15.9: Sensory pathways for equilibrium
and hearing
• Motor responses for equilibrium
1. Automatic movements of eyes
•
Directed by superior colliculi
•
Keep gaze focused on point despite movement
2. Distribution of motor commands to motor nuclei
for cranial nerves controlling eye, head, neck
movements (CN III, IV, VI, XI)
3. Vestibular nuclei relay information to cerebellum
4. Information relayed down vestibulospinal tracts
of spinal cord to adjust peripheral muscle tone
and coordinate with head and neck movements
© 2011 Pearson Education, Inc.
The path of equilibrium information from receptors to the brain stem to muscular effectors
Hair cells of
the vestibule
and
semicircular
ducts monitor
body position
and motion.
Sensory neurons located in
adjacent vestibular ganglia
carry information from the
hair cells. These sensory
fibers form the vestibular
branch of the
vestibulocochlear nerve (VIII).
The vestibular nuclei in the
medulla oblongata integrate
sensory information from both
ears, and relay that information
to the cerebral cortex,
cerebellum, and motor nuclei in
the brain stem and spinal cord.
The automatic movements of
the eyes that occur in
response to sensations of
motion are directed by the
superior colliculi. These
movements attempt to keep
your gaze focused on a
specific point in space,
despite changes in body
position and orientation.
The reflexive motor
commands issued by the
vestibular nuclei are
distributed to the motor
nuclei for the cranial verves
involved with eye, head, and
neck movements (N III, N IV,
N VI, and N XI).
Vestibular
ganglion
The vestibular nuclei relay
information about position
and balance to the
cerebellum.
Semicircular
canals
Vestibule
Cochlear
branch
Vestibulocochlear
nerve (VIII)
Instructions descending in
the vestibulospinal tracts of
the spinal cord adjust
peripheral muscle tone and
complement the reflexive
movements of the head or
neck.
Figure 15.9
© 2011 Pearson Education, Inc.
1
Figure 15.9
© 2011 Pearson Education, Inc.
2
Module 15.9: Sensory pathways for equilibrium
and hearing
• Sensory pathway for hearing
1. Hair cells in specific area of basilar membrane become
stimulated
2. Sensory neurons relay information through cell bodies in
spiral ganglion to cochlear branch of vestibulocochlear nerve
(VIII)
3. Information reaches cochlear nuclei of medulla oblongata and
ascends to midbrain
4. From inferior colliculi of midbrain, auditory sensations
synapse at medial geniculate nucleus of thalamus
5. Projection fibers relay information to different areas of
auditory cortex in temporal lobe
© 2011 Pearson Education, Inc.
Module 15.9: Sensory pathways for equilibrium
and hearing
• Sensory pathway for hearing
•
Most auditory information from one cochlea is
projected to the auditory cortex on the
opposite side
•
Some information from cochlea on the same
side also is received by auditory cortex
•
These interconnections aid in localizing
sounds and reduce functional impact of
damage to a cochlea or ascending pathway
© 2011 Pearson Education, Inc.
Module 15.9: Sensory pathways for equilibrium
and hearing
• Motor responses for hearing
•
Inferior colliculi coordinate a number of
reflexive responses involving skeletal muscles
of head, neck, and trunk
•
Reflexes automatically change position of
head in response to a noise (usually toward
source)
© 2011 Pearson Education, Inc.
Module 15.9 Review
a. Where are the hair cells for equilibrium
located?
b. Which cranial nerves are involved with eye,
head, and neck movements?
c. What is your reflexive response to hearing a
loud noise, such as a firecracker?
© 2011 Pearson Education, Inc.
Section 3: Vision
•
Learning Outcomes
• 15.10 Identify the accessory structures of the
eye and explain their functions.
• 15.11 Describe the layers of the wall of the eye
and the anterior and posterior cavities of
the eye.
• 15.12 Explain how light is directed to the fovea
of the retina.
• 15.13 Describe the process by which images
are focused on the retina.
© 2011 Pearson Education, Inc.
Section 3: Vision
•
Learning Outcomes
• 15.14 Describe the structure and function of
the retina’s layers of cells, and the
distribution of rods and cones and their
relation to visual acuity.
• 15.15 Explain photoreception, describe the
structure of the photoreceptors, explain how
visual pigments are activated, and describe
how we are able to distinguish colors.
• 15.16 Explain how the visual pathways distribute
information to their destinations in the brain.
© 2011 Pearson Education, Inc.
Section 3: Vision
•
Learning Outcomes
• 15.17 CLINICAL MODULE Describe various
accommodation problems associated
with the cornea, lens, or shape of the eye.
• 15.18 CLINICAL MODULE Describe age-related
disorders of olfaction, gustation, vision,
equilibrium, and hearing.
© 2011 Pearson Education, Inc.
Section 3: Vision
•
Eye development
1.
Optic vesicles form in prosencephalon lateral walls
•
2.
Contain cavity continuous with neurocoel
Optic cups form as lateral bulges become indented
•
Remain connected to diencephalon by slender stalks
•
Overlying epidermis pinches off and becomes lens
•
Retina develops
•
Ependymal cells on optic cup outer wall become photo receptors
•
Ependymal cells on optic cup inner wall become pigmented cells
•
Neural tissue on optic cup outer wall becomes neurons, ganglion
cells, and specialized glial cells
© 2011 Pearson Education, Inc.
Section 3: Vision
•
Eye development (continued)
3. Supporting layers of connective tissue develop
from aggregating mesoderm around optic cup
•
Fluid-filled interior chambers develop
© 2011 Pearson Education, Inc.
The formation of optic
vesicles in the lateral
walls of the
prosencephalon
Optic vesicle
Figure 15 Section 3
© 2011 Pearson Education, Inc.
1
Layers of Developing Retina
The formation of optic cups that
remain connected
Optic cup
to the diencephalon
by slender stalks
Developing lens
Week 5
The ependymal cells on the outer wall of the
optic cup develop into the photoreceptors.
The ependymal cells on the inner wall of the
optic cup develop into pigment cells that
absorb light that has passed through the
photoreceptor layer. The separation gradually
decreases until the photoreceptors and
pigment cell layers are in contact.
The neural tissue of the outer wall of the optic
cup forms layers of neurons, ganglion cells,
and specialized glial cells that are responsible
for preliminary processing and integration of
visual information.
Figure 15 Section 3
© 2011 Pearson Education, Inc.
2
The development of interior
chambers filled with fluid
that is continuously
generated and reabsorbed
Optic nerve, N II (follows path of
original connecting stalk)
Lens
Eyelids
Fluid-filled chambers
Connective tissue layers
Week 6
Figure 15 Section 3
© 2011 Pearson Education, Inc.
3
Module 15.10: Eye accessory structures
•
Eye accessory structures
•
Eyelids (palpebrae)
• Eyelashes (hairs that help prevent foreign
particles from reaching the eye)
• Medial canthus (medial connection)
• Lateral canthus (lateral connection)
• Tarsal glands (along inner margin of lids,
secretion keeps eyelids from sticking together)
© 2011 Pearson Education, Inc.
Module 15.10: Eye accessory structures
•
Eyelids (continued)
•
Palpebral fissure (space between eyelids)
•
•
Eye structures seen within:
•
Cornea (transparent anterior surface)
•
Iris (colored part of eye)
•
Pupil (hole that light passes through in center of iris)
Lacrimal caruncle (produces thick secretion
during sleep)
© 2011 Pearson Education, Inc.
The accessory structures of the eye
Cornea
Eyelids and Eyelashes
Eyelashes
Lateral canthus
Palpebra (eyelid)
Medial canthus
Pupil within iris
Palpebral fissure
Lacrimal caruncle
Figure 15.10 1
© 2011 Pearson Education, Inc.
Module 15.10: Eye accessory structures
•
Conjunctiva
•
Epithelium lining surface of eye and inner eyelids
•
Ocular conjunctiva
•
Continuous with thin corneal conjunctiva
•
Palpebral conjunctiva
•
Fornix (pocket where conjunctivae meet)
•
Mucous membrane covered by stratified squamous epithelium
•
Conjunctivitis
•
Swelling associated with conjunctiva due to damage to/irritation
of conjunctiva
•
May be caused by infection, physical, allergic, or chemical
irritation
© 2011 Pearson Education, Inc.
The conjunctiva, the specialized epithelium
covering the inner surfaces of the eyelids
and the outer surface of the eye
Tarsal glands
(Meibomian glands)
Conjunctiva
Palpebral conjunctiva
Ocular conjunctiva
Fornix
Cornea
Figure 15.10 2
© 2011 Pearson Education, Inc.
Conjunctivitis, or pinkeye
Figure 15.10 4
© 2011 Pearson Education, Inc.
Module 15.10: Eye accessory structures
•
Lacrimal apparatus components
•
Lacrimal gland
•
•
Almond-shaped gland that produces tears
(~1 mL/day)
Tear ducts (10–12)
•
•
Deliver tears from gland to under upper
eyelid
Lacrimal puncta
•
Two small pores that drain lacrimal lake
© 2011 Pearson Education, Inc.
Module 15.10: Eye accessory structures
•
Lacrimal apparatus components (continued)
•
Lacrimal canaliculi
•
•
Small canals connecting puncta to lacrimal sac
Lacrimal sac
•
•
Chamber in lacrimal sulcus of orbit
Nasolacrimal duct
•
Delivers tears from lacrimal sac into nasal cavity
inferior and lateral to inferior nasal concha
Animation: The Eye: Accessory Structures
© 2011 Pearson Education, Inc.
The lacrimal apparatus
Superior
rectus muscle
Components of the Lacrimal Apparatus
Lacrimal gland
Tear ducts
Upper eyelid
Lacrimal puncta
Lower eyelid
Lacrimal canaliculi
Orbital fat
Lacrimal sac
Nasolacrimal duct
Inferior rectus muscle
Inferior oblique muscle
Drainage of the nasolacrimal duct
into the inferior meatus
Figure 15.10 3
© 2011 Pearson Education, Inc.
Module 15.10: Eye accessory structures
•
Function of tears
•
Reduce friction
•
Remove debris
•
Prevent bacterial infection with lysozyme and
antibodies
•
Provide nutrients and oxygen to portions of
conjunctiva
© 2011 Pearson Education, Inc.
Module 15.10 Review
a. List the accessory structures associated with
the eye.
b. Explain conjunctivitis.
c. Which layer of the eye would be the first
affected by inadequate tear production?
© 2011 Pearson Education, Inc.
Module 15.11: Eye layers and cavities
•
Three layers of the eye (tunics)
1. Fibrous tunic
•
Outermost layer of eye
•
Consists of cornea (clear) and sclera (white)
•
•
Joined at corneal limbus
Functions
1.
Provides mechanical support and some physical
protection
2.
Attachment site for extrinsic eye muscles
3.
Contains structures assisting in focusing process
© 2011 Pearson Education, Inc.
Module 15.11: Eye layers and cavities
•
Three layers of the eye (continued)
2. Vascular tunic (uvea)
•
Iris (colored part of eye that controls size of pupil)
•
Anterior: incomplete layer of fibroblasts and melanocytes
•
Posterior: pigmented epithelium of neural tunic
•
Color determined by:
1.
Genes influencing density and distribution of
melanocytes
2.
Density of pigmented epithelium
•
Blue: less melanin, light reaches pigmented layer
•
Green, brown, black: increasing melanin
© 2011 Pearson Education, Inc.
Module 15.11: Eye layers and cavities
•
Three layers of the eye (continued)
2. Vascular tunic (continued)
•
Ciliary body (thickened region connecting to lens)
•
Ciliary muscle (smooth-muscle ring)
•
•
•
Ciliary processes (epithelial folds covering muscle)
Suspensory ligaments (attach to lens)
Choroid (vascular layer covered by sclera)
•
Contains extensive capillary network delivering oxygen and
nutrients to neural tissue in neural tunic
Animation: The Eye: Uvea Parts
© 2011 Pearson Education, Inc.
Module 15.11: Eye layers and cavities
•
Three layers of the eye (continued)
3. Neural tunic (retina)
•
Innermost layer of the eye
•
Two layers
1.
2.
Pigmented layer (outer)
•
Absorbs light
•
Ora serrata (jagged anterior edge)
Neural layer (inner)
•
Contains photoreceptors sensitive to light
Animation: The Eye
© 2011 Pearson Education, Inc.
The three tunics of the wall of the eye
Fibrous Tunic
The outermost layer of the eye, consisting of the cornea
and the sclera
Sclera
Corneal limbus
Cornea
Vascular Tunic
The middle layer of the eye; also
called the uvea; contains numerous
blood vessels, lymphatic vessels,
and the intrinsic (smooth) muscles
of the eye
Lens
Optic nerve
Iris
Ciliary body
Choroid
Neural Tunic
The innermost layer of the eye; also known as the retina;
contains photoreceptors
Figure 15.11 1
© 2011 Pearson Education, Inc.
A sectional view showing that the
Cornea
ciliary body and the lens divide the
interior of the eye into a small
Iris
anterior cavity and a large
Ciliary body
posterior cavity
Anterior Cavity
Anterior chamber
Lens
Posterior chamber
Posterior Cavity
Contains the vitreous body
Optic nerve
Figure 15.11 2
© 2011 Pearson Education, Inc.
Module 15.11: Eye layers and cavities
•
Eye cavities
•
Anterior cavity (cornea to lens)
•
Filled with aqueous humor
•
Two chambers
•
1.
Anterior chamber (cornea to iris)
2.
Posterior chamber (iris to ciliary body and lens)
Posterior cavity (main volume of eye)
•
Filled with gelatinous vitreous body (humor)
© 2011 Pearson Education, Inc.
Module 15.11: Eye layers and cavities
•
Aqueous humor
•
Secreted by epithelia of ciliary processes
•
Rate of 1–2 µL/min
•
Similar to CSF
•
Circulates between anterior and posterior chambers
•
Distributes nutrients and wastes
•
Acts as fluid cushion
•
Helps to retain eye shape
•
Reabsorbed at canal of Schlemm (at corneal limbus)
•
Into veins in sclera
•
Reabsorption rate is approximately the same as production
•
Tonometry (measures intraocular pressure)
© 2011 Pearson Education, Inc.
A view of the anterior and posterior cavities of the eye
Site of secretion
of aqueous humor
Iris
Cornea
Ciliary muscle
Anterior
chamber
Canal of Schlemm
Lens
Conjunctiva
Ciliary processes
Posterior cavity
(vitreous chamber)
Ora serrata
Suspensory ligaments
Figure 15.11 3
© 2011 Pearson Education, Inc.
– 4
Module 15.11 Review
a. Name the three tunics of the eye.
b. What give eyes their characteristic color?
c. Where in the eye is aqueous humor located?
© 2011 Pearson Education, Inc.
Module 15.12: Visual axis structures
•
Visual axis
•
Imaginary line drawn from object in view through
structures of the eye to retina
•
Structures
•
Cornea
•
Dense matrix of collagen fibers organized to permit light
•
Has no blood vessels
•
Must get oxygen and nutrients from tears
Animation: The Eye: Light Path
© 2011 Pearson Education, Inc.
Module 15.12: Visual axis structures
•
Visual axis (continued)
•
Structures (continued)
•
Pupil (size controlled by two sets of muscles)
1.
2.
Pupillary dilator muscles
•
Extend radially from pupil edge
•
Contraction enlarges pupil
•
Stimulated by sympathetic nervous system
Pupillary constrictor muscles
•
Concentrically arranged around pupil
•
Contraction constricts pupil
•
Stimulated by parasympathetic nervous system
Animation: The Eye: Interior Parts of Eye
© 2011 Pearson Education, Inc.
Module 15.12: Visual axis structures
•
Visual axis (continued)
•
Structures (continued)
•
Lens
•
•
Concentric layers of cells
•
Cells filled with transparent proteins (crystallins)
•
Give clarity and focusing power
Dense fibrous capsule
•
Connects to suspensory ligaments
Animation: The Eye: Lens and Retina
© 2011 Pearson Education, Inc.
Module 15.12: Visual axis structures
•
Visual axis (continued)
•
Structures (continued)
•
Retina
•
Macula lutea (highest photoreceptor concentration)
•
© 2011 Pearson Education, Inc.
Contains fovea (shallow depression) that is the
site of sharpest vision
A sectional view showing aspects of eye anatomy associated with
positioning the eye and allowing light to reach the
photoreceptors of the retina
Cornea: transparent and lacks blood vessels
Lens: consists of concentric layers of cells
filled with transparent proteins called
crystallins
Suspensory ligaments: resist the
tendency of the lens to assume a
spherical shape
Ciliary body: supports the lens and
controls its shape.
Nose
Retina: contains the photoreceptors, pigment
cells, supporting cells, and neurons
Blood vessels of the choroid: directly or
indirectly provide nutrients to all
structures within the eye.
Sclera (“white of the eye”): stabilizes the
shape of the eye during eye movements
and is site of insertion of the six extrinsic
eye muscles
Optic nerve (N II): carries visual
information to the brain
Figure 15.12 1
© 2011 Pearson Education, Inc.
How the two layers of the pupillary muscles of the iris control the amount of light
entering the eye
Pupillary constrictor
(sphinctor)
The pupillary dilator
muscles extend radially
away from the edge of the
pupil. Contraction of these
muscles enlarges the pupil.
Pupillary dilator
(radial)
Decreased light intensity
Increased sympathetic stimulation
The pupillary constrictor
muscles form a series of
concentric circles around
the pupil. When these
sphincter muscles contract,
the diameter of the pupil
decreases.
Increased light intensity
Increased parasympathetic stimulation
Figure 15.12 2
© 2011 Pearson Education, Inc.
How light passing along the visual axis of the eye
strikes a specific location that contains the
highest density of photoreceptors
Iris
Visual axis of the eye
Nose
Lens
Choroid
Sclera
Neural portion of the retina:
site of the photoreceptors
Fovea in center of macula lutea:
site of sharpest vision
Orbital fat
Figure 15.12 1
© 2011 Pearson Education, Inc.
Module 15.12 Review
a. Which eye structure does not contain blood
vessels?
b. List the structures and fluids that light passes
through from the cornea to the retina.
c. What happens to the pupils when light
intensity decreases?
© 2011 Pearson Education, Inc.
Module 15.13: Focusing at the retina
•
Creating a focused image of an object involves
bending (refracting) light rays together to create
a focal point on the retina
•
Two refracting structures of eye
1. Cornea
2. Lens
•
Focal distance (distance between center of lens and
focal point)
•
•
Determined by distance from object to lens and shape
of lens
Accommodation (changing lens shape to keep focal
distance constant)
© 2011 Pearson Education, Inc.
Focal distance
Focal distance, which is determines by the
shape of the lens and the distance between
the lens and the object being viewed
Light
from
distant
source
(object)
Focal distance
Focal
point
Close
source
Lens
The closer the light source,
the longer the focal distance
Focal distance
The rounder the lens,
the shorter the focal distance Figure 15.13 1
© 2011 Pearson Education, Inc.
Module 15.13: Focusing at the retina
•
Accommodation
•
Lens shape changed by ciliary muscle action
•
For close vision:
•
Ciliary muscle contracts, moving toward lens 
suspensory ligaments’ tension relaxes lens 
lens shape becomes more spherical 
lens increases refractive power
•
Near point of vision (inner limit of clear vision)
•
© 2011 Pearson Education, Inc.
Increases with age as lens becomes inflexible
Module 15.13: Focusing at the retina
• Accommodation (continued)
• Lens shape changed by ciliary muscle action
(continued)
• For distance vision:
• Ciliary muscle relaxes 
suspensory ligament tension increases 
lens becomes flatter  lens decreases refractive
power
Animation: The Eye: Cilliary Muscles
© 2011 Pearson Education, Inc.
Accommodation, the process by which the eye focuses images on the retina by changing the shape of the lens to
keep the focal distance constant
Accommodation when objects viewed are far from
the eye
Accommodation when objects viewed are near the eye
For Close Vision: Ciliary Muscle Contracted,
Lens Rounded
For Distant Vision: Ciliary Muscle Relaxed, Lens
Flattened
Focal point
on fovea
When the ciliary muscle contracts, the ciliary body moves
toward the lens, thereby reducing the tension in the
suspensory ligaments. The elastic capsule of the lens then
pulls it into a more spherical shape that increases the
refractive power of the lens, enabling it to bring light from
nearby objects into focus on the retina.
When the ciliary muscle relaxes, the suspensory ligaments
pull at the circumference of the lens, making the lens flatter
and bringing the image of a distant object into focus on the
retina.
Figure 15.13 2
© 2011 Pearson Education, Inc.
Module 15.13: Focusing at the retina
•
Image formation
•
Most objects are more than just one point
•
Full images invert and reverse passing
through lens
•
Brain compensates during processing
© 2011 Pearson Education, Inc.
The inversion and reversal of images projected onto the retina
A sagittal section through an eye showing
the inversion of the image of a viewed object
A sagittal section through an eye showing
the reversal of the image of a viewed object
Figure 15.13 3
© 2011 Pearson Education, Inc.
Module 15.13 Review
a. Define focal point.
b. When the ciliary muscles are relaxed, are you
viewing something close up or something in
the distance?
c. Why does the near point of vision typically
increase with age?
© 2011 Pearson Education, Inc.
Module 15.14: Retinal layers and cells
•
Retinal layers
•
Pigmented part
•
Absorbs light passing through neural part
•
•
•
Keeps light from bouncing around retina
Has important biochemical interactions with neural part
Neural part
•
Ganglion cells (innermost layer)
•
Axons converge on optic disc
•
•
Also called blind spot since lacking photoreceptors
Axons exit eye through optic nerve
Animation: The Eye: Blind Spot
© 2011 Pearson Education, Inc.
A diagrammatic sectional view through the eye
showing the retina near the origin of the optic nerve
Neural Part of the Retina
Pigmented Part of the Retina
Absorbs light that passes through the neural part,
preventing light from bouncing back and producing
visual “echoes”
Contains photoreceptors, supporting cells,
and neurons that perform preliminary
processing and integration of visual
information
Layer closest to the pigmented part of the
retina; contains the photoreceptors
Ganglion cells
Optic disc (blind spot)
Central retinal vein
Central retinal artery
Optic nerve
Sclera
Blood vessels entering and leaving
the interior of the eye within the
optic nerve
Choroid
Figure 15.14 1
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Module 15.14: Retinal layers and cells
•
Retinal layers (continued)
•
Neural part (continued)
•
Bipolar cells
•
•
Synapse with photoreceptors and ganglion cells
Amacrine cells
•
Facilitate or inhibit communication between ganglion
cells and bipolar cells
© 2011 Pearson Education, Inc.
Module 15.14: Retinal layers and cells
•
Retinal layers (continued)
•
Neural part (continued)
•
Horizontal cells
•
•
Facilitate or inhibit communication between
photoreceptors and bipolar cells
Photoreceptors (outermost layer)
•
Rods (low light, monochromatic vision)
•
Cones (high light, color vision)
Animation: The Eye: The Retina
© 2011 Pearson Education, Inc.
A sectional view showing the retina’s multiple
layers of specialized cells
Pigmented part of retina
Photoreceptors of the Retina
Rods (for vision under dimly
lit conditions)
Horizontal and Amacrine Cells
Facilitate or inhibit communication
between photoreceptors and
ganglion cells, thereby altering the
sensitivity of the retina
Cones (provide color vision
under brightly lit conditions)
Horizontal cells
Amacrine cells
Bipolar cells
Ganglion cells
LIGHT
Figure 15.14 3
© 2011 Pearson Education, Inc.
Module 15.14: Retinal layers and cells
•
Photoreceptor distribution in retina
•
Cones
•
Maximum density at fovea (no rods)
•
•
•
Has highest visual acuity (sharp vision)
~6 million in retina overall
Rods
•
Maximum density at retina periphery
•
~125 million in retina overall
© 2011 Pearson Education, Inc.
Fovea
The retina of each eye contains approximately
Low Density of Cones 6 million cones. The density of cones reaches
High Density of Cones its maximum at the fovea of the macula lutea,
where there are no rods.
Optic disc
Visual acuity
The retina contains approximately 125 million
Low Density of Rods rods. The density of rods is highest at the
High Density of Rods periphery of the retina, where there are very
few cones.
Lateral border
Plot of sharpness of vision (visual acuity) along the
horizontal line; note the direct correlation between
visual acuity and cone density
Fovea
Nasal border
The relative densities of cones and rods on either
side of a horizontal line passing through the fovea
and optic disc of the right eye
Figure 15.14 4
© 2011 Pearson Education, Inc.
Module 15.14 Review
a. Define rods and cones.
b. If you enter a dimly lit room, will you be able
to see clearly? Why or why not?
c. If you had been born without cones in your
eyes, explain why you would or would not be
able to see.
© 2011 Pearson Education, Inc.
Module 15.15: Photoreception
•
Photoreceptors
•
Detect photons of light
•
Light energy also occurs as a wave
•
•
Our visible spectrum of light is 400–700 nm
Contain visual pigments that transduce light
•
Are derivatives of rhodopsin (pigment in rods)
•
Consist of:
•
Retinal (pigment synthesized from vitamin A)
•
Opsin (protein that determines wavelength
absorption of pigment)
© 2011 Pearson Education, Inc.
Rhodopsin
molecule
Retinal
Opsin
The structure of rhodopsin (visual
purple), the visual pigment found
in rods
Figure 15.15 2
© 2011 Pearson Education, Inc.
Module 15.15: Photoreception
•
Photoreceptor structure
•
Outer segment
•
Contains flattened membranous plates or discs
•
Contain visual pigment
•
In cones, are plasma membrane infoldings
•
In rods, each disc is separate entity
•
In cones, tapered
•
In rods, elongate cylinder
•
Inner segment
•
Contains organelles for maintaining cell
© 2011 Pearson Education, Inc.
Structure of Cones
Structure of Rods
Pigment Epithelium
Discs of cones: are infoldings of the
plasma membrane and taper to a
blunt point
The pigment
epithelium absorbs
photons that are not
absorbed by visual
pigments.
Discs of rods: are independent
entities and form an elongated
cylinder
Melanin granules
Outer Segment
The outer segment of a
photoreceptor contains
flattened membranous
plates, or discs, that contain
the visual pigments.
Inner Segment
Discs
Connecting
stalks
Mitochondria
The inner segment contains
the photoreceptor’s major
organelles and is responsible
for all cell functions other
than photoreception.
Golgi
apparatus
Nuclei
Cone
Rods
Each photoreceptor
synapses with a bipolar cell.
Bipolar cell
LIGHT
The major structural features of rods and cones, and the adjacent
pigment epithelium and bipolar cells
© 2011 Pearson Education, Inc.
Figure 15.15 1
Module 15.15: Photoreception
•
•
Steps of phototransduction
1.
Light absorption makes retinal molecule more linear
2.
Opsin activity changes Na+ outer segment permeability and
changes neurotransmitter release to bipolar cell
3.
Bipolar cell activity changes are relayed to ganglion cells
Steps of photopigment regeneration
1.
Pigment breaks down into retinal and opsin (= bleaching)
2.
Retinal converted to original shape (requires ATP)
3.
Converted retinal can recombine with opsin
Animation: Photoreception
© 2011 Pearson Education, Inc.
Module 15.15: Photoreception
•
Color vision
•
Three types of cones
1. Blue cones (16% of all cones)
2. Green cones (10%)
3. Red cones (74%)
•
Combined differential stimulation allows brain
to discern colors
•
All stimulated equally = white
© 2011 Pearson Education, Inc.
Rods
Blue
cones
Light absorption
(percent of maximum)
The range of wavelength
sensitivities for the three
types of cones, each of
which contains a different
form of opsin
Violet
Blue
Green
Red
Green cones
cones
Yellow
Orange
Red
Figure 15.15 4
© 2011 Pearson Education, Inc.
Module 15.15 Review
a. Identify the three types of cones.
b. Compare rods with cones.
c. How could a diet deficient in vitamin A affect
vision?
© 2011 Pearson Education, Inc.
Module 15.16: Visual pathways
•
Visual pathway
•
Receptors  bipolar cells  ganglion cells
•
~1 million ganglion cells converge at optic disc
•
Optic nerves converge at optic chiasm to reach thalamus
•
At thalamus
•
Half of the fibers to lateral geniculate nucleus on same
side
•
Half of the fibers to lateral geniculate nucleus on opposite
side
•
Fibers radiate to visual cortex of occipital lobe
•
= Optic radiation
© 2011 Pearson Education, Inc.
How the visual pathways transmit information
from both eyes to the visual cortex of each
cerebral hemisphere
The Visual Pathways
Combined Visual Field
Left side
Left eye
only
Right side
Binocular vision
Right eye
only
The visual pathways begin at the
photoreceptors in the retina. Each
photoreceptor monitors a specific receptive
field, and when stimulated, passes the
information through a bipolar cell and to a
ganglion cell.
Axons from the approximately 1 million
ganglion cells converge on the optic disc,
penetrate the wall of the eye, and proceed
toward the diencephalon as the optic nerve (II).
Retina
Optic disc
The two optic nerves, one from each eye, reach
the diencphalon at the optic chiasm.
From that point, approximately half the fibers
proceed toward the lateral geniculate nucleus of
the same side of the brain, whereas the other
half cross over to reach the lateral geniculate
nucleus of the opposite side.
Optic tract
From each lateral geniculate nucleus, visual
information travels to the occipital cortex of the
cerebral hemisphere on that side. The bundle of
projection fibers linking each lateral geniculate
nucleus with the visual cortex is known as the
optic radiation.
Diencephalon
and
brain stem
Superior
colliculus
The perception of a visual image reflects the
integration of information that arrives at the
visual cortex of the occipital lobes. Each eye
receives a slightly different visual image,
because (1) the foveae are 5–7.5 cm (2–3.0 in.)
apart, and (2) the nose and eye socket block the
view of the opposite side.
Left cerebral
hemisphere
© 2011 Pearson Education, Inc.
Right cerebral
hemisphere
Collaterals from fibers
synapsing in the lateral
geniculate nuclei
Lateral geniculate
nucleus
Projection fibers
(optic radiation)
Figure 15.16
Module 15.16: Visual pathways
•
Depth perception
•
Interpretation of 3-D relationships of objects in
view
•
Obtained by comparing relative positions of
objects between images from both eyes
•
Perception varies due to position of each eye
•
Each visual cortex receives overlapping visual
field information for both eyes
© 2011 Pearson Education, Inc.
Module 15.16 Review
a. Define optic radiation.
b. Where are visual images perceived?
c. Trace the visual pathway, beginning at the
photoreceptors in the retina.
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.17: Accommodation
problems
•
Accommodation problems
•
Emmatropia (emmetro-, proper + opia, vision)
•
Normal vision
•
Distant image focused on retinal surface
• Ciliary muscle relaxed and lens flattened
•
Myopia (myein, to shut + ops, eye)
•
Nearsightedness
•
Focal distance too short (in front of retina)
•
Cause may be:
• Eyeball too deep
• Resting curvature of lens too great
•
Corrected with diverging (concave) lens in front of eye
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.17: Accommodation
problems
•
Hyperopia
•
Farsightedness
•
Focal distance too long (in back of retina)
•
Cause may be:
•
•
Eyeball is too shallow
•
Lens is too flat
Corrected with converging (convex) lens in
front of eye
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.17: Accommodation
problems
•
Surgical correction
•
Photorefractive keratectomy (PRK)
•
Laser shapes cornea
•
•
Removes 10–20 µm (<10%) of cornea
Laser-assisted in-situ keratomileusis (LASIK)
•
Interior corneal layers reshaped and covered by
normal corneal epithelium
•
~70% of LASIK patients achieve normal vision
•
~10 million people have had corrective procedure
•
Immediate and long-term visual problems can occur
© 2011 Pearson Education, Inc.
Myopia, or nearsighted vision
Myopia
Diverging
lens
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.
Figure 15.17
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.17 Review
a. Define emmetropia.
b. Discuss two surgical procedures for
correcting myopia and hyperopia.
c. Which type of lens would correct hyperopia?
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.18: Disorders of the
special senses
•
•
Olfaction disorders
•
May relate to nerve (N I) or receptor damage
•
Aging issues
•
Number of receptors decline with age
•
Remaining receptors are less sensitive
Gustation disorders
•
Damage to taste buds (mouth infection,
inflammation)
•
Damage to cranial nerves (N VII, IX, X)
•
Problems with olfactory receptors
© 2011 Pearson Education, Inc.
Figure 15.18 1
© 2011 Pearson Education, Inc.
Figure 15.18 2
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.18: Disorders of the
special senses
•
Vision disorders
•
Many mentioned previously
•
Cataracts (opaque lens)
•
Can result from:
•
Injury
•
Radiation
•
Reaction to drugs
•
Aging (senile cataracts)
•
•
Most common
Damaged lens can be replaced by synthetic lens
© 2011 Pearson Education, Inc.
Normal eye
Eye with
cataract
Figure 15.18 3
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.18: Disorders of the
special senses
•
Equilibrium disorders
•
Vertigo (illusion of movement)
•
Caused by conditions that alter function of:
•
Inner ear receptor complex
•
•
Usually affect endolymph
•
Vestibular branch of N VII
•
Sensory nuclei and CNS pathways
Motion sickness (most common cause)
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.18: Disorders of the
special senses
•
Hearing disorders
•
Conductive deafness (issue between
tympanic membrane and oval window)
•
Causes include:
•
Excess wax or trapped water in external ear
•
Scarring or perforation of tympanic membrane
•
Immobility of ear ossicles (fluid or tumor)
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.18: Disorders of the
special senses
•
Hearing disorders (continued)
•
Nerve deafness (issue with cochlea or along
auditory pathway)
•
Damage to receptors
•
20–20,000 Hz in young children but hearing loss later
•
Neural damage to cochlear branch of N VIII
•
Caused by loud noises or pathogenic infections
© 2011 Pearson Education, Inc.
CLINICAL MODULE 15.18 Review
a. Which cranial nerves provide taste
sensations from the tongue?
b. Indentify two common classes of hearingrelated disorders.
c. What causes vertigo?
© 2011 Pearson Education, Inc.