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Chapter 6B
The Peripheral Nervous System:
Special Senses
Outline
• Pathways, perceptions, sensations
• Receptor Physiology
– Receptors have differential sensitivities to various stimuli.
– A stimulus alters the receptor’s permeability, leading to a graded
receptor potential.
– Receptor potentials may initiate action potentials in the afferent
neuron.
– Receptors may adapt slowly or rapidly to sustained stimulation.
– Each somatosensory pathway is “labeled” according to modality
and location.
– Acuity is influenced by receptive field size and lateral inhibition.
– PAIN
– Stimulation of nociceptors elicits the perception of pain plus
motivational and emotional responses.
– The brain has a built-in analgesic system.
• Cortex
– Higher processing
• Basal nuclei
– Control of movement, inhibitory, negative
• Thalamus
– Relay and processing of sensory information
– Awareness, a positive screening center for information
• Hypothalamus
– Hormone secretion, regulation of the internal environment
• Cerebellum
– Important in balance and in planning and executing voluntary
movement
• Brain Stem
– Relay station (posture and equilibrium), cranial nerves,
control centers, reticular integration, sleep control
What did you learn from the vision lab?
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Color blindness
Rod and cone function
What an astigmatism is
After imaging? positive and negative after images
Vision outline
• Anatomy
• Muscles and light control
• Refraction and refractive structures
– Refractive problems
• Retina, photoreceptors, transduction
• Visual fields
• Visual cortical processing
Eye
Countercurrent exchange
• Found in many animal systems
– thermoregulation, and in the kidney
• The transfer of a substance flowing in one direction
to another moving in the opposite direction
• Efficient - gill can remove 80 % of O2
Gills achieve gas exchange in aquatic environments
Gills achieve Gas exchange in aquatic environments
Countercurrent exchange
Eye
• Sensory organ for vision
• Mechanisms that help protect eyes from injury
– Eyeball is sheltered by bony socket in which it is
positioned
– Eyelids
• Act like shutters to protect eye from environmental
hazards
– Eyelashes
• Trap fine, airborne debris such as dust before it can fall
into eye
– Tears
• Continuously produced by lacrimal glands
• Lubricate, cleanse, bactericidal
Eye
• Spherical, fluid-filled structure enclosed by three tissue layers
– Sclera/cornea
• Sclera – tough outer layer of connective tissue; forms visible
white part of the eye
• Cornea – anterior, transparent outer layer through which light
rays pass into interior of eye
– Choroid/ciliary body/iris
• Choroid - middle layer underneath sclera which contains
blood vessels that nourish retina
• Choroid layer is specialized anteriorly to form ciliary body and
iris
– Retina
• Innermost coat under choroid
• Consists of outer pigmented layer and inner nervous-tissue
layer
– Rods and cones
Eye
• Interior consists of two fluid-filled cavities separated
by the lens
– Posterior cavity
• Larger cavity between lens and retina
• Contains vitreous humor
– Important in maintaining the spherical shape of eyeball
– Anterior cavity
• Anterior cavity between cornea and lens
• Contains aqueous humor
– Carries nutrients for cornea and lens
– Produced by capillary network within ciliary body
Eye
• Iris
– Controls amount of light entering eye
– Contains two sets of smooth muscle networks
• Circular (or constrictor) muscle
• Radial (or dilator) muscle
– Pigment in iris is responsible for eye color
– Unique for each individual
• Basis for latest identification technology
• Pupil
– Round opening through which light enters the eye
Eye
• Fovea
– Pinhead-sized depression in exact center of
retina
– Point of most distinct vision
– Has only cones
• Macula lutea
– Area immediately surrounding fovea
– Fairly high acuity
• Macular degeneration
– Leading cause of blindness in western
hemisphere
– “doughnut” vision
Formation and Drainage of Aqueous Humor
Aqueous humor is formed by capillary network in ciliary body, then drains into the
canal of Schlemm, and eventually enters the blood.
Eye
Vision outline
• Anatomy
• Light and muscle control
• Refraction and refractive structures
– Refractive problems
• Retina, photoreceptors, transduction
• Visual fields
• Visual cortical processing
Eye
• Convex structures of eye produce convergence of
diverging light rays that reach eye
Fig. 6-13, p. 194
Eye
• Two structures most important in eye’s refractive
ability are
– Cornea
• Contributes most extensively to eye’s total refractive
ability
• Refractive ability remains constant because curvature
never changes
– Lens
• Refractive ability can be adjusted by changing
curvature as needed for near or far vision
Eye
Focusing on
Distant and Near
Light Sources
What happens to
light rays when
they leave the
light source?
Eye
• Accommodation
– Change in strength and shape of lens
– Accomplished by action of ciliary muscle and
suspensory ligaments
– Age-related reduction in accommodation ability presbyopia
Fig. 6-11, p. 193
Eye
• Macula lutea
– Area immediately surrounding fovea
– Fairly high acuity
• Macular degeneration
– Leading cause of blindness in western
hemisphere
– “doughnut” vision
Lasik
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Eye Health: LASIK Laser Eye Surgery
Laser in-situ keratomileusis, or LASIK, is a popular surgical approach used to correct
vision in people who are nearsighted, farsighted, or have astigmatism.
All laser vision correction surgeries work by reshaping the cornea, or clear front part
of the eye, so that light traveling through it is properly focused onto the retina located
in the back of the eye. LASIK laser eye surgery (laser in-situ keratomileusis) is one of
a number of different surgical techniques used to reshape the cornea.
What Are the Advantages of LASIK Laser Eye Surgery?
LASIK laser eye surgery has many benefits, including:
LASIK laser eye surgery is associated with very little pain.
Vision is corrected nearly immediately or by the next day after LASIK laser eye
surgery.
Recovery is quick and usually no bandages or stitches are required after LASIK
laser eye surgery.
Adjustments can be made years after LASIK laser eye surgery to further
correct vision.
After having LASIK laser eye surgery, most patients no longer need corrective
eyewear.
Mechanics of Accommodation
Far vision
* Light moves towards thick part of lens
Near vision
Vision outline
• Anatomy
• Muscles and light control
• Refraction and refractive structures
– Refractive problems
• Retina, photoreceptors, transduction
• Visual fields
• Visual cortical processing
Emmetropia, Myopia, and Hyperopia
Vision outline
• Anatomy
• Muscles and light control
• Refraction and refractive structures
– Refractive problems
• Retina, photoreceptors, transduction
• Visual fields
• Visual cortical processing
Eye
• Retina – receptor containing portion is actually an
extension of the CNS
• Neural portion of retina consists of three layers of
excitable cells
– Outermost layer containing rods and cones
– Middle layer of bipolar cells
– Inner layer of ganglion cells
• Axons of ganglion cells join to form optic nerve
– Point on retina at which optic nerve leaves is the optic
disc
» Region often called the blind spot because no image
can be detected here because of lack of rods and
cones
Retinal Layers
Photoreceptors
• Rod and cone cells
• Consist of three parts
– Outer segment
• Detects light stimulus
– Inner segment
• Contains metabolic machinery of cell
– Synaptic terminal
• Transmits signal generated in photoreceptor on light
stimulation to next cells in visual pathway
Photoreceptors
Photopigments
• Undergo chemical alterations when activated by
light
• Consists of two components
– Opsin
• Protein that is integral part of disc membrane
– Retinene
• Derivative of vitamin A
• Light-absorbing part of photopigment
Photopigments
• Four different photopigments
– Rod pigment
• Provide vision only in shades of gray
• Rhodopsin
– Absorbs all visible wavelengths
– Cone pigments
• Respond selectively to various wavelengths of light
• Make color vision possible
– Red cones
– Green cones
– Blue cones
Fig. 6-25, p. 202
Parasympathetic stimulation
Sympathetic stimulation
+
Circular
(constrictor)
muscle runs
circularly
Pupillary constriction
+
Circular
muscle
of iris
Radial
muscle
of iris
Pupil
Iris
Radial
(dilator)
muscle runs
radially
Pupillary dilation
Fig. 6-11, p. 193
Properties of Rod Vision and Cone Vision
Rods
Cones
100 million per retina
3 million per retina
Vision in shades of gray
Color vision
High sensitivity
Low sensitivity
Low acuity
High acuity
Night vision
Day vision
Much convergence in retinal
pathways
Little convergence in retinal
pathways
More numerous in periphery
Concentrated in fovea
The sensitivity of the eyes varies
through dark and light
adaptation.
•Dark adaptation
•Can gradually distinguish objects as you enter a
dark area.
•Due to the regeneration of rod photopigments that
had been broken down by previous light exposure.
•Light adaptation
•Can gradually distinguish objects as you enter an
area with more light.
•Due to the rapid breakdown of cone
photopigments.
Vision outline
• Anatomy
• Muscles and light control
• Refraction and refractive structures
– Refractive problems
• Retina, photoreceptors, transduction
• Visual fields
• Visual cortical processing
Hearing outline
• Anatomy
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– Outer, middle, inner
hearing
Transmission of sound waves
Hair cells and transduction
Cochlea and canals/ducts
Pitch and loudness
auditory cortical processing
Ear
• Consists of three parts
– External ear
• Consists of pinna, external auditory meatus, and tympanum
• Transmits airborne sound waves to fluid-filled inner ear
• Amplifies sound energy
– Middle ear
• Transmits airborne sound waves to fluid-filled inner ear
• Amplifies sound energy
– Inner ear
• Houses two different sensory systems
– Cochlea
» Contains receptors for conversion of sound waves into
nerve impulses which makes hearing possible
– Vestibular apparatus
» Necessary for sense of equilibrium
Table 6-6a, p. 223
Ear
Hearing outline
• Anatomy
– Outer, middle, inner
• hearing
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•
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•
•
Transmission of sound waves
Hair cells and transduction
Cochlea and canals/ducts
Pitch and loudness
auditory cortical processing
Hearing
• Neural perception of sound energy
• Involves two aspects
– Identification of the sounds (“what”)
– Localization of the sounds (“where”)
• Sound waves
– Traveling vibrations of air
– Consist of alternate regions of compression and
rarefaction of air molecules
Formation of Sound Waves
Hearing
• Pitch (tone) of sound
– Depends on frequency of air waves
• Intensity (loudness)
– Depends on amplitude of air waves
• Timbre (quality)
– Determined by overtones
Hearing outline
• Anatomy
– Outer, middle, inner
• hearing
• Transmission of sound waves
•
•
•
•
Hair cells and transduction
Cochlea and canals/ducts
Pitch and loudness
auditory cortical processing
Table 6-6b, p. 223
Sound Wave Transmission
• Tympanic membrane vibrates when struck by sound
waves
• Middle ear transfers vibrations through ossicles
(malleus, incus, stapes) to oval window (entrance
into fluid-filled cochlea)
• Waves in cochlear fluid set basilar membrane in
motion
• Receptive hair cells are bent as basilar membrane is
deflected up and down
• Mechanical deformation of specific hair cells is
transduced into neural signals that are transmitted
to auditory cortex in temporal lobe of brain for sound
perception
Transmission of Sound Waves
Fig. 6-33, p. 213
amplification
Transduction to
Auditory nerve
Fig. 6-33c, p. 213
Fig. 6-34a, p. 214
Fig. 6-34b, p. 214
Fig. 6-35, p. 215
Fig. 6-36, p. 216
Sound waves
Vibration of
tympanic membrane
Vibration of
middle ear bones
Vibration of
oval window
Fluid movement
within cochlea
Vibration of
round window
Vibration of
basilar membrane
Dissipation of
energy (no
sound
perception)
In ear
(continue to next slide)
Fig. 6-36, p. 216
Bending of hairs of receptor
hair cells of organ of Corti
as basilar membrane movement displaces these hairs
in relation to overlying
tectorial membrane in which
the hairs and embedded
Graded potential changes
(receptor potential) in
receptor cells
Changes in rate of action
potentials generated in
auditory nerve
Propagation of action
potentials to auditory cortex
in temporal lobe of brain for
sound perception
Fig. 6-36, p. 216
Hearing outline
• Anatomy
– Outer, middle, inner
• hearing
• Transmission of sound waves
• Hair cells and transduction
• Cochlea and canals/ducts
• Pitch and loudness
• auditory cortical processing
Bending of Hairs on Deflection of Basilar
Membrane
Hearing outline
• Anatomy
– Outer, middle, inner
• hearing
• Transmission of sound waves
• Hair cells and transduction
• Cochlea and canals/ducts
• Pitch and loudness
• auditory cortical processing
Equilibrium outline
• Anatomy
– Semicircular canals
• otoliths
Equilibrium
• Vestibular apparatus
– In inner ear
– Consists of
• Semicircular canals
– Detect rotational acceleration or deceleration in any
direction
• Utricle and saccule
– Detect changes in rate of linear movement in any
direction
– Provide information important for determining head
position in relation to gravity
Equilibrium
• Neural signals generated in response to mechanical
deformation of hair cells by specific movement of
fluid and related structures
• Vestibular input goes to vestibular nuclei in brain
stem and to cerebellum for use in maintaining
balance and posture, controlling eye movement,
perceiving motion and orientation
Equilibrium
Fig. 6-38, p. 219
Fig. 6-38a, p. 219
Fig. 6-38b, p. 219
Vestibular
apparatus
Semicircular
canals
Vestibular
nerve
Utricle
Auditory
nerve
Saccule
Endolymph
Perilymph
Ampulla
Oval window
Round window
Cochlea
(Continue to the next slide)
Fig. 6-38, p. 219
Cupula
Hair
cell
Support
cell
Ridge in
ampulla
Vestibular
nerve fibers
Hairs of hair cell;
kinocilium (red)
and stereocilia (blue)
(Continue to the next slide)
Fig. 6-38, p. 219
Fig. 6-38c, p. 219
Chemical Senses
Taste and smell
• Receptors are chemoreceptors
• In association with food intake, influence flow of
digestive juices and affect appetite
• Stimulation of receptors induces pleasurable or
objectionable sensations and signals presence of
something to seek or to avoid
Taste (Gustation)
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•
Chemoreceptors housed in taste buds
Present in oral cavity and throat
Taste receptors have life span of about 10 days
Taste bud consists of
– Taste pore
• Opening through which fluids in mouth come into
contact with surface of receptor cells
– Taste receptor cells
• Modified epithelial cells with surface folds called
microvilli
• Plasma membrane of microvilli contain receptor sites
that bind selectively with chemical molecules
Location and Structure of Taste Buds
Taste
• Tastant (taste-provoking chemical)
• Binding of tastant with receptor cell alters cell’s ionic
channels to produce depolarizing receptor potential
• Receptor potential initiates action potentials within
terminal endings of afferent nerve fibers with which
receptor cell synapses
• Terminal afferent endings of several cranial nerves
synapse with taste buds in various regions of mouth
• Signals conveyed via synaptic stops in brain stem
and thalamus to cortical gustatory area
Taste
• Five primary tastes
– Salty
• Stimulated by chemical salts, especially NaCl
– Sour
• Caused by acids which contain a free hydrogen ion, H+
– Sweet
• Evoked by configuration of glucose
– Bitter
• Brought about by more chemically diverse group of tastants
• Examples – alkaloids, toxic plant derivatives, poisonous
substances
– Umani
• Meaty or savory taste
Taste Perception
• Influenced by information derived from other
receptors, especially odor
• Temperature and texture of food influence taste
• Psychological experiences associated with past
experiences with food influence taste
• How cortex accomplishes perceptual processing of
taste sensation is currently unknown
Smell (Olfaction)
• Olfactory receptors in nose are specialized endings of
renewable afferent neurons
• Olfactory mucosa
– 3cm2 of mucosa in ceiling of nasal cavity
– Contains three cell types
• Olfactory receptor cell
– Afferent neuron whose receptor portion is in olfactory mucosa
in nose and afferent axon traverses into brain
– Axons of olfactory receptor cells collectively form olfactory
nerve
• Supporting cells
– Secrete mucus
• Basal cells
– Precursors of new olfactory receptor cells (replaced about
every two months)
Smell (Olfaction)
• Odorants
– Molecules that can be smelled
• To be smelled, substance must be
– Sufficiently volatile that some of its molecules can
enter nose in inspired air
– Sufficiently water soluble that it can dissolve in
mucus coating the olfactory mucosa
Smell (Olfaction)
• 1000 different types of olfactory receptors
• Odorants act through second-messenger systems to
trigger action potentials
• Afferent signals are sorted according to scent
component by glomeruli within olfactory bulb
Fig. 6-43, p. 225
Olfactory receptor cells
• Enlarged knob bearing several cilia
• Have olfactory receptors
• Odorants
– Must be volatile
– Water soluble
Processing of Scents in Olfactory Bulb
Olfactory processing
• Odors dissected into components
• Each part of a n odor detected by one of a thousand
receptor
• G protein, cAMP, Na channel transduction
• Olfactory bulb
– Above bone layer
– Glomeruli and mitral cells together
• Limbic system in the primary olfactory cortex of the
temporal lobe
• Through the thalamus to the cortex
Processing
• Each odorant molecule activates multiple receptors
and glomeruli
• Odor discrimination based on “patterns” of
glomerular excitation
Vomeronasal Organ (VNO)
• Common in mammals but until recently was thought
to nonexistent in humans
• Located about half an inch inside human nose next
to vomer bone
• Detects pheromones
– Nonvolatile chemical signals passed
subconsciously from one individual to another
• Role in human behavior has not been validated
Fig. 6-15, p. 195
Near
light
source
Diverging rays
Focal point
Fig. 6-17, p. 196
Stronger lens
Near
light
source
Focal point
Fig. 6-17, p. 196
Fig. 6-18, p. 196
Fig. 6-19a,c,d, p. 197
Fig. 6-19a, p. 197
Fig. 6-19b, p. 197
Fig. 6-19c, p. 197
Fig. 6-19d, p. 197
Fig. 6-20, p. 198
Direction of light
Optic nerve
Retina
Pigment layer
Choroid layer
Direction of retinal visual processing
Sclera
Front
of
retina
Back
of
retina
Fibers of
the optic
nerve
Ganglion
cell
Amacrine
cell
Bipolar Horizontal
cell
cell
Cone
Rod
Photoreceptor
cells
Retina
Fig. 6-21, p. 199
Fig. 6-23, p. 200
Back of retina
Cells of
pigment layer
Cone
Outer
segment
Inner
segment
Synaptic
terminal
Rod
Discs
Mitochondria
Outer
segment
Nuclei
Inner
segment
Dendrites
of bipolar
cells
Front
of retina
Direction
of
light
Disc
Light
absorption
Retinence
Opsin
Synaptic
terminal
Rhodopsin in the dark:
retinene in 11-cis form
(inactive)
11-cis form
of retinene
Enzymes
Rhodopsin in the light:
retinene changes shape
to all-trans form
(active)
all-trans form
of retinene
Stepped art
Fig. 6-24, p. 201
Fig. 6-26, p. 204
Blue cone
Green cone
Red cone
Color
perceived
Fig. 6-26, p. 204
Wavelength of light (nm)
Visible
spectrum
Fig. 6-26, p. 204
Fig. 6-27, p. 205
Fig. 6-28, p. 205
Fig. 6-29a, p. 206
Fig. 6-29b, p. 206
Left
(Viewing brain from
above with overlying
structures removed)
Left
eye
Right
Right
eye
1
Optic nerve
Optic chiasm
2
3
Optic tract
Lateral geniculate
nucleus of thalamus
Optic radiation
Optic lobe
Fig. 6-29, p. 206
Visual deficits with specific lesions
1 Left
optic
nerve
2 Optic
chiasm
3 Left
optic
tract (or
radiation)
= Site of lesion
= Visual deficit
Fig. 6-29, p. 206
Table 6-4a, p. 207
Table 6-4b, p. 207
p. 209
Fig. 6-30, p. 210
Pinna of
external ear
Tympanic
membrane
Auditory
ossicles
Semicircular
canals
(eardrum)
Utricle
and saccule
Oval window
Vestibulocochlear nerve
Cochlea
Round window
Eustachian
tube
External
auditory meatus
(ear canal)
External
ear
Middle
ear
Inner
ear
To pharynx
Fig. 6-30, p. 210
Fig. 6-31a, p. 210
Region of
compression
Region of
rarefaction
Fig. 6-31, p. 210
Normal density of air molecules
when tuning fork is at rest
Region of
rarefaction
Region of
compression
Fig. 6-31, p. 210
Fig. 6-31, p. 210
Fig. 6-32, p. 211
Pitch (tone)
depends on
frequency
Same
loudness
Intensity (loudness)
depends on amplitude
Same
note
Timbre (quality)
depends on
overtones
Same loudness,
same note
Fig. 6-32, p. 211
Table 6-5, p. 211
Fig. 6-37, p. 217
Fig. 6-37a, p. 217
Fig. 6-37b, p. 217
Fig. 6-38d, p. 219
Direction of fluid movement
in semicircular canals
Direction of
bending of
cupula and
its hair
Cupula
Hairs
Hair cell
Support cell
Direction of head movement
Fig. 6-39, p. 220
Fig. 6-40, p. 221
Fig. 6-40a, p. 221
Fig. 6-40b, p. 221
Fig. 6-40c, p. 221
Kinocilium
Stereocilia
Otoliths
Gelatinous
layer
Hair cells
Supporting
cells
Sensory
nerve fibers
Fig. 6-40, p. 221
Gravitational
force
Fig. 6-40, p. 221
Fig. 6-40, p. 221
Fig. 6-41, p. 222
Receptors in
eyes
Receptors in
skin
Receptors in
joints and
muscles
Visual input
Cutaneous
input
Proprioceptive
input
Vestibular nuclei
(in brain stem)
Receptors in
semicircular
canals and
otolith organs
Vestibular
input
Coordinated
processing
Output to motor
neurons of limb
and torso muscles
Output to motor
neurons of external
eye muscles
Output to CNS
Maintenance of
balance and desired
posture
Control of
eye movement
Perception of motion
and orientation
Cerebellum
Fig. 6-41, p. 222
Table 6-6c, p. 223