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Chapter 44
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Sensory Systems
Chapter 44
Senses
• Sensory receptors give an organism the
senses of:
– Vision
– Hearing
– Taste
– Smell
– Touch
• Senses provide information about the
environment
3
Overview of Sensory Receptors
• Sensory receptors provide information
from our internal and external
environments that is crucial for survival
and success
• Exteroceptors sense external stimuli
– Some function well on land but not in water,
and vice versa
• Interoceptors sense internal stimuli
– Usually simpler than exteroceptors
4
Overview of Sensory Receptors
• Receptors can be grouped into three
classes
1. Mechanoreceptors are stimulated by
mechanical forces such as pressure
2. Chemoreceptors detect chemicals or
chemical changes
3. Electromagnetic receptors react to heat and
light energy
5
Overview of Sensory Receptors
• Sensory information is conveyed to the
CNS and perceived in a four-step process
1. Stimulation
2. Transduction
3. Transmission
4. Interpretation
6
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Stimulus
Transduction of stimulus
into receptor potential
in sensory receptor
Transmission of
action potential
in sensory neuron
Interpretation of stimulus
in central nervous
system
7
Overview of Sensory Receptors
• Sensory cells respond to stimuli via stimulusgated ion channels in their membranes
– Open or close depending on the sensory system
involved
• In most cases, a depolarization of the receptor
cell occurs
– Analogous to the excitatory postsynaptic potential
(EPSP)
• Referred to as receptor potential
8
Overview of Sensory Receptors
• Receptor potential like a graded potential
– The larger the sensory stimulus, the greater
the degree of depolarization
• The greater the sensory stimulus, the
greater the depolarization of the receptor
potential and the higher the frequency of
action potentials
9
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Voltage (mV)
20
–10
Stimulus
Threshold
–40
Na+
Na+
Stimulus-gated
channels
–70
Time
Stimulus Receptor potential
applied
a.
Voltage (mV)
20
–10
–40
Voltage-gated
channels
–70
Stimulus
applied
b.
Time
Train of action
potentials
To central
nervous system
10
Mechanoreceptors
• Cutaneous receptors
– Receptors in the skin
– Classified as interoreceptors
– Respond to stimuli at the border between
internal and external environments
– Receptors for pain, heat, cold, touch, and
pressure
11
Mechanoreceptors
• Nociceptors
– Transmit impulses perceived as pain
– Sensitive to noxious substances and tissue
damage
– Most consist of free nerve endings located
throughout the body, especially near surfaces
– Transient receptor potential (TRP) ion
channel
• One responds to capsaicin – sensation of heat and
pain
12
Mechanoreceptors
• Thermoreceptors
– Naked dendritic endings of sensory neurons that are
sensitive to changes in temperature
– Contain TRP ion channels that are responsive to hot
and cold
– Cold receptors are stimulated by a fall in temperature
and are inhibited by warming
• Reverse for warm receptors
– Cold receptors are located higher in the skin
– Cold receptors are more numerous than warm
receptors
13
Mechanoreceptors
• Several types of mechanoreceptors in the
skin detect the sense of touch
– Contain sensory cells with ion channels that
open in response to membrane distortions
– 2 types
• Phasic – intermittently activated
– Hair follicle receptors, Meissner corpuscles, Pacinian
corpuscles
• Tonic – continuously activated
– Ruffini corpuscles, Merkel’s disks
14
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1. Merkle
cell
Free
nerve
ending
2. Meissner
corpuscle
3. Ruffini
corpuscle
4. Pacinian
corpuscle
1. Merkle Cell
Tonic receptors located near
the surface of the skin that
are sensitive to touch
pressure and duration.
2. Meissner Corpuscle
Phasic receptors sensitive to
fine touch, concentrated in
hairless skin.
15
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3. Ruffini Corpuscle
Tonic receptors located near
the surface of the skin that
are sensitive to touch
pressure and duration.
4. Pacinian Corpuscle
Pressure-sensitive phasic
receptors deep below the
skin in the subcutaneous
tissue.
16
Mechanoreceptors
• Proprioceptors
– Monitor muscle length and tension
– Provide information about the relative position
or movement of animal’s body parts
– Examples
• Muscle spindles – monitor stretch on muscle –
receptors that lie in parallel with muscle fibers –
knee jerk reflex
• Golgi tendon organs – monitor tension on tendons
– reflex inhibits motor neurons – prevents damage
to tendons
17
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Biceps extension
causes it to stretch
Nerve
Spindle sheath
Specialized
muscle fibers
(spindle fibers)
Motor neurons
Sensory neurons
Skeletal muscle
18
Mechanoreceptors
• Baroreceptors
– Monitor blood pressure
– Located at carotid sinus and aortic arch
– Detect tension or stretch in the walls of these
blood vessels
– When blood pressure decreases, the
frequency of impulses produced by
baroreceptors decreases
• Results in increased heart rate and
vasoconstriction
19
Hearing
• Detection of sound waves
• Sound is the result of vibration, or waves,
traveling through a medium
• Detection of sound waves is possible
through the action of specialized
mechanoreceptors that first evolved in
aquatic organisms
20
Lateral Line System in Fish
• Sense objects that reflect pressure waves and
low-frequency vibrations
– Supplements hearing
• Consists of hair cells within a longitudinal canal
in the fish’s skin that extends along each side of
the body and within several canals in the head
• Hair cells’ surface processes project into a
gelatinous membrane called a cupula
• Hair cells are innervated by sensory neurons
that transmit impulses to the brain
21
Lateral Line System in Fish
• Bending of stereocilia in the direction of
the kinocilium has a stimulatory effect
• Bending in opposite direction is inhibitory
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Canal
Lateral line scale
Nerve
Opening
Inhibition
Excitation
Kinocilium
Stereocilia
Cupula
Hair cell
Cilia
Hair cell
Afferent axons
Lateral
line organ
Stimulation of
sensory neuron
Sensory nerves
Lateral line
a.
22
b.
Hearing Structures in Fish
• Hearing structures in fish
– Called otoliths
– Composed of calcium carbonate crystals
– Contained in the otolith organs of the
membranous labyrinth
– Otoliths vibrate against stereocilia projecting
from hair cells
– Produces action potentials
23
Ear Structure of Land
Vertebrates
• Air vibrations are channeled through the
ear canal of the outer ear
• Vibrations reach the tympanic membrane
causing movement of three small bones
(ossicles) in the middle ear
– Malleus (hammer), incus (anvil), and stapes
(stirrup)
• The stapes vibrates against the oval
window, which leads into the inner ear
24
Ear Structure of Land
Vertebrates
• The inner ear consists of the cochlea
– Bony structure containing part of the cochlear duct
• The vestibular canal lies above this duct, while
the tympanic canal lies below it
• All three chambers are filled with fluid
• Pressure waves travel down the tympanic canal
to the round window, which is another flexible
membrane
• Transmits pressure back to middle ear
25
Ear Structure of Land Vertebrates
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Outer
ear
Middle
ear
Inner
ear
Semicircular
canals
Skull
Auditory
nerve to brain
Oval window
Malleus
Pinna
Stapes
Incus
Auditory
canal
Cochlea
Tympanic
membrane
Round window
Eustachian tube
Eustachian
tube
a.
b.
26
Ear Structure of Land
Vertebrates
• As pressure waves are transmitted through the
cochlea to the round window, they cause the
cochlear duct to vibrate
• Organ of Corti
– Basilar membrane contains sensory hair cells
– Stereocilia from hair cells project into tectorial
membrane
• Bending of stereocilia depolarizes hair cells
• Hair cells send action potentials to the brain
27
Evolution of the mammalian inner ear
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During inner ear
embryology in modern
mammals, ear bones
develop in association
with the lower jaw bone
before moving inward to
the inner ear
Inner Side of Jaw
Outer Side of Jaw
Dog
Early mammal
Ma
Ma
s
Morganucodon
Q
Ar
Ar
Cynodont
Q
Ar
Ar
Q – quadrate
Ar- articular bones
S - stapes
M - malleus
Q
Synapsid
Ar
Ar
28
Ear Structure of Land Vertebrates
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Organ of Corti
Tectorial
membrane
Vestibular canal
Hair
cells
Cochlear duct
Bone
Tympanic
canal
Basilar
membrane
Auditory nerve
c.
Sensory
neurons
To auditory
nerve
d.
29
Ear Structure of Land
Vertebrates
• Basilar membrane of the cochlea consists of
elastic fibers that respond to different
frequencies, or pitch, of sound
• Hair cell depolarization is greatest in region that
responds to a particular frequency
• Afferent axons from that region stimulated more
• Brain interprets that as representing sound of a
particular frequency or pitch
30
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
High Frequency (20,000 Hz)
Malleus Incus Stapes
Oval
window
Vestibular
canal
Tympanic
canal
Cochlear
duct
Basilar
membrane
Apex
Tympanic Round Base
membrane window
a.
Medium Frequency (2000 Hz)
b.
Low Frequency (500 Hz)
c.
31
Navigation by Sound
• A few mammals have the ability to
perceive presence and distance of objects
by sound
– Bats, shrews, whales, dolphins
• They emit sounds and then determine the
time it takes these sounds to return
• This process is called echolocation
• The invention of sonar and radar are
based on the same principles
32
Detection of Body Position
• Most invertebrates can orient themselves
with respect to gravity using a statocyst
– Consists of ciliated hair cells embedded in a
membrane with calcium carbonate stones
(statoliths)
• In vertebrates, the gravity receptors
consist of two chambers in the
membranous labyrinth
– Utricle and saccule
33
Detection of Body Position
• Within the utricle and saccule are hair cells with
stereocilia and a kinocilium
• Processes embedded in the calcium carbonaterich otolith membrane
• Utricle more sensitive to horizontal acceleration
• Saccule more sensitive to vertical acceleration
• Both types of accelerations cause cilia to bend,
thus producing an action potential in an
associated sensory neuron
34
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Semicircular canals
Vestibular nerve
Sensory
neuron
Cochlea
Kinocilium
Stereocilia
Hair cell
Utricle (horizontal
acceleration)
Saccule (vertical
acceleration)
Gravitational
force
Gelatinous
matrix
Otoliths
Signal
Hair cells
Supporting
cells
Stationary
a.
Movement
b.
35
Detection of Body Position
• The utricle and saccule are continuous
with three semicircular canals that detect
angular acceleration in any direction
• At the ends of the canals are swollen
chambers called ampullae
• Groups of cilia protrude into them
• Tips of cilia are embedded within a
gelatinous cupula that protrudes into the
endolymph fluid of each canal
36
Detection of Body Position
• When the head rotates, the semicircular canal
fluid pushes against the cupula, causing the cilia
to bend
• Bending in the direction of the kinocilium causes
a receptor potential
• Stimulates an action potential in the associated
sensory neuron
• Saccule, utricle, and semicircular canals are
collectively called the vestibular apparatus
37
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Semicircular canals
Vestibular nerve
Vestibule
Ampullae
Direction of body movement
Flow of endolymph
Stationary
Movement
Cupula
Endolymph
Cilia of hair
cells
Hair cells
Supporting
cell
Vestibular
nerve
a.
Stimulation
b.
38
Chemoreceptors
• Bind to particular chemicals in the
extracellular fluid
• Membrane of sensory neuron becomes
depolarized and produces action
potentials
• Chemoreceptors are used in the senses of
taste and smell
• Also important in monitoring the chemical
composition of blood
39
Taste (gustation)
• Mixture of physical and psychological
factors
• Broken down into five categories
– Sweet, sour, salty, bitter, and umami (hearty)
• Taste buds are collections of
chemosensitive cells associated with
afferent neurons
40
Taste
• In fish, taste buds are scattered all over
the body surface
• In land vertebrates, taste buds are located
in the epithelium of the tongue and oral
cavity within raised areas called papillae
• Salty and sour tastes act directly through
ion channels
• Other tastes act indirectly by binding to
specific G protein–coupled receptors
41
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Taste bud
Circumvallate
papilla
Foliate papilla
Fungiform
papilla
a.
b.
Support cell
Nerve fiber
c.
d: © Dr. John D. Cunningham/Visuals Unlimited
Receptor cell
with microvilli
Taste bud
Taste pore
544 µm
d.
42
Taste
• Many arthropods have taste chemoreceptors
• Flies have them in sensory hairs located on their
feet
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Signals to brain
Different chemoreceptors
Proboscis
Sensory hair on foot
Pore
43
Smell (olfaction)
• In land vertebrates, involves neurons located in
the upper portion of the nasal passages
• Receptors project into the nasal mucosa, and
their axons project directly into the cerebral
cortex
• Particles must first dissolve in extracellular fluid
before they can activate the olfactory receptors
• Humans can detect thousands of different smells
44
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Olfactory bulb
Nasal passage
Olfactory
bulb
Cribiform
plate
Axon of
olfactory
nerve
Basal cell
Olfactory
receptor
Support cell
Olfactory
hairs
Odor
45
pH
• Peripheral chemoreceptors
– Found in the aortic and carotid bodies
– Sensitive primarily to the pH of plasma
• Central chemoreceptors
– Found in the medulla oblongata of the brain
– Sensitive to the pH of cerebrospinal fluid
• Increased CO2 in blood lowers pH
– Stimulates respiratory control center
46
Vision
• Begins with the capture of light energy by
photoreceptors
• Can be used to determine both the
direction and distance of an object
• Invertebrates have simple visual systems
with photoreceptors clustered in an
eyespot
• Flatworms can perceive the direction of
light but cannot construct a visual image
47
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Photoreceptors
Eyespot
Light
Pigment layer
Flatworm will turn
away from light
48
Vision
• Members of four phyla have evolved welldeveloped, image-forming eyes
– Annelids, mollusks, arthropods, and
chordates
• Although these eyes are similar in
structure, they have evolved
independently
– Example of convergent evolution
49
Vision
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Insect
Mollusk
Chordate
Lenses
Retinular
cell
Optic
nerve
Retina
Optic
nerve
Retina
Lens
Lens
Optic
nerve
50
Structure of the Vertebrate Eye
• Sclera
– White portion of the eye, formed of tough connective
tissue
• Cornea
– Transparent portion through which light enters; begins
to focus light
• Iris
– Colored portion of the eye
– Contraction of iris muscles in bright light decreases
the size of its opening, the pupil
• Lens
– Transparent structure that completes focusing of light
51
onto the retina
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Optic
nerve
Suspensory
ligament
Iris
Pupil
Lens
Cornea
Vein
Artery
Fovea
Ciliary muscle
Retina
Sclera
Ciliary muscle
Lens
Suspensory ligament
52
Structure of the Vertebrate Eye
• The lens is attached to the ciliary muscles
by the suspensory ligament
– Changes shape of lens
• In near vision, ciliary muscles contract
– Lens becomes more rounded and bends light
more strongly
• In distance vision, ciliary muscles relax
– Lens becomes more flattened and bends light
less
53
Structure of the Vertebrate Eye
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Normal Distant Vision
Nearsighted
Farsighted
Nearsighted, Corrected
Farsighted, Corrected
Lens
Iris
Retina
Suspensory
ligaments
Normal Near Vision
a.
b.
c.
• People who are nearsighted or farsighted do not
properly focus the image on the retina
54
Structure of the Vertebrate Eye
• Vertebrate retina contains two types of
photoreceptors
– Rods
• Responsible for black-and-white vision when
illumination is dim
– Cones
• Responsible for color vision and high visual acuity
(sharpness)
• Most are located in the central region of the retina
known as the fovea
• Sharpest image is formed
55
Structure of the Vertebrate Eye
• Rods and cones have same basic
structure
• Both have inner segment rich in
mitochondria and vesicles filled with
neurotransmitter molecules
• Connected by narrow stalk to the outer
segment
• Packed with hundreds of flattened disks
which contain photopigments
56
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Rod
Connecting cilium
Inner segment
Outer segment
Synaptic
terminal
Nucleus
Mitochondria
Pigment
discs
Cone
57
Structure of the Vertebrate Eye
• Photopigment in rods is rhodopsin
• Photopigments of cones are photopsins
– Humans have three kinds of cones
– Each possesses a photopsin consisting of a
cis-retinal bound to a protein with a slightly
different amino acid sequence
– These shift the absorption maximum, the
region of the electromagnetic spectrum that
the pigment best absorbs
58
Light absorption (percent of maximum)
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Blue
cones
Rods
420 nm 500 nm
Green
Red
cones cones
530 nm 560 nm
100
80
60
40
20
400
500
600
Wavelength (nm)
700
59
Structure of the Vertebrate Eye
• The retina consists of three layers of cells
1. External layer contains the rods and cones
2. Middle layer contain bipolar cells
3. Layer closest to eye cavity contains ganglion
cells
• Once photoreceptors are activated, they
stimulate bipolar cells, which in turn
stimulate ganglion cells
• Ganglion cells transmit impulses to brain
via optic nerve
60
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Axons to
optic nerve
Amacrine
cell
Horizontal
cell
Light
Ganglion Bipolar
cell
cell
Cone
Rod
Choroid
• Blind spot – optic nerve exits
61
Sensory Transduction
• In the dark
– Photoreceptor cells release an inhibitory
neurotransmitter that hyperpolarizes the bipolar
neurons
– Prevents the bipolar neurons from releasing
excitatory neurotransmitter to the ganglion cells that
signal to the brain
• In the presence of light
– Photoreceptor cells stop releasing their inhibitory
neurotransmitter, in effect, stimulating bipolar cells
– Bipolar cells in turn stimulate the ganglion cells, which
transmit action potentials to the brain
62
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Dark
Fluid inside disk
Light
11-cis-retinal
Fluid inside disk
Phosphodiesterase
All-transretinal
Phosphodiesterase
Rhodopsin
G protein
K+
Photoreceptor
cell
In the dark cGMP
levels are high and
keep chemically
cGMP gated Na+ channels
open. The Na+
influx depolarizes
Na+ the membrane
causing an influx
of Ca+, which leads
to a release of
inhibitory
neurotransmitter.
This prevents
signaling from the
bipolar cell.
Ca2+
cGMP
K+
Photoreceptor
cell
GMP
When Rhodopsin
absorbs light, 11cis-retinal is
converted to allNa+ trans-retinal. This
causes rhodopsin
to activate a G
protein that
stimulates
phosphodiesterase,
which converts
cGMP to GMP. The
reduced levels of
cGMP close the
Na+ channels
2+
Ca hyperpolarizing
the membrane.
This prevents the
release of
inhibitory
neurotransmitter
allowing bipolar
cells to fire.
( – ) Inhibitory
neurotransmitter
Bipolar
cell
Bipolar
cell
63
Visual Processing
• Action potentials in the optic nerves are
relayed from the retina to the lateral
geniculate nuclei of the thalamus
• They are then projected to the occipital
lobes of the cerebral cortex
• Each hemisphere of the cerebrum
receives input from both eyes
64
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Right eye
Left eye
Optic nerves
Optic chiasm
Optic tracts
Lateral
geniculate
nuclei
Occipital lobes
of cerebrum
(visual cortex)
65
Visual Acuity
• Relationship between receptors, bipolar
cells, and ganglion cells varies in different
parts of the retina
– Fovea – one-to-one connections = high acuity
– Outside of fovea, many rods converge on a
single bipolar cell and many bipolar cells
converge on a single ganglion cell
• More sensitive to dim light
• At the expense of acuity and color vision
66
Visual Processing
• Color blindness is due to an inherited lack
of one or more types of cones
• People with normal vision are trichromats
– Have all three cones
• Color blind individuals are dichromats
• Sex-linked recessive trait
– More common in men
67
Binocular vision
• Primates and most predators have two eyes,
one located on each side of the face
• 2 fields of vision overlap
• Parallax permits binocular vision
– Ability to perceive 3-D images and depth
• In contrast, prey animals generally have eyes
located to the sides of the head
– Prevents binocular vision, but enlarges the overall
receptive field
68
Diversity of Sensory
Experiences
• Sensing infrared radiation
– The only vertebrates that can sense infrared
radiation are several types of snakes
– Pit vipers
• Have a pair of heat-detecting pit organs on either
side of the head between the eye and nostril
• Locate heat sources in the environment, including
prey in darkness
• Paired pits appear to be stereoscopic
• To some extent can form a thermal image
69
Diversity of Sensory Experiences
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Pit
Outer chamber
Inner chamber
Membrane
© Leonard Lee Rue III
70
Diversity of Sensory
Experiences
• Detect electrical currents
– Elasmobranchs (sharks, rays, and skates)
have electroreceptors called the ampullae of
Lorenzini
– Can sense electrical currents generated by
the muscle contractions of their prey
• Detect magnetic fields
– Eels, sharks, bees, and many birds appear to
navigate along the magnetic field lines of the
Earth
71