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CHAPTER 45
LECTURE
SLIDES
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Sensory Systems
Chapter 45
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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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Hearing Structure 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
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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
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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
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Ear Structure of Land Vertebrates
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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
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Ear Structure of Land Vertebrates
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Taste
• Many arthropods have taste chemoreceptors
• Flies have them in sensory hairs located on their
feet
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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
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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
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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
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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
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Vision
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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
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onto the retina
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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
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Structure of the Vertebrate Eye
• People who are nearsighted or farsighted do not
properly focus the image on the retina
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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
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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
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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
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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
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• Blind spot – optic nerve exits
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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
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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
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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
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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
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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
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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
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Diversity of Sensory Experiences
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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
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