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Sensory Systems
Chapter 46
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Sensory Systems
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All nerve impulses arrive
to CNS as Action
Potential
They reach different brain
regions so different
senses sensed
Intensity depends upon
number of action
potentials received.
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Categories of Sensory Receptors
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Sensory information is conveyed to the CNS
and perceived in a four-step process.
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3 Main Classes of Sensory Receptors
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Mechanoreceptors – Pressure,
Gravity, Inertia, Sound, Touch,
Vibration
Chemoreceptors – Taste,
Smell, Humidity
Photoreceptors – Light, Heat,
Electricity, Magnetism
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Categories of Sensory Receptors
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Sensory receptors transduce stimuli into
graded depolarizations which stimulates the
production of action potentials.
– Exteroceptors sense stimuli that arise in
the external environment.
– Interoceptors sense stimuli that arise from
within the body.
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Sensory Transduction
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Sensory cells respond to stimuli because
they possess stimulus-gated ion channels in
their membranes.
– Sensory stimulus produces a change in
the membrane potential.
 receptor potential
– greater the sensory stimulus, the greater
the depolarization of the receptor potential
and higher frequency of action potentials
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Events in Sensory Transduction
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Cutaneous receptors
–
–
Thermoreceptors – naked dendritic nerve endings
 Cold – stimulated by fall – inhibited by increase
 Heat –
Nociceptors – transmit pain signal to brain
Some sense tissue damage others more sensitive
Mechanoreceptors – Fine touch – face and fingertips

hair follicle receptors - with hair
 Meissner’s corpulse – no hair – fingers, palms, nipples
 Ruffini endings - duration and extent of touch
 Merkel cells - duration and extent
 Pacinian Corpuscle – deep subcutaneous - pressure
Mechnoreceptors – measure force applied to membrane
 Proprioreceptors – measure stretch – reflex Knee-jerk
 Baroreceptors – measure stretch in arteries adjust B.P.

–
–
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Muscle Spindle
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Sensing Taste, Smell, and Body Position
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Chemoreceptors contain membrane proteins
that can bind to particular chemicals in the
extracellular fluid.
Taste
– Taste buds mediate taste in vertebrates.
 located in epithelium of tongue and oral
cavity within raised papillae
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Taste
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Sensing Taste, Smell, and Body Position
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Smell
– Olfaction involves chemoreceptors located
in the upper portion of the nasal passage.
 New research suggests there may be as
many as a thousand different genes
coding for different receptor proteins for
smell.
Internal chemoreceptors
– detect variety of chemical characteristics
of blood or fluid derived from blood
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Smell
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Sensing Taste, Smell, and Body Position
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Lateral line system
– made up of sensory structures within a
longitudinal canal in the fish’s skin
 hairlike processes at their surface
project into gelatinous membrane
(cupula)
 vibrations in the environment produce
movements of the cupula, which
cause hairs to bend
 stimulates sensory neurons
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Sensing Taste, Smell, and Body Position
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Gravity and angular acceleration
– statocyst – invertebrates generally
consists of ciliated hair cells with the cilia
embedded in a gelatinous membrane
containing crystals of calcium carbonate
– Cilia bend with change in position
– Tilt to the right cilia on right side bend
activate sensory neurons
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Sensing Taste, Smell, and Body Position
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In vertebrates – fluid filled membranous
chamber – labyrinth = organ of equilibrium
and hearing
Gravity receptors = two chambers utricle and
saccule – possess hair cells similar to the
lateral line system
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Sensing Taste, Smell, and Body Position
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Inner ear
– Receptors consist of utricle and saccule.
 hairlike processes embedded within a
gelatinous membrane containing
calcium carbonate crystal (otolith
membrane)
 utricle more sensitive to horizontal
acceleration
 saccule more sensitive to vertical
acceleration
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Sensing Taste, Smell, and Body Position
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Utricle and saccule are continuous, with three
semicircular canals oriented in different planes. Detect angular acceleration at any angle
– ampullae - swollen chambers at end of canals
 group of cilia protrude into ampullae
 tips of cilia embedded within wedge of
cupula that protrudes into the endolymph
fluid of each semicircular canal
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Utricle and Saccule
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Structure of Semicircular Canals
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The Ears and Hearing
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Structure of the ear
– In terrestrial vertebrates, vibrations in air
may be channeled through the ear canal
to the eardrum (tympanic membrane).
 Vibrations of the tympanic membrane
cause movement of three small bones
(ossicles) in the middle ear.
 malleus
 incus
 stapes
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Middle
ear
Outer ear
Inner
ear
Auditory canal
Pinna
Eustachian tube
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Malleus Incus
Stapes
Semicircular
canals
Skull
Auditory
nerve
to brain
Tympanic
membrane
Oval
window
Round
window
Cochlea
Eustachian tube
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The Ears and Hearing
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The middle ear is connected to the throat by
the Eustachian tube which equalizes the air
pressure between the middle ear and the
external environment.
Inner ear is composed of the cochlea.
– The cochlear duct is located in the center
of the cochlea.
 The area above is the vestibular canal
and the area below is the tympanic
canal.
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Bone
Cochlear
duct
Vestibular
canal
Auditory
nerve
Tympanic
canal
Organ of Corti
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The Ears and Hearing
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Transduction in the cochlea
– bottom of the cochlear duct, basilar
membrane, quite flexible and vibrates in
response to pressure waves
 cilia of sensory hair cells project into
tectorial membrane
 organ of Corti
 cilia of hair cells bend in response to the
movement of the basiliar membrane
relative to the tectorial membrane
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Hair cells
Basilar
membrane
Tectorial
membrane
Sensory
neurons
To auditory
nerve
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The Ears and Hearing
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Frequency location in cochlea
– When a sound wave enters the cochlea
from the oval window, it initiates a
traveling motion of the basilar membrane.
 Flexibility of the basilar membrane limits
the frequency range of human hearing
to between approximately 20 and
20,000 cycles per second (in children).
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Stapes Oval window
Incus
Basilar
Malleus
Vestibular canal
membrane
Cochlear
Tympanic
duct
membrane
Apex
Round
Tympanic canal
window Base
High frequency (20,000Hz)
Medium frequency (2000Hz)
Low frequency (500Hz)
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Sonar
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Some mammals such as bats emit sounds
and then determine the time it takes for the
sound to return.
– locate themselves in relation to other
objects such as prey
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Evolution of the Eye
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Structure of the vertebrate eye
– sclera - white portion of the eye, formed of
tough connective tissue
– iris - colored portion of the eye
 Contraction of the iris muscles in bright
light decreases pupil size.
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Evolution of the Eye
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–
Light enters the eye through a transparent
cornea which begins to focus the light.
Light then passes through the pupil to the
lens, a transparent structure that
completes the focusing of light onto the
retina.
 lens attached by suspensory ligament to
the ciliary muscles
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Human Eye
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Vertebrate Photoreceptors
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Vertebrate retina contains two photoreceptors.
– rods - black and white vision
 photopigment - rhodopsin
– cones - sharpness and color vision
 Both have an inner segment rich in
mitochondria, with numerous vesicles
filled with neurotransmitter molecules.
 photopigment - photopsins
 red, blue, and green cones
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Color Vision
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Vertebrate Photoreceptors
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Sensory transduction in photoreceptors
– Inverse to the usual way stimuli are detected
– In the dark, photoreceptors release inhibitory
neurotransmitter the hyperpolarizes the
neurons
– Light inhibits the photoreceptors from
releasing their inhibitory neurotransmitter,
and thus stimulates the bipolar cells and the
ganglion cells, which transmit action
potentials to the brain.
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Structure of the Retina
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Visual Processing in the Vertebrate Retina
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Action potentials propagated along the
axons of ganglion cells are relayed through
the lateral geniculate nuclei of the thalamus
and projected to the occipital lobe of the
cerebral cortex.
– The brain interprets this information as
light in a specific region of the eye’s
receptive field.
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Visual Processing in the Vertebrate Retina
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Color blindness
– inherited lack of one or more types of
cones
– more common in men due to sex-linkage
Binocular vision
– ability to perceive three-dimensional
images and sense depth
 each eye sees object at a slightly
different angle
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Pathway of Visual Information
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Diversity of Sensory Experiences
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Heat
– pit vipers
– Pits have membrane
that is warmed by
infrared radiation
stimulating
thermal receptors
Electricity
– Elasmobranchs – ampullae of Lorenzini
Magnetism
– eels, sharks, bees, and birds – navigate in magnetic
field of earth
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