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Sensory Systems
Chapter 45
Overview of Sensory Receptors
Axon nerve endings respond to stimuli...Axon
nerve endings are not dendrites!!! And they
can be afferent or efferent
-Exteroceptors
-Interoceptors
1. Mechanoreceptors are stimulated by
mechanical forces such as pressure
2. Chemoreceptors detect chemicals or
chemical changes
3. Energy-detecting receptors react to
electromagnetic and thermal 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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Mechanoreceptors
Afferent Nerve undergoing
depolarization & “local current
flow”
Nociceptors = pain receptors
-Free nerve endings,
are located where
damage is most likely to occur
Temperature extremes affect the transient receptor
potential (TRP) ion channel
-Produces depolarization by an inward flow of Na+ and
Ca2+, which in turn causes the sensory neuron to fire
-Leads to a release of glutamate and an EPSP in neurons
in spinal cord, which ultimately produces the pain
response
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Mechanoreceptors
Receptors in the skin contain sensory
Cells with ion channels that open in
Response to mechanical distortion...
Touch detectors
-Phasic = Intermittently activated
-Tonic = Continuously activated
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Mechanoreceptors
Proprioceptors become
activated when muscle
is stretched
-Provide information about
the relative position or
movement of animal’s
body parts
-Examples:
-Muscle spindles sensory neuron
innervates thin group of muscle fibers
-Golgi tendon organs Monitors tension
in tendons
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Mechanoreceptors
Baroreceptors monitor blood pressure
-A highly branched network of afferent neurons
located in the carotid sinus (blood to the brain)
aortic arch (near the heart)
-Detect tension or stretch in the walls of these
blood vessels
-When blood pressure decreases, the frequency of
impulses produced by baroreceptors
decreases...Sympathetic Division of Autonomic
Nervous system Creates Vasoconstriction
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Hearing
Hearing is the detection of sound waves
-Sound is the result of vibration, or waves,
traveling through a medium
Auditory stimuli travel farther and more quickly
than chemical ones
Auditory receptors provide better directional
information than chemoreceptors
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Lateral Line System in Fish
“Distant Touch”
Canals running the length of the fish’s body
-Contain hair cells with cilia that project into a
gelatinous cupula(moves around)...Detect
pressure waves. Allows fish to swim in
schools, hunt...
Bend cilia toward kinocilium = stimulate
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Ear Structure of Land Vertebrates
Air vibrations are channeled through the auditory 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... The inner ear consists of the
cochlea, a bony structure
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Ear Structure of Land Vertebrates
Pressure vibrations enter into the oval window...
which opens into vestibular canal crosses through the
cochlear duct into the tympanic canal...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 (So no echo!!!)
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But How do We Get Sound to the Brain??
The organ of Corti, which transduces sound in the cochlea,
consists of:
-Basilar membrane: Bottom of cochlear duct
-Hair cells with associated sensory neurons & stereocilia
-Tectorial membrane: Overhanging, gelatinous membrane
(holds top of stereocilia in place) Stereocilia of hair
cells bend in response to vibrations of the basilar membrane
-Hair cells are depolarized or hyperpolarized and signals are sent
to brain via afferent auditory nerve... You hear a sound
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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 (sound navigation and
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ranging) relied on echolocation principles
Detection of Body Position
Most invertebrates can orient themselves with
respect to gravity due to a sensory structure
called a statocyst
-Consists of ciliated hair cells embedded in a
gelatin with calcium carbonate stones called
statoliths (the stones are called 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
-Embedded in the calcium carbonate-rich 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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Chemoreception Used in Taste
& Smell
Chemoreceptors
-Membrane of sensory neuron becomes
depolarized and produces action potentials
Taste (gustation)
-Broken down into five categories:
-Sweet, bitter, and umami sensations are
transmitted via G-protein coupled receptors on
receptor cells...Salty tastes are detected via ion
channels that take in Na+...Sour tastes are
detected through H+ ion channels
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Taste
Flies taste with hairs in their feet and fish can taste from all
over their body surface
In land vertebrates, taste buds are located in the epithelium of
the tongue and oral cavity within raised areas called
papillae... That contain taste buds that are a epithelial cells
associated with afferent neurons.
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Smell
In land vertebrates, the sense
of smell (olfaction) involves
neurons located in the upper
portion of the nasal passages....
Chemoreceptor neurons have dendrites,
with G-protein coupled receptors & cilia
Projecting into nasal mucosa.
Axons synapse with Olfactory
bulb that project into cerebral cortex
Humans can discern 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
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Vision
Vision begins with the capture of light energy by
photoreceptors
-Visual information is 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
The members of four phyla have evolved well-developed,
image-forming eyes that use the same lightcapturing molecule
-Annelids, mollusks, arthropods, and chordates
Although these eyes are similar in structure, they have
evolved independently
-An example of convergent evolution.
-
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Structure of the Vertebrate Eye
Optic
nerve
Sclera = White
portion of the eye, formed
of tough connective tissue
Suspensory
ligament
Iris
Pupil
Lens
Cornea
Ciliary muscle
Vein
Cornea = Transparent Artery
portion through which Fovea
light enters;
begins to focus light
Retina
Sclera
Iris = Colored portion of the eye
-Contraction of iris muscles
in bright light decreases
the size of its opening, the pupil
Lens = A transparent structure
that completes focusing of
light onto the retina
Ciliary muscle
Ciliary Muscle bends lens
Lens
Suspensory ligament
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The Vertebrate Retina contains two types of Photoreceptors:
Rod
Synaptic
terminal
Connecting cilium
Inner segment
Outer segment
Nucleus Mitochondria
Rhodopsinblack-and-white
vision
Pigment
discs
Cone
color vision
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Photopsin
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Light absorption (percent of maximum)
Humans have three kinds of cones
Blue
cones
420 nm
Green
Rods cones
500 nm 530 nm
Red
cones
560 nm
Each cone possesses a
photopsin consisting of
a cis-retinal bound to a
protein with slightly different
amino acid sequences
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80
60
40
400
500
600
Wavelength (nm)
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Sensory Transduction in the Eye
When rhodopsin absorbs light,
11 cis-retinal is isomerized into
all-trans-retinal
-Activates a G protein that
stimulates the enzyme
phosphodiesterase, which
converts cGMP to GMP
-Na+ channels close, leading
to a hyperpolarization of the membrane
-Prevents the release of the
inhibitory neurotransmitter
onto bipolar cells
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...Bipolar cells fire and then ganglion
cells send signal to optic nerve and
brain Horizontal
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Amacrine
cell
Axons to
optic nerve
cell
Light
Ganglion Bipolar Cone Rod Choroid
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cell
cell
AND THEN YOU SEE THE LIGHT
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