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Sensory Pathways
• Functions of sensory pathways: sensory
reception, transduction, transmission, and
integration
• For example, stimulation of a stretch
receptor in a crayfish is the first step in a
sensory pathway
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Membrane
potential (mV)
Slight bend:
weak
stimulus
Weak
receptor
–50 potential
–70
Dendrites
2
1
3
1 Reception
Membrane
potential (mV)
Muscle
Large bend:
strong
stimulus
–50
Action potentials
0
–70
0 1 2 34 5 6 7
Time (sec)
Stretch
receptor
Strong receptor
potential
–70
2 Transduction
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Brain perceives
slight bend.
4
Axon
Membrane
potential (mV)
Membrane
potential (mV)
Fig. 50-2
Brain
Action potentials
Brain perceives
large bend.
0
–70
0 1 2 34 5 6 7
Time (sec)
3 Transmission
4 Perception
Sensory Systems
• Sensations and perceptions
– Begin with sensory reception, the detection of
stimuli (physical or chemical) by sensory
receptors
– Intergration of sensory information by brain is
Perception
• Exteroreceptors
– Detect stimuli coming from the outside of the
body.
• Interoreceptors
– Detect internal stimuli
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Functions Performed by Sensory Receptors
• All stimuli represent forms of energy
• Sensation involves converting this energy
– Into a change in the membrane potential of sensory
receptors
Sensory transduction is the conversion of stimulus
energy into a change in the membrane potential of a
sensory receptor
• This change in membrane potential is called a
receptor potential which is then transmitted to other
parts of the nervous system for processing and
interpretation
• Many sensory receptors are very sensitive: they are
able to detect the smallest physical unit of stimulus
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• Therefore sensory receptors (sensory cells) are
excitable ( capable of generating a charge
across the membrane called the receptor
potential): Why?
Sensory receptors could be modified neurons
or special cells capable of generating a
receptor potential and then releasing
neurotransmitters that in turn stimulate the
nervous system
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Transmission
• After energy has been transduced into a
receptor potential, some sensory cells
generate the transmission of action
potentials to the CNS
• Sensory cells without axons release
neurotransmitters at synapses with sensory
neurons
• Larger receptor potentials generate more
rapid action potentials
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• Integration of sensory information begins
when information is received
• Some receptor potentials are integrated
through summation
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Perception
• Perceptions are the brain’s construction of
stimuli
• Stimuli from different sensory receptors
travel as action potentials along different
neural pathways
• The brain distinguishes stimuli from different
receptors by the area in the brain where the
action potentials arrive
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Amplification and Adaptation
• Amplification is the strengthening of
stimulus energy by cells in sensory
pathways
• Sensory adaptation is a decrease in
responsiveness to continued stimulation
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Types of Sensory Receptors
• Based on the energy they transduce, sensory
receptors fall into categories
– Mechanoreceptors
– Chemoreceptors
– Electromagnetic receptors
– Thermoreceptors
– Pain receptor
– Photoreceptors
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Figure 45.1 Sensory Cell Membrane Receptor
Proteins Respond to Stimuli
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Figure 45.4 Olfactory Receptors Communicate
Directly with the Brain
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Figure 45.5 Taste Buds Are Clusters of Sensory
Cells
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Figure 45.6 The Skin Feels Many Sensations
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Figure 45.7 Stretch Receptors Are Activated when
Limbs Are Stretched
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Figure 45.10 Structures of the Human Ear
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Hearing
• Vibrating objects create percussion waves in
the air
– That cause the tympanic membrane to vibrate
• The three bones of the middle ear
– Transmit the vibrations to the oval window on
the cochlea
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• These vibrations create pressure waves in the
fluid in the cochlea
– That travel through the vestibular canal and
ultimately strike the round window
Cochlea
Stapes
Axons of
sensory
neurons
Oval
window
Vestibular
canal
Perilymph
Base
Figure 49.9
Round
window
Tympanic
Basilar
canal
membrane
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Apex
• The pressure waves in the vestibular canal
– Cause the basilar membrane to vibrate up and
down causing its hair cells to bend
• The bending of the hair cells depolarizes their
membranes
– Sending action potentials that travel via the
auditory nerve to the brain
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• The cochlea can distinguish pitch
– Because the basilar membrane is not uniform
along its length
Cochlea
(uncoiled)
Apex
(wide and flexible)
Basilar
membrane
1 kHz
500 Hz
(low pitch)
2 kHz
4 kHz
8 kHz
16 kHz
(high pitch)
Figure 49.10
Base
(narrow and stiff)
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Frequency producing maximum
vibration
• Each region of the
basilar membrane
vibrates most
vigorously
– At a particular
frequency and
leads to
excitation of a
specific
auditory area of
the cerebral
cortex
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Figure 45.8 The Lateral Line System Contains
Mechanoreceptors
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Figure 45.9 Organs in the Inner Ear of Mammals
Provide the Sense of Equilibrium
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Similar mechanisms underlie vision throughout
the animal kingdom
• Many types of light detectors
– Have evolved in the animal kingdom and may
be homologous
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Vision in Invertebrates
• Most invertebrates
– Have some sort of light-detecting organ
• One of the simplest is the
eye cup of planarians
– Which provides information
about light intensity and
direction but does not form
images
Light
Light shining from
the front is detected
Photoreceptor
Visual pigment
Ocellus
Nerve to
brain
Screening
pigment
Light shining from
behind is blocked
by the screening pigment
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Figure 45.15 Ommatidia: The Functional Units of
Insect Eyes
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Figure 45.16 Eyes Like Cameras
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• The human retina contains two types of
photoreceptors
– Rods are sensitive to light but do not
distinguish colors
– Cones distinguish colors but are not as
sensitive
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Figure 45.18 Rods and Cones
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Figure 45.12 Rhodopsin: A Photosensitive Molecule
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Figure 45.14 Light Absorption Closes Sodium
Channels
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Figure 45.13 A Rod Cell Responds to Light
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Figure 45.19 Absorption Spectra of Cone Cells
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Figure 45.20 The Retina
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• Signals from rods and cones
– Travel from bipolar cells to ganglion cells
• The axons of ganglion cells are part of the optic
nerve
– That transmit information to the brain
Left
visual
field
Right
visual
field
Left
eye
Right
eye
Optic nerve
Optic chiasm
Lateral
geniculate
nucleus
Figure 49.24
Primary
visual cortex
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