Sensory Receptors
General Properties
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The Study of the Senses
All that we know about the world around us is obtained
through our senses (“Esse est percipi”):
 “For there is no conception in a man’s mind, which hath
not at first, totally, or in parts, been begotten upon the
organs of the Sense” (Thomas Hobbes, 1558-1679).
 “What is real? How do you define real? If you’re talking
about what you can feel, what you can smell, what you
can taste and see, then real is simply electrical signals
interpreted by your brain… this is the world that you
know” – Morpheus to Neo in The Matrix, 1999.
Thomas Hobbes
Goals:
•
How does the nervous system encode and process
sensory stimuli?
•
How does activity and circuitry in the nervous system
account for sensory experience at the behavioral level?
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Sensation and Perception
Sensation:
• Related to the physical interaction of a stimulus with
a sensory receptor.
• Law of Specific Nerve Energies (Johannes Müller,
1829): sensation depends on the type of receptor
activated, not the form of activation.
Perception:
• Conscious awareness and interpretation of
sensation.
• Related to the particular sensory pathway in the
nervous system that processes information from a
receptor.
• Influenced by “top down” processes: cognition,
attention, experience.
• Not directly related to the actual stimulus activating
the receptor: it is the brain’s interpretation of
sensation, based on its internal model of reality.
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Sensation ≠ Perception
Parallel!
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Stimulation of Sensory Receptors
 Stimulation: Stimulus
interacts with primary sensory
receptor.
 Accessory structures:
 Shapes the input to the receptor.
 Examples:
 Cornea, iris, and lens of eye
 Outer/middle/inner ear structures
 Pacinian corpuscle touch receptor.
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Transduction and Transmission
 Transduction: the conversion of
stimulus energy into electrophysiological response.
 Transduction current: membrane
currents that result from sensory
transduction.
 Receptor Potential: Change in
membrane potential produced by
transduction currents.
 Transmission:
 Direct: transduction currents generate
action potentials (typical of “long”
neuronal receptors).
 Indirect: transduction currents cause
transmitter release, activating primary
afferent fibers ( “short” epithelial
receptors).
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Receptor Classification: Processes
 Neuronal (“long" receptors):
 Have dendritic and axonal processes.
Somatosensory
Auditory Visual
 Transmit information via action
potentials.
 Examples: most somatosensory and
olfactory receptors.
 Epithelial (“short" receptors):
Interneuron
Ganglion
cell
 NO axonal process.
Ganglion
cell
 "Synaptomimetic":


functionally and morphologically similar to
presynaptic nerve endings.
transduction currents modulate release of
neurotransmitter, in some cases generating APs in
receptor cell.
Ganglion
cell
 Examples: visual, auditory/vestibular,
taste receptors.
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Receptor Classification: Stimulus Energy
 Mechanoreceptors
•
•
•
•
•
Touch/pressure
Pain
Hearing
Balance
Joint/muscle proprioception
 Chemoreceptors
•
•
•
•
Taste
Smell
Pain
Itch
 Electromagnetic receptors
• Vision
• Electroreception
• Magnetoreception
 Thermoreceptors
• Warm
• Cold
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Receptor Classification: Stimulus Origin
 Exteroceptive:
• Located at or near the surface of the body
• Provide information about the external environment
(senses)
 Interoceptive:
• Found in blood vessels, connective tissues and organs
• Provide information about the internal environment
(organs, blo0d chemistry)
 Proprioceptive:
• Located in muscles, tendons, joints, muscles and the
internal ear (vestibular)
• Provide information about body and limb positions,
skeletal muscle movements, balance
 Nociceptive:
• Located throughout the body
• Provide information about pain
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Types of Transduction: Direct
Stimulus interacts directly with
ion channels (analogous to
ionotropic synaptic receptors).
Hair Cells
(Auditory/vestibular mechanoreceptors)
Transduction channels
(stretch activated, cation selective)
Transduction current
(carried by K+)
Ca2+ influx
(via Cav channels
Synaptic transmission
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Types of Transduction: Indirect
 Stimulus acts on G-protein coupled
receptor
 E.g., photoreceptors have lightsensitive photopigments that regulate
ion channels via second messenger
(analogous to metabotropic synaptic
receptors).
Photoreceptors
Transduction channels: (cGMP gated Na
channel).
“Dark Current”: Transduction channels
open at rest, net inward Na+ current,
depolarizes photoreceptor. When photons
captured by pigment, then breakdown of
cGMP is catalyzed, dark current decreases
and cell hyperpolarizes.
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Information Coded by Sensory Systems
 Modality: What form does the stimulus take?
 Intensity: How strong is the stimulus?
 Timing: When did the stimulus occur?
 Location: Where is the stimulus in the world, and
what is it’s spatial relation to other stimuli?
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Determinants of Sensory Modality
Modality
Hearing,
Vision balance
Taste
Smell
Proprioception
touch,
pain,
temp
Receptor
 Modality determined by
 Form of energy in the
stimulus.
 Type of sensory receptor
specialized to respond
best to the form of energy
in stimulus (“labelled
line”).
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Submodality
Cones
 Each modality subdivides sensory
“space” to extract different attributes
of a stimulus.
 Examples:
 Taste: Taste receptors specialized
for sweet, sour, salty, bitter,
umami
 Vision: Photoreceptors express
different photopigments selective
for particular wavelengths of light.
 Hearing: Hair cells in cochlea are
tuned for sound frequency by
location and mechanical
properties of the sensory
epithelium.
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Rods
Hair cells
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Sensory Coding: Intensity
SA Touch Receptor :
(tonically active, slowly
adapting)
 Electrophysiological response (i.e.
change in receptor potential) is
proportional to stimulus amplitude.
 Most basic code for amplitude is firing
rate of sensory neurons or primary
sensory afferents (but code may change
centrally).
 Adaptation: Response often declines
under constant stimulation (often
interpreted as evidence that change is
more important than steady-state).
(Note slowing of firing rate over time in
illustrated touch receptor)
Stimulus Intensity (Skin Deflection)
 Intensity proportional to magnitude of
energy in stimulus.
- Response
- Stimulus
Time
Firing rate encodes intensity
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Sensory Coding: Time
• Temporal properties of a stimulus are
encoded by changes in the discharge
pattern of peripheral sensory neurons.
• Latency: time from stimulus onset to
response (basis for reaction times).
• Onsets and offsets: transient
spiking.
• Duration.
• Stimulus “shape”.
Pacinian Corpuscle
• Pattern codes for mimicking stimulus
features may be converted to a
“rate/place” code centrally.
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stimulus
•Phasic on and off response.
•rapidly adapting
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Sensory Coding: Spatial Location
Receptive field: the region of
“sensory space” over which a
receptor integrates energy.
Retinal circuitry underlying an
“opponent” visual receptive field
Skin innervation creating
an opponent tactile
receptive field
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Explicit Space Mapping
 Physical space directly (explicitly)
“mapped” by spatial pattern
(topography) of activated receptors.
 Vision
 Optics of the eye project visual scene
onto a 2D sensory epithelium (retina).
 Visual stimulus directly encoded by
locus of stimulated photoreceptors.
 Somatosensation
 Stimulus location is encoded by
location of sensory receptors on the
body surface.
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Implicit Space Mapping
 Hearing
 Sensory epithelium represents sound
frequency, not space.
 Space perception derived from neural
analysis of spatial cues (neural
computation)
 Spatial cue disparities at the two ears
generated by head (horizontal plane).
 Directionally-dependent spectral
filtering of incoming sounds by the ears
(vertical plane).
 Smell
 Sensory epithelium sensitive to odorants,
not space.
 Space computed centrally based upon
reconstruction of odor “trails”.
Porter et al, Nat. Neuro. (2007)
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Sensory Discrimination
“Minimum Audible Angle”
(MAA)
 Acuity: a form of “two-point
discrimination” (Ernst Weber,
1795-1878):
 What is the minimal perceptible
difference (just-noticeable
difference or JND, difference
limen) between two stimuli?
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Acuity
 Acuity is related to
 Size of the receptive field
i.e., diameter of the
photoreceptor, relative to
the projected image on the
retina: Smaller is better.
 Spatial density
(receptors/mm2) of
receptors in the retina:
higher is better.
400 receptors
3,600 receptors
14,400 receptors
160,000 receptors
 Examples: density of
pixels on a TV screen or
image sensor of a digital
camera.
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