Sensory%20Systems

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Sensory Systems

• Dr. Audrey Ettinger
• April 10, 2006

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Neuronal signaling
Synapse Chemical signal Receptors
Axon electrical signal
Figure 39-10Page 755
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Examples of the Variety of Neurons Found in the
Human Nervous System
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Examples of the Variety of Neurons Found in the
Human Nervous System
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Human (and animal) senses

• Vision
• Hearing
• Touch
• Taste
• Smell
• Proprioception (the secret sixth sense)

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What is the purpose of having senses?

• Answer
• Senses bring information about the outside world
  into the body

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Sensory systems

• Convert sensory information into neural signals
• Process is called sensory transduction

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What kind of information should the sensory
system transduce for the rest of the nervous
system?
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Transduction process must include two kinds of
information

• What kind of signal?
• Red or blue sweater?
• Salty or sweet taste?
• More generally a taste or a color?
• How much signal?
• Loud or soft music?

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General principles of sensation

• What kind information is transmitted by which
  neurons respond to the signal
• How much information is transmitted by the
  number of action potentials sent
• The action potential is an all or none signal

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Characteristics of sensory neurons

• Shape suited to function
• Receptor type (molecule) specific to sense
• Chemoreceptors, mechanoreceptors, photoreceptors
• GRADED receptor potential
• Not all-or-nothing action potential
• Synapse onto neuron that fires regular action
  potentials

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Taste

• Chemoreceptors are similar to neurotransmitter
  receptors
• Only four (or five) tastes recognized by humans
• Sweet, salty, bitter, sour, umami (MSG)
• One kind of receptor for each taste

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Fig. 41-10a,bPage 797
Taste (Gustation)
Epithelial cells
Taste receptor cell
Taste pore
Papillae
Taste bud
50 µm
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Figure 41-10cPage 797
Sugar binds to a taste receptor
Sugar molecule
K channel open
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2
3
Adenylyl cyclase
G protein
GTP
Receptor
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activates
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K channel closes
Protein kinase A
5
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Taste

• Stronger taste results from more sugar
  molecules binding
• Closing potassium channels depolarizes cell
• Gustatory neuron synapses onto another neuron to
  carry information into the brain

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Smell

• Chemoreceptors are similar to neurotransmitter
  receptors
• 10,000 odorants recognized by humans
• 1,000 kinds of odorant receptors
• Each odorant activates a subset of receptors
• Pattern of receptors bound indicates odorant

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Fig. 41-11a,b Page 798
Olfactory receptors
Neurons of the olfactory bulb
Olfactory tract to brain centers for smell
Olfactory bulb
Olfactory bulb
Sinuses
Receptor cells
Nonsensory epithelium
Cilia
Wall of nasal cavity
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Figure 41-11c Page 798
Odorant binding to a receptor
Odor molecule
1
2
3
Na channel closed
Adenylyl cyclase
G protein
GTP
Receptor
4
5
Na channel opens
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Smell

• Stronger smell results from more odorant
  molecules binding
• Opening sodium channels depolarizes cell
• Olfactory neuron synapses onto another neuron to
  carry information into the brain

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Touch (and pressure, and pain)

• Six kinds of mechanoreceptors
• Respond to different types of input
• Different adaptation patterns

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Figure 41-2a,bPage 790
Ruffini corpuscle (pressure)
Meissner corpuscle (touch, pressure)
Hair
Free nerve endings (pain)
Merkel disc (touch, pressure)
Epidermis
Dermis
Subcutaneous tissue
Pacinian corpuscle (deep pressure, touch)
Hair follicle receptor (hair displacement)
500 µm
Mechanoreceptors
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Pressure directly opens sodium channels in the
Pacinian corpuscle
Pressure
Sodium channel opens
Sodium channel closed
Figure 41-2cPage 790
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Touch, pressure, pain

• Opening sodium channels depolarizes cell
• Mechanoreceptor neurons synapse onto another
  neuron to carry information into the brain

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Vision

• Best understood of all the senses
• Retina contains light-sensitive cells
• Four types of photoreceptors 3 cones, 1 rod
• Four additional types of neurons are present in
  the retina

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The human eye
Figure 41-14Page 801
Retina
Iris
Lens
Pathway of light
Pupil
Optic nerve
Cornea
Blind spot
Fovea
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The retina
Ganglion cell
Bipolar cell
Retina
Light rays
Figure 41-16aPage 802
Pigmented epithelium
Cone cell
Optic nerve fibers
Rod cell
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Electron microscopy of rods and cones
10 µm
Cone cell
Rod cell
Figure 41-16bPage 802
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Vision

• Photoreceptor types are responsive to different
  light signals
• Brighter light causes bigger response
• More photoreceptors are recruited to see bigger
  objects

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Figure 41-18Page 803
Light REDUCES signaling from the rods
Discs
Discs
Na channel closes
Na channel open
Plasma membrane of rod
Photon
Plasma membrane of disc
Rod
G protein
Rhodopsin
Esterase
Disc interior
In the light, the rod cell becomes hyperpolarized
In the dark, the rod cell is depolarized
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Cells of the retina
Cone
Rod
Discs
Horizontal cell
Bipolar cell
Amacrine cell
Figure 41-17Page 802
Ganglion cell
To optic nerve
Light
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The light signal travels into the brain
Lateral geniculate nucleus of the thalamus
Optic chiasm
Right primary visual cortex
Optic nerves
Left primary visual cortex
Figure 41-19Page 804
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Vision

• Several neuron types process vision in the retina
• Visual information crosses to the opposite side
  of the brain
• Visual information travels to the thalamus
  (lateral geniculate nucleus) and then to primary
  visual cortex

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Hearing

• Auditory receptors are complex mechanoreceptors
• Pitch and loudness signals are transduced by the
  same receptors

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The Organ of Corti lies within the ear
Cochlear nerve, division of the
vestibulocochlear (VIII) nerve
Oval window
Organ of Corti
Figure 41-9aPage 795
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Auditory receptors
Tectorial membrane
Organ of Corti
Tectorial membrane
Basilar membrane
Stereocilia
Force
Force
Hair cell
Cochlear nerve
Basilar membrane
Figure 41-9b,cPage 795
Fluid vibrations
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Auditory receptors
Tectorial membrane
Organ of Corti
Tectorial membrane
Basilar membrane
Stereocilia
Force
Force
Hair cell
Cochlear nerve
Basilar membrane
Figure 41-9b,cPage 795
Fluid vibrations
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Hearing

• Receptors responsive to different wavelengths are
  arranged spatially
• Louder sounds move the stereocilia farther
• Ion channels are mechanically opened to
  depolarize the hair cells

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If this material interests you...

• NEU/PSY/BIO 220 Sensation and Perception (Spring
  course required for Neuroscience major, Biopsych
  concentration, elective for Biology majors)
• NEU 200 Introduction to Neuroscience (Spring
  course)
• Neuroscience Club!