Sensory Systems

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Sensory Systems
1. Visual
Distal senses
2. Auditory
Proximal senses
acoustic
vestibular
3. Somatosensory
cutaneous
proprioceptive
4. Gustatory
5. Olfactory
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Somatosensory Systems
cutaneous proprioceptive
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Adequate Stimulus A stimulus of a quality and
of sufficient intensity to excite a sensory
receptor.
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Adequate Stimuli for Somatosensation
Thermal (infrared radiation, contact)
Touch (light touch, pressure, vibration)
Pain and Itch (chemical, thermal, mechanical)
Proprioception (mechanical stretch or pressure)
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Cutaneous subsystems
epicritic location vibration texture shape pr
otopathic pain temperature itch and tickle
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Receptive field
That part of the periphery to which a cell
responds.
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Meissners
Merkels
60 hz vibration
Pressure
Pacinian
Ruffinis
Free nerve ending
Pain
Stretch
200 hz vibration
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Summation of responses of different receptors
(spatial summation).
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Coding of intensity by increased rate (temporal
summation).
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Epricritic, or non-pain Somatosensation
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As in the retina, receptive fields vary in
size. Smaller receptive fields greater
acuity two-point discrimination
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Center-surround organization of
cutaneous receptive fields results in lateral
inhibition. Serves to enhance contrast
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Protopathic, or pain Somatosensation
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Pain Receptors
Called Nociceptors

• Free nerve endings that respond to
• mechanical stimuli
• thermal stimuli
• chemical stimuli, or
• all three
• (polymodal receptors)

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Free nerve endings of unmyelinated C fibers
or thinly myelinated Ad fibers
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Cutaneous classified by conduction velocity
Proprioceptive classified by axon diameter
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Substance Effect Potassium activation
Bradykinin activation Histamine activation
Prostaglandins sensitization Substance
P sensitization
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Gate control theory of pain control
Transcutaneous electrical nerve stimulation may
act via gate control
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Referred Pain
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CN V and VII
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Parallel Processing in the Somatosensory System
Lemniscal System (non-pain epicritic)
Extralemniscal System (pain protopathic) Spinot
halamic pathways Neospinothalamic Paleospinoth
alamic Spinomesencephalic
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Neospinothalamic Pathway
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Paleospinothalamic Pathway
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Spinomesencephalic Pathway
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Descending control of pain
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Sensory System Summary
1. Sensory systems detect change
over space (lateral inhibition to enhance
contrast) over time (rapidly adapting)
2. Detect features
3. Structures are laminated (cells in layers)
4. Parallel pathways
5. Hierarchical processing
6. Topographical organization
7. Non-uniform receptive fields
8. Extreme sensitivity, wide dynamic range
9. Non-linear response