Anatomical Substrates of Somatic Sensation

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Anatomical Substrates ofSomatic Sensation
John H. Martin, Ph.D. Center for Neurobiology
Behavior Columbia University CPS
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• The 2 principal somatic sensory systems
• Dorsal column-medial lemniscal system
• Anterolateral system

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Dorsal Column-Medial Lemniscal System

• Mediates mechanical sensations
• -touch, limb position sense, vibration sense
• Well established clinical experimental

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Anterolateral System

• Mediates protective sensations
• -Pain
• -Temperature (cold warmth)
• -Itch
• Not as definitively established as is the DC-ML
  system for touch

Why? Pain w/o tissue trauma Trauma w/o
pain Cultural pathological
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Perspective

• Peripheral somatic sensory receptors are
  sensitive to different stimulus qualities
• Mechanical
• Thermal (warm, cold)
• Noxious (mechanical, thermal, polymodal)
• Different receptor classes provide input to
  different somatic sensory pathways
• Differential thalamic and cortical localization

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Dorsal column-medial lemniscal system Mechanical
sensations
1 somatic sensory cortex
Dorsal column
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Anterolateral system Pain, Thermal, Itch
2. Anterolateral system Spinothalamic tract
Spinoreticular tract Spinomesencephalic tract
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Mechanoreceptors are encapsulated
Merkels receptor
Meissners corpuscle
Pacinian corpuscle
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Nociceptors, thermoreceptors, itch receptors
are bare nerve endings
Bare nerve ending
Merkels receptor
Meissners corpuscle
Pacinian corpuscle
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Dorsal root ganglion neuron
PNS CNS
Receptive
Conductive - 1 afferent fiber
Output
Integrative
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Key Characteristics of DRG Neurons

• Peripheral receptive field
• Spatial area within which stimulationactivates
  the sensory neuron

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Receptive field
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CNS Neurons have Receptive Fields
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Key Characteristics of DRG Neurons

• Peripheral receptive field
• Spatial area within which stimulationactivates
  the sensory neuron

• Response to constant stimulation

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Mechano-receptorreceptivefields
Rapidly adapting
Slowly adapting
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Mechano-receptorreceptivefields
Rapidly adapting
Slowly adapting
Meissner's
Merkel
Pacinian
Ruffini
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Nociceptors, thermoreceptors, itch receptors
are bare nerve endings
Bare nerve ending
Merkels receptor
Meissners corpuscle
Pacinian corpuscle
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Nociceptorsrespond tonoxiousstimuli
Bluntprobe
Pin
Serratedforceps
Low force VERY NOXIOUS LARGE RESPONSE
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Peripheral nervecrosssection
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Fiber Histogram Sensory axon innervating the skin
IV (C)
III (A-d)
II (A-b)
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Fiber Histogram Sensory axon innervating a muscle
IV (C)
I (A-?)
III (A-d)
II (A-b)
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SC
Sensory Dorsalroot
Motor Ventralroot
Spinal nerve
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Dermatomes
Area of skin innervatedby all sensory
fibersw/in single dorsal root
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Dermatomesoverlap
Skin
Peripheral nerve
Dorsal root
Ventral root