Anatomical Substrates of Somatic Sensation
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Anatomical Substrates ofSomatic Sensation
John H. Martin, Ph.D.Center for Neurobiology & BehaviorColumbia University CPS
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The 2 principal somatic sensory systems:
1) Dorsal column-medial lemniscal system
2) 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 sensorycortex
Dorsal columnMedial lemniscus 2. Dorsal column
nuclei
3. Thalamus:Ventral posteriornucleus
1. MechanoreceptorDorsal rootganglion
Peripheral nerve
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Anterolateral system:
Pain, Thermal, Itch
2. Anterolateral system: •Spinothalamic tract •Spinoreticular tract •Spinomesencephalic tract
Dorsal horn
1. NociceptorThermoreceptorItch/histamine
Peripheral axon
Dorsal root ganglion
Cingulatecortex
& Medial dorsal nuclei.
1° somatic sensorycortex
3. Thalamus:Ventral posterior…
Insular cortex
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Mechanoreceptors are encapsulated
Merkel’s receptorMeissner’s corpuscle Pacinian
corpuscle
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Nociceptors, thermoreceptors, & itch receptors
are bare nerve
endings
Merkel’s receptorMeissner’s corpuscle Pacinian
corpuscle
Bare nerveending
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Dorsal root ganglion neuron
ReceptiveIntegrative
Conductive - 1° afferent fiber Output
PNS CNS
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Key Characteristics of DRG Neurons
• Peripheral receptive field– Spatial area within which stimulation
activates 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
– Rapid adaptation
– Slow adaptation
Off On
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Mechano-receptorreceptivefields
Rapidly adapting Slowly adapting
Meissner's
Pacinian
Merkel
Ruffini
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Mechano-receptorreceptivefields
Rapidly adapting Slowly adapting
Meissner's
Pacinian
Merkel
Ruffini
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Nociceptors, thermoreceptors, & itch receptors
are bare nerve
endings
Merkel’s receptorMeissner’s corpuscle Pacinian
corpuscle
Bare nerveending
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Nociceptorsrespond tonoxiousstimuli
Low force; VERY NOXIOUS; LARGE RESPONSE
Bluntprobe
Pin
Serratedforceps
High force; NOT NOXIOUS; NO RESPONSE
Low force; NOXIOUS; RESPONSE
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Peripheral nervecross
sectionUnmyelinated
Small myelinated
Large myelinated
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Fiber Histogram: Sensory axon innervating the skin
II (A-)
III (A-)IV (C)
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Fiber Histogram: Sensory axon innervating a muscle
II (A-)
III (A-)
IV (C)
I (A-)Mechanoreceptors
Protective
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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
Peripheral nerve
Dorsalroot
Ventralroot
Skin
Pain dermatomesoverlap < touch Dermatomal boundariesvary
Dermatome facts: