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Anatomical Substrates of Somatic Sensation. John H. Martin, Ph.D. Center for Neurobiology & Behavior Columbia University CPS. The 2 principal somatic sensory systems: Dorsal column-medial lemniscal system Anterolateral system. Dorsal Column-Medial Lemniscal System.
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Anatomical Substrates ofSomatic Sensation John H. Martin, Ph.D. Center for Neurobiology & Behavior Columbia University CPS
The 2 principal somatic sensory systems: Dorsal column-medial lemniscal system Anterolateral system
Dorsal Column-Medial Lemniscal System •Mediates mechanical sensations -touch, limb position sense, vibration sense •Well established; clinical & experimental
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
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
3. Thalamus: Ventral posterior nucleus 2. Dorsal column nuclei Medial lemniscus Peripheral nerve 1. Mechanoreceptor Dorsal root ganglion 1° somatic sensory cortex Dorsal column-medial lemniscal system: Mechanical sensations Dorsal column
1° somatic sensory cortex Cingulate cortex 3. Thalamus: Ventral posterior… Insular cortex & Medial dorsal nuclei. Peripheral axon 1. NociceptorThermoreceptorItch/histamine Dorsal root ganglion Dorsal horn Anterolateral system: Pain, Thermal, Itch 2. Anterolateral system: •Spinothalamic tract •Spinoreticular tract •Spinomesencephalic tract
Mechanoreceptors are encapsulated Merkel’s receptor Meissner’s corpuscle Pacinian corpuscle
Nociceptors, thermoreceptors, & itch receptors are bare nerve endings Bare nerve ending Merkel’s receptor Meissner’s corpuscle Pacinian corpuscle
Dorsal root ganglion neuron PNS CNS Receptive Conductive - 1° afferent fiber Output Integrative
Key Characteristics of DRG Neurons • Peripheral receptive field • Spatial area within which stimulationactivates the sensory neuron
Slow adaptation • Rapid adaptation Off On Key Characteristics of DRG Neurons • Peripheral receptive field • Spatial area within which stimulationactivates the sensory neuron • Response to constant stimulation
Meissner's Merkel Pacinian Ruffini Rapidly adapting Slowly adapting Mechano-receptorreceptivefields
Rapidly adapting Slowly adapting Mechano-receptorreceptivefields Meissner's Merkel Pacinian Ruffini
Nociceptors, thermoreceptors, & itch receptors are bare nerve endings Bare nerve ending Merkel’s receptor Meissner’s corpuscle Pacinian corpuscle
High force; NOT NOXIOUS; NO RESPONSE Low force; NOXIOUS; RESPONSE Nociceptorsrespond tonoxiousstimuli Bluntprobe Pin Serratedforceps Low force; VERY NOXIOUS; LARGE RESPONSE
Unmyelinated Small myelinated Large myelinated Peripheral nervecrosssection
Fiber Histogram: Sensory axon innervating the skin IV (C) III (A-d) II (A-b)
Protective Mechanoreceptors Fiber Histogram: Sensory axon innervating a muscle IV (C) I (A-) III (A-d) II (A-b)
SC SensoryDorsalroot MotorVentralroot Spinal nerve
Dermatomes Area of skin innervatedby all sensory fibersw/in single dorsal root
Dermatome facts: Pain dermatomesoverlap < touch Dermatomal boundariesvary Dermatomesoverlap Skin Peripheral nerve Dorsal root Ventral root