1 Anterolateral System Mediates protective sensations -Pain - PDF document

Anatomical Substrates of Somatic Sensation John H. Martin, Ph.D. Center for Neurobiology & Behavior Columbia University CPS The 2 principal somatic sensory systems: 1) Dorsal column-medial lemniscal system 2) Anterolateral system Dorsal Column-Medial Lemniscal System •Mediates mechanical sensations -touch, limb position sense, vibration sense •Well established; clinical & experimental 1

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 Dorsal column- 1° somatic sensory medial lemniscal cortex system: Mechanical sensations 3. Thalamus: Ventral posterior nucleus 2. Dorsal column Medial lemniscus nuclei Dorsal column Peripheral nerve 1. Mechanoreceptor Dorsal root ganglion 2

Anterolateral system: 1° somatic sensory Cingulate cortex Pain, Thermal, cortex Itch 3. Thalamus: Ventral posterior… Insular cortex & Medial dorsal nuclei. Peripheral axon 2. Anterolateral system: 1. Nociceptor •Spinothalamic tract Thermoreceptor •Spinoreticular tract •Spinomesencephalic tract Itch/histamine Dorsal root ganglion Dorsal horn Mechanoreceptors are encapsulated Merkel’s receptor Meissner’s Pacinian corpuscle corpuscle Nociceptors, thermoreceptors, & itch receptors are bare nerve endings Bare nerve ending Merkel’s receptor Meissner’s Pacinian corpuscle corpuscle 3

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 stimulation activates the sensory neuron Receptive field 4

CNS Neurons have Receptive Fields Key Characteristics of DRG Neurons • Peripheral receptive field – Spatial area within which stimulation activates the sensory neuron • Response to constant stimulation – Slow adaptation – Rapid adaptation Off On Rapidly adapting Slowly adapting Mechano- receptor receptive fields Meissner's Merkel Pacinian Ruffini 5

Rapidly adapting Slowly adapting Mechano- receptor receptive fields Meissner's Merkel Pacinian Ruffini Nociceptors, thermoreceptors, & itch receptors are bare nerve endings Bare nerve ending Merkel’s receptor Meissner’s Pacinian corpuscle corpuscle Nociceptors respond to Blunt probe noxious High force; NOT NOXIOUS; NO RESPONSE stimuli Pin Low force; NOXIOUS; RESPONSE Serrated forceps Low force; VERY NOXIOUS; LARGE RESPONSE 6

Peripheral nerve cross section Unmyelinated Small myelinated Large myelinated Fiber Histogram: Sensory axon innervating the skin IV (C) III (A- δ ) II (A- β ) Fiber Histogram: Sensory axon innervating a muscle Protective IV (C) Mechanoreceptors I (A- α ) III (A- δ ) II (A- β ) 7

Sensory Dorsal root Motor Ventral Spinal nerve root Dermatomes Area of skin innervated by all sensory fibers w/in single dorsal root Dermatomes Skin overlap Dermatome facts: Peripheral nerve Pain dermatomes overlap < touch Dorsal Dermatomal root boundaries Ventral vary root 8

Dermatomes Skin overlap Peripheral nerve Dorsal root Ventral root Dermatomes Skin overlap Peripheral nerve Dorsal root Ventral root Dorsal column Dorsal root ganglion Lateral column “Anterolateral” column Ventral column Dorsal root Spinal Dorsal horn nerve Intermediate zone Ventral horn Ventral root 9

Anterolateral system Dorsal column- medial lemniscal system Bare nerve ending Merkel’s receptor Meissner’s Pacinian corpuscle corpuscle Reminder: Rexed’s laminae Somatotopic organization of the dorsal columns Rostral Demyelination Caudal 10

Lamina 5 Lamina 5 11

Spinal Hemisection Ipsilateral loss of touch … Contralateral loss of pain … (2-3 segments caudal to injury) Mechanoreceptor Nociceptor Syringomyelia Site of lesion • Bilateral loss of pain & thermal senses • Preservation of mechanosensations Somatotopy of spinal paths 12

Dorsal column- Anterolateral medial lemniscal system system Postcentral Postcentral gyrus / Insular/ gyrus / 1° SScx Cingulate cortex Dorsal Also: column •Reticular formation nuclei •Superior colliculus Internal Pain, (mesencephalon) arcuate thermal, Mechano… fibers itch Dorsal Spinal horn commissure Somatic Sensory Thalamus Medial dorsal Ventral posterior medial-VPM (trigem) Ventral posterior lateral-VPL (spinal) Touch Pain Ventral posterior (VMpo) Leg area Arm area Face area Internal capsule Ventral posterior nucleus 13

• Cell-stained section 1° Visual Cortex (Nissl) • most 6 cell layers Layer 1 Layers 2 & 3 Layer 4 Layer 5 Layer 6 •neuron density varies Brodmann’s cytoarchitectonic areas Over 50 areas Defined on basis of histology Somatic sensory areas: 1° somatic sen. cx = 1, 2, 3a, 3b 14

Somatic Sensory Cortical Areas for Mechanosensations 1° somatic Area 5 (3° SScx) sensory cortex Postcentral Central Postcentral Skin gyrus Area 7 sulcus sulcus receptors (Posterior Parietal cortex) 2° somatic sensory cortex Deep receptors Pyramidal neuron: projection neuron Layer 1 Layers 2 & 3 Layer 4 Stellate Neuron: interneuron Layer 5 Layer 6 from to Other cortical back to Thalamus areas Thalamus to Subcortical areas 1° Somatic Sensory Cortex Output Systems VP lateral VP medial Mechanosensation 15

Cortical Pain Representations Thalamus Cortex Behavior Emotional/salience/ Medial dorsal Anterior cingulate valence Ventral med. Post. Mid-insular Limbic (VM po) cortex cortex Behavioral Amygdala Autonomic Ventral posterior 1° SS ??Localization/ (VPL/VPM) Cortex Discrimination Cingulate gyrus: emotional/valence Postcentral gyrus: localization Insular pain representation: behavioral (autonomic; react) Summary • Early morphological specialization of DRG neurons sets stage for separate mechanosensory and pain/temp/itch systems • Different ascending pathways to distinct subcortical and cortical sites • Single thalamic mechanosensory nucleus and 1° ctx • Multiple thalamic pain nuclei and cortical areas • Parietal lobe projections may play role in stimulus localization, esp. for touch • Cortical pain representations closely tied to emotions 16