Pain and Temperature Objectives 1. Describe the types of sensory receptors that transmit pain and temperature. 2. Understand how axon diameter relates to transmission of pain and temp information. 3. Describe the basis of the flexion (withdrawal) reflex for pain. 4. Describe the general location of the reticular formation in the brainstem. 5. Describe the names and locations of the cell bodies and axons in the pathways that transmit fast and slow pain. Describe the cortical areas involved. 6. Describe how information is transmitted from first order to second order neurons in this pathway and its clinical consequences. 7. Understand the role of the postcentral gyrus in pain perception. 8. Describe the effects of lesions in the spinal cord, brainstem, and cortex on pain sensation 9. Describe: thalamic pain syndrome, the effects of damage to the postcentral gyrus, Brown-Sequard Syndrome, Syringomyelia
Pain and Temperature Sensory Stimuli: pain, temperature, itch Sensory Receptors: (free nerve endings) Nociceptors: mechano (intense pressure) thermal (< 5 C or > 45 C) polymodal (mechano, thermo, chemical) Thermal receptors: cool, warm http://neuroscience.uth.tmc.edu/s2/chapter07.html Chris Cohan, Ph.D. Dept. of Pathology/Anat Sci University at Buffalo
Diameter of Primary Axons An action potential carrying a pain signal traveling from my finger to my spinal cord would require 30 ms for the fastest and 2 sec for the slowest pain fibers.
Components of Acute Pain The FAST component of pain transmitted by larger (Ad) fibers mechanonociceptors signal brief, sharp sensation. The SLOW component of pain transmitted by smallest (C) fibers polymodal nociceptors signal a second, longer, burning sensation. Ad, C fibers use specific Na channel isoform. Mutations affect pain perception Pharmacological research: develop specific blocker
P/T Pathways Several pathways transmit P/T info. 1. Lateral Spinothalamic Tract - fast pain 2. Spinoreticulothalamic Tract - slow pain Relays in: Thalamus Reticular Formation of medulla/pons/midbrain forms the central core of the posterior half of brainstem the area outside of easily distinguished tracts/nuclei where fibers and cell bodies are loosely associated Reticular Formation.
Pathway Comparison Pain Pathway Tactile Pathway Receptor Naked nerve ending Meissner s, merkel, pacinian, ruffini Axon Size Small Medium/Large 2 neuron Posterior horn of sp cord Caudal medulla Crossing CNS Spinal cord Caudal medulla Destinations Reticular formation, periaqueductal grey, sup colliculus, hypothalamus, thalamus, cortex Thalamus, cortex Thalamic Relay VPL, Intralaminar nuclei Cortical area Somatosensory cortex, insula, cingulate gyrus, orbitofrontal Modulation Initiates descending activity from brainstem to spinal cord VPL Somatosensory cortex Little
Lateral Spinothalamic Tract Mediates FAST component of pain Uses larger (Ad) fibers Indicates the location and intensity of a pain stimulus REFLEXES Ad fibers mediate the fast pain (withdrawal/flexion) reflex via local connections in the spinal cord
Flexion/Withdrawal Reflex Dorsal root to extensors Ventral root Multi-synaptic, multi-level reflex pathway Activates flexor, inhibits extensor motoneurons Affects opposite side spinal cord for postural stability Shows how reflexes are wired-in, sophisticated.
Lateral Spinothalamic Tract Postcentral gyrus Hi neighbor! midbrain pons medulla VPL med lemniscus 1 2 Cell Body Dorsal Root Ganglion Spinal cord Axon Dorsal Root CROSSES CNS medulla ALS
Lateral Spinothalamic Tract midbrain VPL pons medulla medulla ALS
Lateral Spinothalamic Tract Axons of neuron #2 travel rostrally as they cross the midline. ALS L1 ALS L2 L3 L4 L2 L3 L4 A lesion in the ALS produces a complete loss of P&T sensation on the opposite side starting 2 levels below.
2nd order neurons I- Marginal Zone III-V- Nucleus Proprius Marginal zone/lamina I III to Nucleus Proprius/Lam III-V dorsolateral fasciculus Anterior white commissure 2 nd order neurons are located in 2 areas of dorsal horn: marginal zone and nucleus proprius
Cortical Localization Lateral spinothalamic tract terminates in postcentral gyrus (somatosensory cortex) It conveys information about the location of a pain stimulus Stimulation of the postcentral gyrus during neurosurgery produces tingling sensations on the body, as expected from the dorsal column pathway, but no pain sensation!
Somatotopic Orientation in Spinal Cord S LT C
Spino-reticulo-thalamic Tract Mediates SLOW component of pain Uses smallest (C) fibers Carries information about unpleasant qualities of pain burning, throbbing, long-lasting Multisynaptic brainstem - reticular formation different thalamic nuclei - intralaminar different cortical areas
Spino-reticulo-thalamic Tract Substantia Gelatinosa/lamina II ALS Substantia gelatinosa increases integration and modulation of slow pain signals In addition to marginal zone and nuc proprius, substantia gelatinosa provides an intermediary relay for slow pain fibers. 1 st order fiber subst gelatinosa nuc proprius ALS
Cortical Areas These are areas associated with emotion. Insula Cingulate Gyrus
How are these pathways activated? emotional, physiological, and behavioral states Pain Modulatory Pathways Descending pathways from brainstem to spinal cord can decrease activity in the ALS! These pathways begin in several brainstem areas and eventually terminate in substantia gelatinosa where they block transmission of pain activity from 1st-order to 2 nd order neurons in spinal cord. PAG Brainstem locations: periaqueductal grey, locus ceruleus Periaqueductal grey reticular formation
Transmitted by splanchnic nerves Visceral Pain Poorly localized low nociceptor density, convergence What does it feel like: 1. Deep body sensation - eg stomach ache. Transmitted via ALS to visceral region of postcentral gyrus. 2. Referred Pain* - pain from body dermatome different in location from pain source. eg pain in Lt arm from heart ischemia. Here, visceral 1 fibers converge onto 2 somatic ALS neurons in spinal cord. T1-4 *visceral pain info hijacked by dermatome pain pathway!!
Summary of Pain Pathways Lateral Spinothalamic T Spinoreticulothalamic T P/T ALS
Clinical Implications of Lesions can lead to neuropathic pain; eg. Phantom limb pain Peripheral Nerve damage Spinal Cord/Brainstem damage VPL Damage - Postcentral gyrus damage if complete- pure sensory loss Recovery- thalamic pain syndromeepisodes of intense pain Inability to localize pain because fast component of pain carried by lateral spinothalamic tract terminates there. Syringomyelia Brown-Sequard P/T.
Syringomyelia You examine a 40 year-old man who has burned both hands, but he says he never felt it. Pin prick sensation is absent on hands, arms, and shoulders of both sides, but normal elsewhere. Tactile sensation is intact.
Syringomyelia affects only crossing fibers effects limited to levels of lesion ALS
Syringomyelia formation of a cavity in the spinal cord running longitudinally for a variable number of segments. In this case it is around the central canal. It can arise congenitally or from trauma, infection, tumors, or ischemia.
Syringomyelia Dermatomes affected: C4 - T2 Cavity: C2 C8 ALS C2 T2 C8
Case Study A 24-year-old man was brought to the emergency room after he was shot during an argument outside a bar. On exam, he had the following symptoms: Loss of touch, vibration, position senses in Left lower limb. Loss of pinprick sense in Right lower limb. R L
Case Study Which sensory systems affected Which pathways involved. Why did he lose some sensation on Left side but others on Right. Where is Lesion. P/T Touch R L General Strategy: 1. Clues to Level?- unique symptoms 2. Draw pathways, find where adjacent
Brown-Sequard syndrome loss of tactile sensation on one side of body, loss of P&T on opposite side of body. unique symptoms indicating spinal cord location L