Biopsychology of Pain - Lecture Notes PDF

BIOPSYCHOLOGY OF PAIN 18 Oct 2024 Today’s Plan What is Pain? Sensory Systems and Nociception Theoretical Explanations: Gate Control Theory of Pain Central Processes Cognitive, Affective and Social Factors Learning Outcomes Explain Nociception within somatic sensation Describe the neural mec...

BIOPSYCHOLOGY OF PAIN 18 Oct 2024 Today’s Plan What is Pain? Sensory Systems and Nociception Theoretical Explanations: Gate Control Theory of Pain Central Processes Cognitive, Affective and Social Factors Learning Outcomes Explain Nociception within somatic sensation Describe the neural mechanisms behind the Gate Control Theory of Pain Analyse the role of descending factors Discuss the psychological dimension of Pain INTRODUCTION: WHAT IS PAIN? What Is Pain? According to the IASP (16 July 2020) ‘Unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage’ (Raja et al., 2020) IASP also stated that: Personal experience influenced to varying degrees by biological, psychological and social factors Pain and nociception are different phenomena. Pain cannot be inferred solely from activity in sensory neurons Shaped by context, experience, expectations SENSORY SYSTEMS AND NOCICEPTION Nociceptive Systems Sensation of Pain is triggered by nociceptive systems Nociceptors activated by tissue damage Send neural information to central processing Perception of Pain organised in the Brain More to pain than just tissue damage… Antinociceptive Systems Also influenced by antinociceptive system Reduces nociceptive input to Brain Pain more than tissue damage and noxious stimuli… Complex and integrated sensory-emotional experience Influenced by number of diverse factors Afferent and Efferent Neurons Afferent Neurons Periphery to CNS Efferent neurons CNS to periphery Afferent and Efferent Neurons Sensory Receptors: Afferents Sensory Receptors: Afferents  Primary Afferent Neurons Axon Conduction Sensory Function Receptor Type Afferent Axon Type Diameter Velocity Axon Proprioception Muscle spindle 13 – 20 μm 80 – 120 m/s Myelin Ia, II Touch Merkel, Meissner, Pacinian 6 – 12 μm 35 – 75 m/s and Ruffini Cells Aβ Pain, temperature Free nerve endings 1 – 5 μm 5 – 30 m/s Aδ Pain, temperature, itch Free nerve endings 0.2 – 1.5 μm 0.5 – 2 m/s C Nociceptors and Nociception Aδ and C synapse on Dorsal Roots (Substantia Gelatinosa) 1. With Excitatory T Cells 2. And Inhibitory Interneurons (Enkephalin) But Aβ also synapse with T Cells and Interneurons Input to Brain reflects this ‘competition’… Processing in Spinal Cord Effects on T Cells and Interneurons depend on Type of neuron on synaptic connection a. Aδ and C excite T Cells and inhibit Interneurons b. Aβ excite Interneurons and inhibit T Cells Combined (ratio) excitation and inhibition Modulates experience of Pain Nociceptors to Brain Lateral Spinothalamic Tract Stimulation causes Pain Lesions reduce sensation of Pain Perception constructed by the Brain Sensation, emotion, cognition More than tissue damage Summary Nociceptors detect tissue insult Convey information to CNS Enter CNS in Spinal Cord Synapse with T Cells and Interneurons Brain constructs perception of Pain But input processed in Spinal Cord THE GATE CONTROL THEORY OF PAIN (AND OTHER EXPLANATIONS) Specificity Theory of Pain Dedicated sensory receptors Impulses Impulses – Detect specific sensations Of which Pain is one example Innocuous Noxious Intensity Pain experience Noxious Skin DRG Nociceptor Nociceptive Neurons Stimulus – Follows adequate stimulation Pattern Theory of Pain No specific nociceptors Impulses – Quality of sensation produces specific neural firing pattern Innocuous Noxious Innocuous Noxious Intensity Intensity Pain associated with patterns DRG DRG Nociceptor Nociceptor Skin Skin Nociceptive Nociceptive of neural activity Noxious Stimulus Cell 1 Neurons Neurons Innocuous Stimulus Cell 2 Cell 3 Gate Control Theory (Melzack & Wall, 1965) Pain not unaltered sensation from stimulus to Brain – Can be modified by number of factors Shaped by activity in metaphorical ‘gates’ – Based on ratio of activity from different types of neurons Aδ, C, Aβ, T Cells, and Interneurons Gate Control Theory (Melzack & Wall, 1965) When Aδ and C neurons are stimulated – Inhibit interneurons: do not release Enkephalin Nociception is transmitted (via T Cells) When Aβ neurons are stimulated – Excite interneurons: Enkephalin is released Nociception is blocked (or reduced, depends on ratio) Gate Control Theory (Melzack & Wall, 1965) Interneurons in the Spinal Cord – Influence opening and closing of ‘gate’ The greater the activity in Interneurons – The greater the inhibition of nociceptive pathways Gate Control Theory (Melzack & Wall, 1965) Neurochemistry of Anti-nociception Bottom-up and top-down modulation – Ascending and descending inhibitory pathways Both based on Enkephalin – Opioid receptors in Aδ, C, T Cells, and Brain regions Both pathways have effect on gating system Gate Control Theory: Main Ideas 1. Outcome of nociceptors determined by ‘gate’ – When gate is open Aδ and C neurons excite T Cells – When gate is closed Aδ and C neurons do not excite T Cells Gate Control Theory: Main Ideas 2. Factor determining opening/closing of gate – Ratio of Aβ to Aδ and C neurons activity in same regions Aβ neurons contribute to closing the gate Aδ and C neurons contribute to opening the gate Gate Control Theory: Main Ideas 3. Gates also influenced by descending pathway – Descending anti-nociceptive system (Midbrain) Inhibitory synapses with nociceptive neurons – Inhibitory link through Interneuron Gate Control Theory: Main Ideas 4. Descending pathways influenced by – Cognitive, social, affective factors… Pain more than direct result of noxious stimulus – A psychological experience  Gate Control Theory of Pain T Cell Touch Gate Control Theory: Evaluation Theory capable of explaining – Indirect relationship pain-tissue damage – Production of anti-nociception effect by CNS Interaction between CNS structures involved in Pain – Key in determining experience of Pain Not just result of afferent input BRAIN PROCESSES Brain Processes Complex sensorial, emotional, and cognitive experience – Ultimately organised by the Brain When nociceptive input reaches the Brain – Activates circuits of interacting regions Creates perception of, and response to Pain Brain Processes Processes more than nociception a. Sensory (discriminative quality) b. Affective (emotional quality) c. Cognitive (evaluative quality) Reflects complexity of Pain  Termination of Ascending Pathways Lateral Spinothalamic Tract – First synapses in Midbrain Periaqueductal Gray Area (PAG) – Then synapses in the Thalamus Ventrocaudal Mediodorsal Nucleus (MDvc) Posterior Ventromedial Nucleus (VMpo) Ventroposterolateral Nucleus (VPL) Termination of Ascending Pathways From VMpo and MDvc nuclei to – Insula and Anterior Cingulate Cortex From VPL nucleus to – Somatosensory Cortex Biological basis of Pain formed by – The Pain Matrix Termination of Ascending Pathways Brain Processes: Descending Pathways Stimulation of descending pathways – Modulates experience of Pain Connection between ACC and PAG – Cognitive information modulates PAG PAG projects to Dorsal Horn – Nociceptive neurons Brain Processes: Descending Pathways Brain Processes: Ascending and Descending Pathways Summary Nociceptive input reaches Brain – Specialised Pain functions Sensory, affective, cognitive dimensions of pain – Insula, Cingulate Cortex, and Somatosensory Cortex Descending information modulated – Psychological dimension COGNITIVE AND SOCIAL FACTORS Cognitive Targeting Cognitive distractions and positive mood – Can reduce Pain Focusing on Pain and negative mood: opposite effect Cognitive interventions focus on – Interpretation of implications of Pain Relaxation, positive focus, self-efficacy, language… Social Factors Pain behaviour can be conditioned – As it is rewarding e.g., lying in certain position to avoid pain Verbal and behavioural manifestations can also be learned – Reinforced by others’ concern and sympathy (context) Can increase experience of pain Placebo Effects Partly explained by classical conditioning – But placebo in absence of learning? Simply told to expect Pain relief… Supported by nocebo effect – Pain induced by expectation Contextual factors associated with Pain Placebo Effects and the Brain Placebo lowers activity in Pain Matrix – Effect reduced by Naloxone (opioid antagonist) Mostly linked to ACC – Connections to OFC: goals and expectations Pain relief based on cognitive processing Increases PAG activity: cortical input Summary Pain related to, but not reducible to, noxious stimulus – There is also a psychological dimension Cognitive and social factors Complex and subjective experience modulated by – Combination of sensation, affect, and cognition