Pain Pain Pain: One Definition Classic Paths A new Theory Pain and Drugs According to the international Association for the Study (Merskey & Bogduk, 1994), Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. However, these researchers continue to point out that Pain is always subjective Pain is always unpleasant, thus Pain is also an emotional experience However, these researchers continue to point out that Pain may be reported when there is no tissue damage If reported in same way as pain due to tissue damage, Then should be regarded as pain. Thus, pain should not be tied to a particular stimulus However, these researchers continue to point out that Pain is a psychological experience Information received from nociceptors and nociceptive pathways Is not pain 1
Classic Paths Nociceptor activity can be stimulated by a number of substances The peptides Bradykinin The amines Serotonin Histamine The arhindonic acid derivative Prostaglandin Classic Paths Nociceptor activity can be stimulated by a number of substances These substances often generated by injury to cells Classic Paths Given the large number of substances that stimulate these receptors A central mechanism is required to modulate and interpret these signals This means that a number of factors influence our perception and interpretation of pain E.g., learning The particular situation Our expectations Classical Pain Theories Specificity Theory Cells within the nervous system Respond and relay signals received from nociceptors Specificity Theory Cells within Laminae 1, 2, and 5 of dorsal horn in the spinal cord Are responsive to A-delta A and C- C Coming into central nervous system from The periphery Viscera Joints Muscles tendons Specificity Theory Release neurotransmitter Substance P Considered classic pain-eliciting neurotransmitter 2
Specificity Theory When pain signals conducted up spinal cord Usually ascend in Lateral Spinothalamic tract Classical Pain Theories Another Classic Pain Theory Pattern Theory Pain is produced by particular patterns of stimulation We perceive pain when Stimulation of different receptors reaches a critical level Classical Pain Theories Classical theories could not account for all pain phenomena E.g., Phantom limb pain Psychological influence on perception of pain Proposed by Ronald Melzack and Patrick Wall Ronald Melzack 1929- Patrick Wall 1925-2001 In order to overcome inadequacies in earlier theories Melzack and Wall proposed Gate Control Theory A-delta C- 3
Receive afferent inputs from A-delta A These differ in a number of ways Low threshold Higher threshold They differ in diameter and myelination Large in diameter Myelinated Small in diameter unmyelinated They differ in diameter and myelination They differ in the duration of their response delta Fast adapting nociceptors Slow adapting nociceptors They differ in diameter and myelination They differ in the duration of their response They have different destinations Two branches Synapse at level of the spinal cord Immediate ascension to the thalamus Synapse only at the level of the spinal cord 1. Spinal gating mechanism modulates transmission of nerve impulses Inhibits transmission of impulses from afferent Provides a Gate for pain impulses Where is the gating mechanism found? In the dorsal horn of spinal cord 4
1. Spinal gating mechanism modulates transmission of nerve impulses 2. Gating mechanism influenced by relative amounts of activity in large-diameter and small diameter Large-diameter fiber activity inhibits transmission Closes the gate Small-diameter fiber activity facilitates transmission Opens the gate A-delta Large Diameter Fibers C- Small Diameter Fibers 3. Descending impulses from brain Influence gating mechanism 3. Descending impulses from brain Influence gating mechanism 4. A specialized system of large-diameter, rapidly conducting (the central control trigger) activates selective cognitive processes These then influence spinal gating mechanism By way of descending 3. Descending impulses from brain Influence gating mechanism 4. A specialized system of large-diameter, rapidly conducting (the central control trigger) activates selective cognitive processes 5. When output of spinal cord transmission (T) cells exceeds critical level, Action System activated 1. Action System = neural areas that underlie the complex, sequential patterns of behaviour experienced as pain The location of the gate? In the Substantia Geletinosa of the Spinal cord 5
The location of the gate? In the Substantia Geletinosa of the Spinal cord The large A-delta A and small C play a highly specialized and important role in pain processes They activate the T cells directly And contribute to their output During intense stimulation May be important in raising T cell output above critical level necessary for perception of pain The small C- C Facilitate transmission of pain They open the gate Provide the basis for Summation Prolonged activity Spread of pain to other body areas What was the most important contribution of the gate control theory? According to Melzack (1993) it was The emphasis on CNS Mechanisms Most important contribution of Gate Control Theory? The emphasis on CNS Mechanisms Pain could no longer be considered only as a peripheral system Brain clearly an active system that Filters, selects, and modulates input 6
Most important contribution of Gate Control Theory? The emphasis on CNS Mechanisms Dorsal horns also play part in active nature of the system Not merely passive transmission stations Instead considered sites of dynamic activity E.g., inhibition Excitation modulation Controlling Pain with the Gate Control Theory Gate Control Theory has implications on methods of pain control Stimulating large A-delta A Fibers without stimulating small C-C Should close gate and reduce pain Successful techniques include Acupuncture Acupuncture Controlling Pain with the Gate Control Theory Acupuncture Has been used successfully to relieve pain Another method Electrical stimulation As effective as acupuncture at controlling pain Most common electrical stimulation method TENS Transcutaneous Electrical Nerve stimulation TENS 7