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EXTENSION FOR COMMUNITY HEALTHCARE OUTCOMES
WORKING TO BRING SPECIALTY HEALTHCARE TO ALL PEOPLE
Basic Pain Anatomy and Physiology
George D. Comerci, Jr., MD, FACP, AAPM-DiplomateProfessor of Internal Medicine
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ObjectivesAt the end of this session the participant will:
1. Review basic definitions related to thediscussion of pain
2. Describe the function of a nociceptor3. Describe basic pain pathways in the CNS4. Discuss mechanisms of pain inhibition and
facilitation
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Basic DefinitionsPain:
• an unpleasant sensory and emotional experience associated with actual or potential tissue damage
Chronic Pain: • pain that continues beyond the expected time of tissue healing
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Basic DefinitionsSomatic Pain:
• Pain associated with thermal, chemical or mechanical stimuli-(producing tissuedamage)
Neuropathic Pain:• pain that arises as a direct consequence of damage to the somatosensory nervous
system
Existential Pain: pain that occurs upon questioning and doubting the value of one’s ongoing existence
as a living, sencient being -gc
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The Nociceptor
A transducer…converts one form of energy to anotherSpecialized neuron that responds to mechanical, thermal and/or chemical
stimuli
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Types of Sensory Neurons
C fiber: non-myelinated A-delta fiber: lightly myelinated
A-beta: thickly myelinated (light touch/ proprioceptive
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Cross Section Peripheral Nerve(Dept of Physiol. Univ. of Sydney)
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The Nociceptor (Nature.2001)
http://www.nature.com/nature/journal/v413/n6852/fig_tab/413203a0_F3.html#figure-title
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The Nociceptor (J Clin. Invest.2010)
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Nociceptor Function
Stimulus transduction• The receptor (gate) is triggered to open by a noxious stimulus causing sodium to enter
the cell and an electrical “spike” to occur
Action Potential Generation• The “spike”, if strong enough, causes the nerve to depolarize by means of a flood of
sodium entering the cell
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Nociceptor Function
Action Potential Propagation:• The wave of sodium entry/depolarization rushes down the length of the neuron
towards the presynaptic membrane
Transmitter Release• At the presynaptic membrane, the wave of depolarization causes packets of
neurotransmitters to be released into the synaptic space
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The Nociceptor
Important neurotransmitters• Calcitonin Gene related peptide• Substance P• Glutamate • Aspartate• Many more…..
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The Nociceptor (J Clin. Invest.2010)
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The Synapse
The junction of the nociceptor with the 2nd order (spinal) neuron Second order neuron relays information to the
supraspinal structures of the brainThe synapse of the nociceptor with the spinal neuron
occurs in the Laminae of Rexed
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Laminae (J Am Acad Ortho Surg.2012)
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Pain Pathways from the Spine to the Brain
Spinoreticular Tract: spinal neurons that synapse in the Periaqueductal
Gray and Nucleus Raphe Magnus-[role in descendingpain modulation]
Spinal neurons to the medial thalamus-[role in memory and affective components ofpain]
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Pain Pathways from the Spine to the Brain
Spinothalamic Tract:spinal neurons that synapse in the Ventroposterolateral (VPL) thalamus-[role in pain
localization-below the head]
(Trigeminothalamic Tract): spinal neurons that synapse in the VPL lateral
thalamus-[role in pain localization-face and head]
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Spinal Pathways
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The Thalamus
Ventroposterolateral (VPL) Nucleus: Synapse with neurons going to somatosensory cortex with the role of localization of pain
Medial (Centromedian) NucleusSynapse with neurons in the limbic system
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SpinothalamicPathways
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The Thalamus
Functions not only as a “switching station” but also has an important role in certain chronic pain conditions
Storage of memory recall of the sensory and affective components of pain that is long past
• These may be kept “at bay” by inhibitory neurons• Thalamic Syndrome
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Pain and the Cerebral Cortex
Somatosensory CortexPain Localization
Secondary Somatosensory Cortex
Anterior Cingulate CortexPain Experience
Insular Cortex
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Modulation of Nociceptive Input
Endogenous Excitatory Mechanisms• Amplification of nociceptor firing• Amplification of secondary neuron firing (NMDA)• Amplification of descending facilitory neurons
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Modulation of Nociceptive Input
Endogenous Inhibitory Mechanisms
Nociceptor• Inhibition of nociceptor firing (NSAIDS, anesthetics/antiepileptic drugs)
Spinal • Inhibition of secondary neuron firing (GABA)
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Modulation of Nociceptive Input
Brainstem
• Amplification of descending inhibitory neuronsfrom the periaqueductal gray (TCAs/NSRIs)
Cortex
• Cognitive manipulations (distraction, hypnosis,expectation)
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Risk Factors for the development of chronic pain
Gender and SexAgeEfficacy of Endogenous pain modulationGeneticsEnvironmental FactorsPsychologic Factors
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Summary
We’ve discussed:NociceptionPain pathways in the CNSFacilitory and Inhibitory Factors
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References1. Marchand S. The Physiology of Pain Mechanisms: From the Periphery to the
Brain. Rheum Dis Clin N Am. 2008;34: 285-3092. Bonica’s Management of Pain. 4th Ed. 2010.pp24-73 . Walters
Kluwer/Lippincott Williams and Wilkins. Philadelphia.