Pain Physiology PDF

Pain  Physiology of pain – Least rapidly adapting sensation – Types of pain Sharp or first pain – Sensed quickly, short duration – Very well localized Fast conducting larger fibers (Aδ fibers) – Sensation highly localized and well defined – Stimulation i...

Pain  Physiology of pain – Least rapidly adapting sensation – Types of pain Sharp or first pain – Sensed quickly, short duration – Very well localized Fast conducting larger fibers (Aδ fibers) – Sensation highly localized and well defined – Stimulation is mechanical  Dull, burning, aching (second) pain – Longer lasting “soreness” Two phases – Short initial phase – Longer second phase – Small slowly conducting fibers (Type C) – Unmyelinated – Diffusely located – Sends impulses to limbic system This diagram shows the pathways of pain and touch sensations through the nervous system. Touch Pathway: Signals from the Ruffini end organ (sensitive to stretch) are transmitted through A-beta fibers (first-order afferent fibers) to the dorsal root ganglion and then to higher brain centers. Pain Pathway: Signals from free nerve endings (pain receptors) are transmitted via A-delta (fast, sharp pain) or C fibers (slow, dull pain) to the dorsal root ganglion. The pain signals cross the midline in the spinal cord and ascend to higher brain centers through second-order afferent fibers. Pain receptors exhibit little to no adaptation, meaning they continue to send signals as long as the stimulus persists. Other receptors, like those for touch (Meissner corpuscles), adapt quickly, reducing their firing rate over time. Pain signals begin at receptors in the skin (first-order neurons) and are relayed through the spinal cord to the thalamus (second-order neurons). Finally, third-order neurons transmit these signals to the somatosensory cortex for processing. Specific regions of the cortex correspond to specific parts of the body, and their size in the brain correlates to the sensitivity or motor control of that body part. Larger body parts (like hands, lips, and tongue) indicate higher sensitivity and more cortical representation in the somatosensory cortex.  Pain mediators – Often released into EC space from damaged tissue Activate nociceptor endings – Histamine, substance P, bradykinin, prostaglandins , acetylcholine , H+ ions, leukotrienes , etc – Substances have other responses » Ie prostaglandins : inflammation, bradykinin: vasodilation, etc  Hyperalgesia and allodynia – Hyperalgesia Primary nociceptors may become sensitized after tissue injury – More readily activated » Reduced threshold – Fire at higher rates with given stimuli Surrounding area becomes tender – Noxious stimuli evokes more pain than normal – Allodynia Normally innocuous stimuli evokes pain – Tactile or thermal Continued signaling results in synaptic changes in the dorsal horn causing increased frequency of firing at lower levels of stimuli. Can progress to LTP – Gate theory Stimulation of fibers that transmit non-painful stimuli can block pain pathways in the dorsal horn – Stimulation of Aβ fibers (touch) can dominate and block pain (close the gate) – Rubbing a toe that was stubbed Usefulness of massage therapy TENS units , acupuncture – Innocuous input may prevent pain transmission  Neuromodulators – Endogenous pain killers – Released by dorsal horn, brain stem and tissue fibers Endogenous opioids Inhibit pain fibers – Dynorphin, beta endorphin, enkephalin Levels vary among individuals  Neuropathic pain – Pain in absence of nociceptor stimuli – Original injury healed Damage to peripheral nerves or CNS often – CNS damage » Areas involved in transmitting nociceptive info Peripheral neuropathic pain – Damage to peripheral nerves Central neuropathic pain – Damage to central regions » Lesions of thalamus Phantom pain  Clinical presentation – Burning, pins and needles, stinging, aching, grinding, itching, constant – Distal region of phantom Rarely follows path of severed nerve – Report limb as in awkward position May attempt to move limb or reach for objects – Telescoping common – Last sensations to disappear have highest cortical representation – Pain often resembles pain that was present prior to amputation Somatosensory memory  Complex – Theories Neuromas – Sprouts have increased excitability – Conduction block does not eliminate SPC hypersensitivity – NMDA upregulation Imprinted memory of prior pain Mismatch between output and visual feedback Cortical reorganization – Sensations from stump referred to limb – Adjacent cortical areas » Stroking cheek elicits sensation in limb » Modality specific Phantom limb pain No phantom limb pain Amputated Intact side Amputated Intact side the body we perceive is in large part built into our brain-it's not entirely learned. In fact, you do not need the body to feel the body  Acute and chronic pain – Acute Serves biological purpose Pain for limited time Associated with tissue damage Stimulates SNS – Increased heart rate, resp. rate, sweating, etc – Chronic Outlives its usefulness Often not correlated with tissue damage Prolonged> 6 months Continuous or intermittent Complex and difficult to manage Depression often  Chronic pain syndrome (pathological or neuropathic pain) – Debilitating, affects millions, helplessness Medical care without success, stigma, family issues , depression – No survival advantage or value Nervous system damage or long-term changes – Tx: Pain is undertreated – NSAIDS Effective for conditions that are inflammatory in origin – Hard on stomach, cardiovascular issues, fluid retention, renal failure with long term use – Not for long term continuous use , Opioids Bind opiate receptors, Inhibit ascending pathways – Controversies? » Addiction, tolerance, need for dose escalation – Useful in some cases – Tolerance vs addiction – Risk of addiction low for true chronic pain pts. » Respiratory depression and confusion in elderly, Constipation, N/V – Opiate induced hyperalgesia is an issue – Antiseizure drugs : Lyrica, Gabapentin – Very useful for neuropathic pain – Cannabinoids – Antidepressants ( Cymbalta and others) – Conservative approaches – PT, Tens, acupuncture, traction,