Pain Physiology Flashcards
Pain definition
Unpleasant sensory & emotional experience associated w or resembling that associated w actual or potential tissue damage
Nociception
Neuronal process of encoding noxious stimuli:
- reception/conduction & CNS processing of nerve signals generated by stimulation of nociceptors
Consequences of encoding nocicpetion
Autonomic (elevated BP)
Behavioral (motor withdrawal reflex)
Pain sensation is not necessarily implied
Nociceptive pathway
Primary afferent (incoming) tissue nociceptor
Crosses at spinal cord
2nd projects to brain
Tertiary projects to cortex/higher structure
Ascending pain pathway
Perception (cerebral cortex)
Modulation (spinal cord)
Transmission (sensory nerves)
Transduction (sensory nerve endings, nociceptors)
Transduction
Alpha-delta nociceptors
Composed of mechanoreceptors & mechanothermal receptors
- low thresholds (<75%)
- high thresholds (<25%)
Discharge of alpha-delta nociceptors
Discharged at higher rate than C-fiber nociceptors
- provide more discriminative info to CNS
- responsible for pricking & sharp qualities of 1st pain
C-fiber nociceptors
Almost all high threshold & respond to different types of stimulation (polymodual)
Activation is responsible for slow-onset (2nd) pain that occurs after the initial insult
secondary pain to c-fiber nociceptors
Burning & aching qualities
Signals tissues damage & inflammation that initiates self-preservation behaviors such as avoidance, guarding and disuse
Silent of sleeping nociceptors in transduction
Present in both A-delta fibers & C-fibers
High threshold activated by tissue damaging events
Important role in peripheral sensitization
Pathway of transducing pain to action potential
Lesion stimulates bradykinin, serotonin, prostaglandin, K+, histamine (from mast cell)
Sends activation of nociceptors
Depolarization of cell membrane
Action potential
Origination of electrical signals
ES are transmitted by nociceptive fibers to dorsal root ganglion at spinal cord
A-deltoid fibers receive electrical signals
Small diameter 1-4um
Myelinated
Fast conduction
Transit well, localized pricking, sharp pain
“First pain”
C-fibers features for receiving electrical signals
Small diameter 0.4-1.2um
Unmyelinated
Slow conduction
Transmit poorly localized dull or aching pain
“Second pain”
Tracts for transmitting signals
Signal cross from gray to white matter & travels through ascending tracts
Spinothalamic
Spinocervical
Spinoreticular
Spinohypothalamic
Spinothalamic tract
Most important in transmission of nociceptive information
Spinocervical tract
Important role in carnivores
Modulation
Dorsal horn of the spinal horn
Amplification (activation) or suppression (inhibition) of the nociceptive impulses
Regulation of NMDA receptor
Modulating the release of substances like P and GABA
Activation of modulation
Excitatory or inhibition Interneurons which transmit sensory information for short distance in spinal cord
Activation of modulation - withdrawal reflex
Propriospinal neurons involved in segmented reflex activity extend to Supra spinal centers (midbrain & cortex) through ascending tracts to synapse the 3rd order neurons located in medulla, pons, midbrain, thalamus, hypothalamus and cerebral cortex
Location of 3rd order neurons
Medulla, pons, midbrain, thalamus, hypothalamus and cerebral cortex
Function of perception
End result of neuronal activity of pain transmission. Pain becomes a conscious multidimensional experience
Cortical areas activated during perception
Reticular area
Somatosensory cortex
Limbic system
Response of reticular system
Autonomic & motor response & warning to do something (move away from insult)
Response during activation in Somatosensory cortex
Identifies intensity, type and location of pain, relates to past experience & memory
Response to activation in limbic system
Emotional & behavioral responses to pain (attention, mood, motivation)
Inhibitory mechanisms of descending pathways
Noradrenergic pathway
Serotonergic pathway
Opioid system
Location & mechanisms of noradrenergic pathway
Locus coeruleus to dorsal horn
Stimulates adrenergic receptors (alpha 2)
Decrease in release of P
- hyperpolarizing 2nd order neuron
- depolarizing the GABAergic neurons to inhibit pain transmission
Mechanism & function of serotonergic pathways
Nucleus raphe magnus to dorsal horn
Excitatory effects (5-HT2A and 5-HT3 receptors)
Release of enkephalins
- inhibit release of sub P
- hyperpolarizing 2nd order neuron
Opioid system locations
Periaqueductal gray
Raphe magnus
Dorsal horn of SC
Function of opioid system at periaqueductal gray
Glutamate excites enkephalins
Inhibits GABA
Activate glutamatergic neuron
Opioid system at raphe magnus
Allows glutamate to excite enkephalins
Inhibits GABA
Allows excitation of serotonin neuron
Opioid system at dorsal horn of SC
Acts as presynaptic opioid receptor
- inhibits Ca influx = inhibits release of subs P
Postsynaptic promotes K efflux = hyperpolarization = inhibits pain transmission
Enhances inhibition of GABA in descending modulatory neuron
Peripheral sensitization activators
Primary hyperalgesia
Sub p
Neurokinin A
Calcitonin gene related peptide (CGRP)
——>
Peripheral sensitization results
Excitability of sensory & sympathetic fibers
Promote vasodilation
Extravasation of plasma proteins
Recruitment of inflammatory cells
Peripheral sensitization - sensitizing soup
Cytokines
Serotonin
Prostaglandins & leukotrienes
Hydrogen & potassium ions
Nitric oxide
Bradykinin
Histamine
Others
Sensitizing soup ions function
Lower response threshold for A-delta and C fiber activation
Activation of silent norciceptors
Central sensitization
Brain (thalamus)
Exaggerated response of normal inputs
Wind up
Wind up includes
Alterations in dorsal horn neuronal function
Reduction of neuronal firing threshold
Increase in neuronal response associated w the given stimulus
After-discharge or spontaneous neuronal signaling
Spread of increased sensitivity to adjacent neurons
Pathophysiological effects of treating pain
^ anxiety, depression, HR, RR, coagulability
V metabolism, immune function
Pathophys effects - immune system
^ cortisol levels impair wound healing and v immune system function
^ ADH, catecholamines, renin, angiotensin II, aldosterone
V insulin and test
Pathophys effects - neuroedocrine
Activated by pain
Glucogenesis is favored - hyperglycemia
Impaired metabolism results in increased protein catabolism, lipolysis and cachexia
Pathophys effects - GIT
Sympathetic stimulation can cause shunting of blood, v motility, v mucosal integrity
Pathophys effects - cardio system
Activation of Renin-angiotensin system
Fluid retention (water & sodium)
Elevated BP
V renal perfusion
^ HR, SV, CO and myocardial O2 consumption
Pathophys effects - resp t
Atelectasis
Ventilation perfusion mismatch
Arterial hypoxemia
Hypercarbia
Pathophys effects - coagulation system
^ platelet adhesiveness
V fibrinolysis
Hypercoagulation
Psychological effects of treating pain
Patients quality of life is decreased w pain
Owners can feel frustration or guilty of procedures
Caregivers feel guilt or lack of compassion if unable to treat pain
