Dr.
Nurhayu Ab Rahman
Aim
Describe the physiology and mechanisms of pain
Definition
Pain is an unpleasant sensory and emotional experience with actual or potential tissue damage or described in terms of such damage
-International Association for Study of Pain
Pain
Unique in the sense that :
Behavioral response Autonomic response Emotional response
Pain Why?
Signaling/ screening mechanism- to prevent tissue
injury Learning tool Initiate healing response Necessary for survival Annoying neuralgia
Classification of Pain
Duration Acute or chronic Region & System Headache, Backache Cause Somatic
Nociceptive, neuropathic
Psychogenic
Pain Pathways
Different route from non-noxious ,other type of
sensory modalities (e.g touch, pressure, thermal, vibration, stretch, etc) Ascending Neural Pathways
Tissue damage nociceptor nerve spinal
cord/brainstem thalamus higher centre
Descending Neural Pathways Motor mechanisms e.g restriction of movement Pain modulation pathways Descending Inhibitory Pathways (Analgesic system) Endogenous Opioid System
Chemical basis of nociception
Noxious stimulus release of chemical mediators Histamine Bradykinin K+ , H+ Prostaglandins Leukotrienes Serotonin Substance P
Activation of Nociceptor
3 types of pain receptor : A-delta fibers
Small diameter (2-5um), Myelinated fast conducting Response to mechanical and thermal stimuli that cause tissue damage Transmit initial wave acute ,sharp pain
C-polymodal Neurones (C-PMN) Small diameter (0.3-3 um),Unmylinated, slow , dull ache Respond to strong/intense mechanical, thermal, and chemical stimuli
Silent nociceptor Mechanically insensitive Only active in presence of tissue injury
Nociceptor poor adaptation to stimulation
A. Shelley, A. Cross. Pathophysiology of pain. Symposium on pain management, part 1. Mayo Clin Proc. 1994;69:375-83.
Classic pain pathway
Sensory afferent fibers onward transmission via spinal
ganglia ( dorsal nerve root) - First Order Neurone
Or trigeminal ganglion (orofacial)
Synapse at Spinal Cord ( Substantia Gelatinosa) - Second
Order Neurone
Or V Brainstem Complex (orofacial )
Transmitted to Thalamus ( Third order Neurone) Ventroposterior Lateral Nucleus (VPL) (neck, trunks and limbs) Ventroposterior medial Nucleus (VPM) (orofacial) Transmitted to Cerebral Cortex
Sympathetic activation
The afferent nociceptive fibers that carry pain impulse are in
close proximity to sympathetic nerve fibers Autonomic response therefore one of the manifestation that accompany pain sensation
nd 2
Order Neurones
Receive both noxious ( C and A-delta) and non-noxious ( tactilelarge fiber sensory afferent) Wide receptive field Only receive input from C and A-delta fibers Excited only high intensity noxious stimulus
Categorized as WDR wide dynamic neurones
NS Nociceptive- specific neurones
Ability to localise, detect, discriminate and grade the
intensity of noxious stimuli
Thalamic Projection
Project to Primary sensory area of cerebral cortex Play a role in the sensory discriminative dimension of pain
Other part as well Hypothalamus neuroendocrine response Anterior cingulate cortex ( implicated in the affective or motivational dimension of pain
It is now well accepted that the sensation of pain that
is experienced arises in the CNS by means of two pathways
Sensory Discriminative- System ( Spatio- Temporal
Analysis
Burning, prickling, location, intensity, duration
Motivational Affective- System ( Central Intensity
Monitor)
Give rise to unpleasant character of painful sensation
Gate Control Theory
The transmission of nerve
impulse from afferent fibers to spinal cord central transmission cell is modulated by gating mechanism depended on relative amount of activity in LARGE diameter fiber and SMALL diameter fiber L fiber close gate S fiber open gate The Gating mechanism also influenced by descending input from the brain When input exceeds threshold, it activates the action system
Pain Modulation
Descending Inhibitory Pathway (Analgesic System) Periaqueductal grey matter (PAG) Nucleus Raphe Magnus (NRM) Descending neurone that terminate in Substantia Gelatinosa
Stimulation a/w release of Central Serotonin and Endorphin -induce inhibition of pain transmission anti-depressant
Endogenous Opioid System Endorphine bind to morphine receptor to suppress pain Repeated injections will cause tolerance and dependance
Referred Pain
Pain that is felt in an area
that is innervated by a different nerve from the one that cause the primary pain ( original site of tissue damage) Convergence input theory multiple afferent sensory input synapse at single second order neurone to carry impulse to higher centre. Difficulty in determine location
Pain Management
Pharmacological
NSAIDS Corticosteroid Local Anaesthetics Capsaicin Opioids Serotonin Agonist Anticonvulsant
Physical
Stretch Therapy Jaw Exercise Massage Heat application TENS Acupuncture
Psychological
Information Counselling Education Relaxation Hypnosis Psychotherapy
Summary
THE END
