How is pain perceived?
Sanjay Manohar (Oxford 2015)
Definition; Roles of pain in behaviour
Adaptive function. Phylogenetically old.
1. Receptors = free nerve endings, polymodal but some specificity
- mechanoreceptors - channels
- thermoreceptors - channels (Thermal grill illusion)
- chemoreceptors (itch)
- damage / stress - acid release -- TRP receptor channels
2. Fibres. cell body in DRG. glutamatergic.
light myelination unmyelinated
up to 15 m/s <2 m/s
complete adaptation slow adapting
more modality specific more polymodal
Anoxia experiment local anaesthetics (lignocaine, VGNaC)
3. Spinal cord
lamina I/V -> lissauer tract (2 segments)
substantia gelatinosa lamina II
decussation, lateral and ventral spinothalamic tract
4. VPL/VPM -> S1 postcentral gyrus
Spinohypothalamic - hormonal and autonomic-visceral responses
Spinoreticular tract -> reticular formation -> cortex: arousal
Referred pain - larger RFs; convergence?
Subjective intensity is variable
Potentiation by damage, sensitisation by chronic pain
- hyperalgesia, allodynia, neuropathic pain
prostaglandin, bradykinin, Nociceptin (NSAIDs)
Substance P (a tachykinin) = signals cell damage, potentiates afferent, also central cotransmitter.
Top down control -
Gelatinosa & Periaqueductal grey: mu and kappa opioid receptors (DBS)
dynorphins, enkephalins, endorphins: natural neurotransmitter peptides (->opiate drugs);
G-protein coupled - modulate transmission. Tachyphylaxis, addiction
Manipulate expectation socially/visually; hypnosis, placebo
Bottom up control
Melzack and Wall 1960s gate theory:
Abeta tactile -> inhbitory interneuron. TENS
Effect: reflex withdrawal, arousal, perception, learning.
compare the injured war hero with the chronic pain sufferer
adaptive vs maladaptive pain
scale! arguably the humanity's biggest problem?