“Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Pain is always subjective. Each individual learns the application of the word through experience related to injury in early life. It is unquestionably a sensation in a part of the body but it is also always unpleasant and therefore also an emotional experience.” (Definition of pain; International Association for the Study of Pain, 1979)

“A Canadian woman born with an indifference to painful stimuli had no other sensory deficits & was quite intelligent. Despite early training to avoid damaging situations, she developed progressive degeneration of her joints & spinal vertebrae, leading to skeletal deformation, degeneration, infection, &, finally, death at age 28.Apparently, low levels of nociceptive activity are important during everyday tasks to tell us when a particular movement or prolonged posture is putting too much strain on our body. Even during sleep, nociception may be the prod that makes us toss & turn enough to prevent bedsores or skeletal strain.People with a congenital absence of pain reveal that pain is a separate sensation, not simply an excess of the other sensations. Such people usually have a normal ability to perceive other somatic sensory stimuli.”

From: Bear MF, Connors BW, Paradiso MA (2001) Neuroscience: Exploring the Brain. 2nd Edn.Lippincott, Williams & Wilkins, Baltimore.

“The index case for the present study was a 10-year-old child, well known to the medical service after regularly performing ‘street theatre’. He placed knives through his arms & walked on burning coals, but experienced no pain. He died before being seen on his 14th birthday, after jumping off a house roof… All 6 affected individuals had never felt any pain, at any time, in any part of their body… All had injuries to their lips (some requiring later plastic surgery) &/or tongue (with loss of the distal third in 2 cases), caused by biting themselves in the first 4 yr of life. All had frequent bruises & cuts, & most had suffered fractures or osteomyelitis, which were only diagnosed in retrospect because of painless limping or lack of use of a limb. The children were considered of normal intelligence by their parents & teachers, & by the caring physicians…”

From: Cox JJ et al. (2006) An SCN9A channelopathy causes congenital inability to experience pain. Nature 444, 894-898.

• Pain is a distinct modality

• Pain is sensed by specific receptors (nociceptors) projecting through distinct pathways

• Nociceptors respond to noxious (damaging or potentially damaging) stimuli

• The main submodalities of pain are “fast” and “slow” pain

[Pain] [Touch]

From: Payne R, Allen RR (1998) Pain. Scientific American Medicine

Excitatory synapses drive the postsynaptic neuron’s membrane potential towards threshold, increasing the probability of an action potential

Inhibitory synapses drive the postsynaptic neuron’s membrane potential away from threshold, decreasing the probability that an action potential will be generated.

Emotion

Learning &Memory

LIMBIC SYSTEM

Painful stimuli evoke protective reflexes as well as sensations & emotions

Pain can be controlled at several levels

TERMS

• Analgesia: loss of sensibility to pain (e.g. Aspirin is an analgesic)

• Anaesthesia: total loss of sensation (e.g. Barbital is an anaesthetic)

ANAESTHESIA

• Local anaesthesia: produced by injection of an anaesthetic drug in a limited area (e.g.injection of novocaine near a branch of the trigeminal nerve for tooth extraction)

• General anaesthesia: loss of consciousness in addition to loss of sensation (e.g. inhalation of fluothane for abdominal surgery)

TRAUMAmechanical

thermalchemical

NOCICEPTORSENSITISATION

NOCICEPTORSTIMULATION

DORSALHORN

THALAMUS

ASSOCIATION CORTEXpain perception

Prostaglandinsleukotrienessubstance P

aspirin

K+, 5-HT, histamine,bradykinin

hyperalgesia

A-delta fibres, small, myelinated,5-30 m/sec, sharp, pricking pain

C fibres, small, unmyelinated,0.5-2 m/sec, dull, aching pain

xylocaine

morphine opioids Acupuncture,cutaneousstimulation

spinothalamic

SOMATOSENSORYCORTEX

location, intensityLIMBIC SYSTEMemotional aspects

HYPOTHALAMUSautonomic responses

?

general anaesthetics

IC Bruce, 2000

Pain

From: Payne R, Allen RR (1998) Pain. Scientific American Medicine

Endogenous pain control via opioids

DISSOCIATIVE ANAESTHETICS

• Central analgesia with an anaesthetic-like state of altered consciousness

• Produced by compounds like phencyclidine (“Angel Dust”) and ketamine

REFERRED PAIN

• Pain perceived as coming from a source remote from its actual origin

• Characteristic of pain in the viscera

• e.g. heart pain “referred” to the left arm in angina pectoris

• pain in the diaphragm “referred” to above the clavicle in pleurisy

From: Payne R, Allen RR (1998) Pain. Scientific American Medicine