Pain perception

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Pain perception

Pain can arise from the central nervous system, the autonomic nervous system or from the periphery. The definition of pain includes pain as being both a physiological (sensory) and an emotional experience ( influenced by psychological and emotional factors).

Descriptions of pain:
Paraesthesia - ‘pins and needles’ (an abnormal sensation that is not unpleasant)
Causalgia – ‘burning’ or ‘searing’ sensation
Dysaesthesia – ‘burning’, ‘numbness’, ‘tingling’ (an unpleasant abnormal sensation)
Formication – ‘insects crawling on the skin’
Hyperpathia – painful response to a light touch (eg clothing)

Pain receptors:
Nociceptors:
free noncorpuscular peripheral nerve endings
stimulated by intense thermal, mechanical or chemical stimuli
when stimulated  release neuropeptides (eg substance P)  convert stimulus into electrical activity
most nociceptors lie dormant  inflammation sensitises them

Skin nociceptors  two afferent nerve fibres (C fibres and A-delta fibres)

A-delta nociceptors:
group III afferents
high threshold mechanoreceptors
small diametre fibres
transmit more rapidly than C fibres
transmits the ‘sharp immediate pain’  serves as a warning of impending tissue damage

C-polymodal nociceptors:
• group IV afferents
• polymodal – responds to either thermal, mechanical or chemical stimulus
• small diametre unmyelinated fibres
• transmit impulses relatively slowly – the ‘second pain’
• transmits the prolonged ‘burning pain’

Muscle nociceptors:
Mostly free nerve endings in the walls of arterioles and connective tissue. Two types of nociceptors in skeletal muscle (group III and group IV)

Central pathways:
Three type of neurons transmit sensory information to the CNS – non-nocioceptive neurons (transmit proprioceptive information); nocioceptive specific neurons (transmit A-delta nocioceptive inputs from the skin); wide dynamic range neurons (transmit a wide range of stimuli)

Nocioceptive information is transmitted via two pathways:
• neospinothalamic pathway (arises in dorsal horn where A-delta nocioceptive fibres terminate; ascends in contralateral tract to lateral part of thalamus; responsible for the localisation and identification of noxious stimuli)
• spinoreticular tract/paleo-spino-reticulo-diencephalic pathway (arises in dorsal horn from where the non-myelinated C fibres terminate; ascends next to neospinothalamic pathway; terminates into brainstem’s reticular tissue and cerebral cortex)

Experience of pain:
A persons pain tolerance is dependent on a number of factors. Eg:
• emotional response
• psychological factors
• cultural background
• ethnic origin
• attitude of parents towards pain during childhood
• mood (anxiety and depression reduced pain tolerance)
• stressful environment (eg severe wounds in war may not be noticed)
• distractions (eg music)
• past experiences of pain

Measurement of pain:
The verbal questioning of pain on a 0-10 scale is often not sufficient due to the multidimensional aspects of pain.

Verbal Description Scale (VDS):
0=no pain; 1=mild pain; 2= discomforting; 3=distresssing; 4=intense; 5=excruciating
Most popular system

Visual Analogue Scale (VAS):
10cm line on which the patient marks the spot for pain intensity and this is measured. One end of the line represents ‘No pain’ and the other represents ‘worst possible pain’.

Happy Faces rating Scale:
Used with children.
Has poor validity.

McGill Pain Questionnaire (MPQ):
Includes descriptors of pain (chosen from 30 groups of adjectives) and a pain drawing.

Measurement of the impact of pain:
eg disability indexes/questionnaires

Emotional aspects of pain:
Emotional aspects of pain are the negative feelings that mediate the behavioural responses to the pain.

Acute pain serves a useful protective purpose (eg evokes flexor withdrawal reflex); main emotional response to acute pain is anxiety, but this can be mediated by the experiences of pain noted above.

Chronic pain has no useful purpose; those with chronic pain tend to be more anxious, depressed, irritable, angry and resentful . Serotonin activity in CNS is decreased  this may play a role in insomnia, depression and a decreased pain tolerance (may also be due to a lowering of endorphin levels)

Behavioural aspects of pain:
Pain influences behaviour. Examples of behaviours include sleep disturbances; verbal attacks on others; decreased activity levels; abdication of social roles

Many chronic pain patients present with varying degrees of psychological and behavioural problems that exacerbate pain.

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