Pain

Nocioception: response to noxious or damaging stimuli.
Nocioceptors: receptors which respond to potentially harmful stimuli. Exhibit plasticity (can adapt or remember painful events).

Pain threshold

Pain perception threshold - similar for all
Pain tolerance threshold (pain becomes unbearable) - controlled by limbic system.

Primary Afferent Nocioceptive (PAN) fibres

Ad fibres: myelinated, produce fast rapid phasic pain.
C fibres: unmyelinated, produce slow persistent tonic pain. Release substance P, prostaglandins, bradykinin leucotrienes, serotonin, histamine.

Mechanoreceptors

Do not distinguish between noxious and non-noxious stimuli.

Dorsal laminae

Gate theory

TENS/heat/massage work via A-fibres to supress signal from C-fibres, therefore closing the gate.

Opioids

C nocioceptors (long-acting) are sensitive to morphine. Ad fibres are not.

There are 3 main receptor sites

Mu receptorsAgonists bind here (morphine) Naloxone is a specific antagonist at this site.
Kappa receptorsAnalgesia but no respiratory depression. Morphine does not bind here. Pentazocine does ( a partial agonist).
Sigma receptorsVasomotor stimulation (hallucinations/paranoia)

Increasing endogenous opiates:

acupuncture (effect reversed by naloxone)
placebos
exercise

Stimulation-produced analgesia (SPA)

superficial massage
pressure massage
vibration
heat/cold
TENS
acupuncture

A fibres degenerate more rapidly with age than C fibres, therefore elderly less likely to get relief from TENS/physio as more C fibre impulses manage to pass through the gate:

Gate

Assessment of pain

Pain rating scales eg McGill questionnaire: 4 areas of description
- Sensory (descriptive)
- Affective (feelings generated)
- Evaluative (severity)
- Miscellaneous (descriptive)
Behavioural assessment
- Somatic intervention (medication/surgery)
- Impaired functioning (Reduced mobility/occupational)
- Pain complaints (verbal and non-verbal communication)

Managing pain Motivational Affective Cognitive "Medical" Pain management plan Medication Specific aids eg massage Exercise "hurt not harm" Rest Relaxation Focussing on things other than pain Healthy habits "building a life around health rather than the pain" 10% have treatable depression