Physiology of pain
- Pain is an unpleasant sensory and emotional experience
- Associated with potential or actual tissue damage
- It is a complex interaction of
- Sensory factors
- Emotional factors
- Behavioural factors
- Stimuli activate the nociceptive system
- Conveyed to brain by an adaptable pathway
- Pain is only experienced in the conscious brain
Somatic pain
First pain
- First or 'fast' pain is a protective response
- Allows rapid withdrawal from a painful stimulus
- Due to stimulation of high threshold thermo / mechanical receptors
- Transmitted by fast myelinated A fibres
- Enter dorsal horn of spinal cord
- Secondary fibres in spinothalamic tract transmit stimulus to
posterior thalamic nuclei
- Tertiary fibres transmit stimulus to somatosensory post-central
gyrus
Secondary pain
- Secondary or 'slow' pain is responsible for delayed sensation of
pain
- Elicits behaviour to protect damaged tissue
- Initiates reflex responses to pain such as tachycardia, hypertension
and increased respirator rate
- Due to stimulation of high threshold polymodal receptors
- Respond to mechanical, thermal and chemical stimuli
- Transmitted by slow unmyelinated C fibres
- Enter dorsal horn
- Secondary fibres in palaeospinothalamic tract transmits stimulus to
medial thalamic nuclei
- Collateral transmit to midbrain, medullary reticular formation and
hypothalamus
- Further information is transmitted to forebrain limbic system
Visceral pain
- Fewer visceral nociceptors than somatic receptors
- Cortical mapping is less concentrated
- Visceral pain is poorly localised
- Pain is also qualitatively different due to progressive stimulation
and summation
- May also be referred to site away from source of stimulation
Physiology
Peripheral activation
- Most pain originates after tissue damage
- Due to the local release of inflammatory mediators
- Mediators involved include:
- Leukotrienes D4 and B4
- Bradykinin
- Histamine
- 5HT
- Activate or sensitise high threshold nociceptors
- Results in primary hyperalgesia
Spinal level
- Occurs in dorsal horn of spinal cord
- Complex interaction between excitatory and inhibitory interneurons
- Also involves descending inhibitory tracts in spinal cord
- Gate Control Theory explains non-linear relation injury and response
- Suggests that pain can be 'gated-out' in dorsal horn by other
stimuli
- Neurotransmitters involved are:
- Excitatory amino acids
- Neuropeptides
- Some neuropeptides increase nociception - Substance P,
bombesin, VIP
- Other neuropeptides reduce nociception - galanin, somatostatin, GABA
Supraspinal level
- Perception of pain is associated with activity in:
- Thalamus
- Primary and secondary cortex
- Various regions are involved with descending inhibition
- Originate at level of cortex and thalamus
- Mediated via relays in brainstem and dorsal columns to dorsal horns
- Mediators involved:
- Noradrenaline
- 5HT
- Endogenous opioids
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