2 Neuroscience2.7 Neural circuits
2.7.1.5 Pain and chronic pain
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Written by Resident Doctor Committee · Published
Pain and nociception
Pain is an unpleasant sensory and emotional experience, and nociception is neural processing of potentially damaging stimuli. Either can occur without the other.
Primary afferent fibres
Fibre | Myelin | Speed | Pain carried |
|---|---|---|---|
A-delta | Thinly myelinated | Fast | First pain: sharp, well-localised |
C | Unmyelinated | Slowest | Second pain: dull, burning, diffuse |
Wind-up is the progressive amplification of dorsal horn responses by repeated C fibre input.
Spinothalamic fibres cross near entry and ascend contralaterally, so cord hemisection reduces pain and temperature sensation on the opposite side below the lesion.
Descending modulation
Descending pathways through the periaqueductal grey and rostroventral medulla can inhibit or facilitate dorsal horn transmission.
Supraspinal (brainstem and midbrain) endogenous opioid analgesia acts principally through mu receptors.
Kappa and delta receptors act mainly at spinal level, and sigma is not an opioid receptor.
Gate control theory holds that large fibre touch input and descending influences regulate small fibre nociceptive input, which is the rationale for rubbing an injury.
Sensitisation and chronic pain
Peripheral sensitisation lowers nociceptor thresholds through inflammatory mediators.
Central sensitisation increases responsiveness within nociceptive circuits and can spread beyond the injured area. NMDA-dependent calcium signalling contributes.
Term | Meaning |
|---|---|
Allodynia | Pain from a normally non-painful stimulus |
Hyperalgesia | Increased pain from a normally painful stimulus |
In chronic pain, pain, low mood and poor sleep reinforce each other.