Primary and secondary hyperalgesia
After a tissue injury, the area around the injury becomes hypersensitive and light touch can elicit pain. There are two aspects to the hyperalgesia: primary hyperalgesia is defined as the hypersensitive area at the site of the injury. It is due to an 'inflammatory soup' of endogenout algesic agents.
Secondary hyperalgesia develops in uninjured
tissue surrounding the site of injury, and is thought to be a result of sensitization in the central nervous system, including the dorsal horn.
The diagram opposite indicates that mechanisms that increase synaptic strength within the dorsal horn
may be responsible. Upregulation of the production of peptide neurotransmitters, changes in gene transcription in the cells of the dorsal horn and changes in the effects of descending contol pathways may all play a part in the responses.
The lower diagram opposite indicates that in hyperalgesic states there is a leftward shift in the stimulus response curve, indicating that pain is induced at lower stimulus intensities.
Allodynia is the mild pain that is present in response to light touch stimuli, and hyperalgesia is the hightened response to a painful stimulus.