In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Abnormal processing of somatosensory inputs in the central nervous system ( central sensitization ) is the mechanism accounting for the enhanced pain sensitivity in the skin surrounding tissue injury ( secondary hyperalgesia ) .
### Output Text:
Abnormal processing of somatosensory inputs in the central nervous system ( central sensitization ) is the mechanism accounting for the enhanced pain sensitivity in the skin surrounding tissue injury ( secondary hyperalgesia ) . 