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: The purpose of this investigation was to extend these findings to a persistent pain model which could be distinguished from acute pain models on the basis of the nociceptive fibers activated , neurochemical substrates , and duration of the nociceptive stimulus .
### Output Text:
The purpose of this investigation was to extend these findings to a persistent pain model which could be distinguished from acute pain models on the basis of the nociceptive fibers activated , neurochemical substrates , and duration of the nociceptive stimulus . 