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: Population spikes ( PS ) , which were evoked by stimulation of the LPP , were recorded in the DG region .
### Output Text:
Population spikes ( PS ) , which were evoked by stimulation of the LPP , were recorded in the DG region . 