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: Animal studies suggest that incontinence secondary to serotonergic antidepressants could be mediated by the 5HT4 receptors found on the bladder .
### Output Text:
Animal studies suggest that incontinence secondary to <span class="chemical">serotonergic antidepressants</span> could be mediated by the 5HT4 receptors found on the bladder . 