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: These tachykinins , in turn , initiate a micturition reflex by stimulating NK1 and NK2 receptors .
### Output Text:
These <span class="chemical">tachykinins</span> , in turn , initiate a micturition reflex by stimulating NK1 and NK2 receptors . 