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: Increased expression of neuronal nitric oxide synthase in bladder afferent pathways following chronic bladder irritation .
### Output Text:
Increased expression of neuronal <span class="chemical">nitric oxide</span> synthase in bladder afferent pathways following chronic bladder irritation . 