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 results indicate that neuronal gene expression in visceral sensory pathways can be upregulated by chemical irritation of afferent receptors in the urinary tract and / or that pathological changes in the urinary tract can initiate chemical signals that alter the chemical properties of visceral afferent neurons .
### Output Text:
These results indicate that neuronal gene expression in visceral sensory pathways can be upregulated by chemical irritation of afferent receptors in the urinary tract and / or that pathological changes in the urinary tract can initiate chemical signals that alter the chemical properties of visceral afferent neurons . 