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: Therefore , rostral ventrolateral medulla neurons appear to play a significant role in maintaining hypertension in conscious spontaneously hypertensive rats .
### Output Text:
Therefore , rostral ventrolateral medulla neurons appear to play a significant role in maintaining hypertension in conscious spontaneously hypertensive rats . 