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: A role for nerve growth factor ( NGF ) in contributing to increased voiding frequency and altered sensation from the urinary bladder has been suggested .
### Output Text:
A role for nerve growth factor ( NGF ) in contributing to increased voiding frequency and altered sensation from the urinary bladder has been suggested . 