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: CONCLUSIONS : Sural nerve SAP amplitude reduction is a reliable and sensitive marker of degeneration and recovery of sensory fibres .
### Output Text:
CONCLUSIONS : Sural nerve SAP amplitude reduction is a reliable and sensitive marker of degeneration and recovery of sensory fibres . 