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: In contrast , the conduction velocity , the number and size of axons in the optic nerve , and the specific localization of Na ( v ) 1 . 6 channels in the nodes of Ranvier were unchanged .
### Output Text:
In contrast , the conduction velocity , the number and size of axons in the optic nerve , and the specific localization of <span class="chemical">Na</span> ( v ) 1 . 6 channels in the nodes of Ranvier were unchanged . 