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: The integral of the compound action potential in optic nerve was significantly reduced , and the threshold for action potential generation was increased , indicating a reduction in the level of functional plasma membrane sodium channels .
### Output Text:
The integral of the compound action potential in optic nerve was significantly reduced , and the threshold for action potential generation was increased , indicating a reduction in the level of functional plasma membrane <span class="chemical">sodium</span> channels . 