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: Since astrocytes were always damaged in parallel with neurons in SNR it is proposed that the anatomical and functional interrelationship between neurons and astrocytes is particularly tight in SNR .
### Output Text:
Since astrocytes were always damaged in parallel with neurons in SNR it is proposed that the anatomical and functional interrelationship between neurons and astrocytes is particularly tight in SNR . 