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: Nissl - staining and antibodies against the neuron - specific calcium - binding protein , parvalbumin , served to detect neuronal damage in SNR .
### Output Text:
Nissl - staining and antibodies against the neuron - specific <span class="chemical">calcium</span> - binding protein , parvalbumin , served to detect neuronal damage in SNR . 