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: Centrally acting alpha - 2 adrenergic agonists are one of several pharmacologic agents used in the treatment of spasticity related to disorders of the central nervous system .
### Output Text:
Centrally acting alpha - 2 adrenergic agonists are one of several pharmacologic agents used in the treatment of spasticity related to disorders of the central nervous system . 