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: It is known that beta - adrenoceptor antagonists are effective in the treatment of akathisia , one of the extrapyramidal side effects that occur during neuroleptic treatment .
### Output Text:
It is known that beta - adrenoceptor antagonists are effective in the treatment of akathisia , one of the extrapyramidal side effects that occur during neuroleptic treatment . 