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: However , neither alterations in prejunctional beta - adrenoceptor influences nor alterations in neuronal and extraneuronal uptake mechanisms appear to be responsible for this alteration .
### Output Text:
However , neither alterations in prejunctional beta - adrenoceptor influences nor alterations in neuronal and extraneuronal uptake mechanisms appear to be responsible for this alteration . 