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: The pathophysiology of painful temporomandibular disorders is not fully understood , but evidence suggests that muscle pain modulates motor function in characteristic ways .
### Output Text:
The pathophysiology of painful temporomandibular disorders is not fully understood , but evidence suggests that muscle pain modulates motor function in characteristic ways . 