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 activation of spinal N - methyl - D - aspartate receptors may contribute to degeneration of spinal motor neurons induced by neuraxial morphine after a noninjurious interval of spinal cord ischemia .
### Output Text:
The activation of spinal <span class="chemical">N - methyl - D - aspartate</span> receptors may contribute to degeneration of spinal motor neurons induced by neuraxial <span class="chemical">morphine</span> after a noninjurious interval of spinal cord ischemia . 