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: We investigated the relationship between the degeneration of spinal motor neurons and activation of N - methyl - d - aspartate ( NMDA ) receptors after neuraxial morphine following a noninjurious interval of aortic occlusion in rats .
### Output Text:
We investigated the relationship between the degeneration of spinal motor neurons and activation of <span class="chemical">N - methyl - d - aspartate</span> ( <span class="chemical">NMDA</span> ) receptors after neuraxial <span class="chemical">morphine</span> following a noninjurious interval of aortic occlusion in rats . 