Neurotransmitters nociceptive afferent fibers

Two neurotransmitters are released by the nociceptive afferent fibers in the dorsal horn of the spinal cord. These neurotransmitters, which stimulate the... 

Opioid-induced analgesia is produced through the action of opioid receptors on presynaptic terminals of the C-fibers and A-delta fibers. These fibers, which transmit nociceptive messages, are inhibited by the indirect effects of opioids, which in turn reduce the release of neurotransmitters such as substance P, CGRP, and glutamate. This effect occurs in the peripheral nervous system as well as at the primary afferent terminals in the spinal cord. 

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