In the spinal cord, opioid receptors primarily influence pain transmission by acting on which components?

In the spinal cord, opioid receptors primarily influence pain transmission by acting on which components?

• A. Motor neurons in the ventral horn
• B. Primary afferent neurons in the dorsal horn and interneurons
• C. Cortical neurons in the prefrontal cortex
• D. Sensory neurons in the dorsal root ganglion

Answer: (B)

Opioids in the spinal cord mainly dampen nociceptive transmission at the dorsal horn circuitry. They do this by acting on presynaptic terminals of primary afferent neurons and on local interneurons there. Activation of mu receptors on the presynaptic terminals reduces calcium influx, which decreases release of excitatory neurotransmitters like glutamate and substance P from C and A-delta fibers. At the same time, mu receptors on dorsal horn neurons enhance inhibitory influences, hyperpolarizing second-order neurons and curbing their firing. This combination effectively limits the signal that would be sent up the spinal cord to higher centers. This is why the action is best described as targeting the primary afferent terminals in the dorsal horn and the interneuronal network there. It’s not primarily on motor neurons in the ventral horn, nor on cortical neurons in the prefrontal cortex, and while opioid receptors can be present in the dorsal root ganglion, the central mechanism for spinal analgesia centers on the dorsal horn synapses rather than the DRG cell bodies.

Opioids in the spinal cord mainly dampen nociceptive transmission at the dorsal horn circuitry. They do this by acting on presynaptic terminals of primary afferent neurons and on local interneurons there. Activation of mu receptors on the presynaptic terminals reduces calcium influx, which decreases release of excitatory neurotransmitters like glutamate and substance P from C and A-delta fibers. At the same time, mu receptors on dorsal horn neurons enhance inhibitory influences, hyperpolarizing second-order neurons and curbing their firing. This combination effectively limits the signal that would be sent up the spinal cord to higher centers.

This is why the action is best described as targeting the primary afferent terminals in the dorsal horn and the interneuronal network there. It’s not primarily on motor neurons in the ventral horn, nor on cortical neurons in the prefrontal cortex, and while opioid receptors can be present in the dorsal root ganglion, the central mechanism for spinal analgesia centers on the dorsal horn synapses rather than the DRG cell bodies.