The pharmacological effects of the opioid analgesics are derived from their complex
interactions with three opioid receptor types (μ, δ, and κ; morphine is an agonist at the μ
opioid receptor). These receptors are found in the periphery, at presynaptic and postsynaptic
sites in the spinal cord dorsal horn, and in the brain stem, thalamus, and cortex, in what
constitutes the ascending pain transmission system, as well as structures that comprise a
descending inhibitory system that modulates pain at the level of the spinal cord. The cellular …