Figure 4: Diagrammatic representation of the dorsal horn of the spinal cord (a) Primary sensory afferents carrying nociceptive sensation ends in the dorsal horn of the spinal cord. (b) The presynaptic terminals of the primary sensory afferents contain excess substance P following morphine + fosaprepitant treatment (red). Neurokinin 1 receptor (v) is present on the postsynaptic terminal. μ- receptor is present on both the presynaptic and postsynaptic terminals