The spread an opiate within the epidural space.

A opioid is seen being injected through an epidural catheter. The drug spreads laterally through the epidural space and enters the spinal canal particularly in the region of the nerve roots. It then spreads to exert its principal effects on pre- and post-synaptic receptors in the substantia gelatinosa of the dorsal horn. This is in contrast to the effect of local anaesthetic solutions whose effects are exerted primarily in the region of the emerging spinal nerves.