Effects of morphine on electrical activity of the rectum in man

1. The effects of morphine on the electrical activity of the human rectum were investigated. 2. In healthy volunteers, morphine (0.04-0.16 mg/kg, i.v.) induced spike activity that could become cyclical. 3. All the effects of morphine were antagonized by naloxone (0.03-0.2 mg/kg, i.v.), but not blocked by atropine (0.007-0.014 mg/kg, i.v.). 4. In patients with spinal cord injury, morphine was observed to have similar excitatory effects. Spinal cord transection was complete in all patients, so that participation of supraspinal nervous structures in these effects could be ruled out. The sacral parasympathetic nervous centres could not have been involved in two patients in whom the medullary cone was also destroyed. The thoracolumbar sympathetic nervous centres were not completely destroyed in any of the patients, however, so that the possibility that these centres may have been involved cannot be entirely ruled out. 5. Morphine failed to activate an aganglionic rectum in a patient with Hirschsprung's disease, indicating that it had no direct effect on smooth muscle cells. It is therefore probable that morphine may have an effect on the intrinsic innervation.