Effects of intrathecal morphine and spinal anaesthesia on sympathetic nerve activity in humans.

Microneurography with direct registration of efferent sympathetic nerve activity to muscle or skin was used to study the effect of morphine and procaine given intrathecally to patients scheduled for transurethral resection of the prostate. A lumbar spinal catheter was inserted and multiunit postganglionic sympathetic activity was recorded in a muscle or skin fascicle of the peroneal nerve. After injection of 0.4 mg morphine, sympathetic nerve activity to muscle (n = 6) or skin (n = 2) was recorded for 30 min. Compared with a 10-min control period prior to the injection no change in activity could be observed. In contrast, spinal anaesthesia (procaine 75-100 mg) with an upper level of sensory blockade at T3-T11 completely eliminated intraneurally recorded sympathetic activity within 5 min (n = 8). Skin vasodilation and loss of sudomotor responses in the foot accompanied the neural blockade.