The development of a technique to estimate the number of motor units in the urethral sphincter

Abstract

An electromyographic technique is presented that has been developed in an effort to quantitate the number of motor units in the urinary sphincter. Sphincter contraction was provoked by electrical stimulation and recorded with a catheter mounted electrode. Incremental stimulation produced corresponding increments of sphincter contraction that were recorded as the magnitude of the sphincter electromyogram. Estimates of the number of motor units in the sphincter were made by analysis of variance of a Poisson distribution. Using this method, the mean number of motor units in the sphincter of five dogs was estimated to be 129 with values ranging from 80 to 182. Anatomical correlation was sought by infiltrating this sphincter with horseradish peroxidase, which is taken up by axonal transport to stain the motor neurons in the spinal cord. The mean number of motor neurons was 229 with values ranging from 116 to 358. There are technical and physiologic explanations for the observed differences between the predicted and the actual number of motor neurons stained and this is addressed in the discussion.