Quantification of motor pathways to the pelvic floor in humans

The motor innervation of the pelvic floor plays a major role in defecation disorders such as fecal incontinence. It consists of central motor pathways and peripheral nerve fibers. Transcranial magnetoelectric stimulation of the brain and magnetoelectric stimulation of the lumbosacral motor roots were performed in 10 healthy volunteers. Motor evoked potentials were recorded from the external anal sphincter. This procedure allowed differentiation between a predominantly central and a solely peripheral component of the motor innervation of the external and sphincter. To compare these recordings with well-established data, motor evoked potentials were also recorded from the anterior tibial muscle. The central motor conduction time was 20.9 +/- 2.4 ms to the external anal sphincter and 14.8 +/- 2.3 ms to the anterior tibial muscles. Central motor conduction velocities were 40.7 +/- 5.2 and 55.5 +/- 7.6 m/s, respectively. This showed that conduction in the central fibers to the external anal sphincter was significantly slower than in those to the anterior tibial muscle. We conclude 1) that magnetoelectric stimulation allows differentiation between central and peripheral portions of the motor innervation of the pelvic floor, and 2) that central motor pathways innervating the pelvic floor differ significantly in their physiological properties from those innervating limb muscles.