Up and down the spinal cord: afferent and efferent innervation of the human external anal sphincter

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Abstract
Afferent and efferent pathways from the anal canal to the external anal sphincter muscle have been investigated in healthy volunteers by means of scalprecorded somatosensory‐evoked potentials following electrical stimulation within the anal canal and by means of motor‐evoked potentials recorded from the external anal sphincter muscle following transcranial magneto‐electrical stimulation of the motor cortex. Control recordings were obtained following posterior tibial‐nerve stimulation on the one hand and recordings form the anterior tibial muscle on the other hand. Latencies of both somatosensory‐evoked potentials following anal stimulation and motor‐evoked potentials recorded from the external anal sphincter were similar to those of the tibial muscle/nerve despite a longer distance. This may be due to slower conduction of the sensory and motor pathways from and to the pelvic floor in comparison to those pathways innervating the limbs, or to the fact that pelvic floor pathways comprise a larger number of neurons and synapses as has been shown in the motor pathways to the pelvic floor in animals.