Neurobiological aspects of the pelvic floor muscles involved in defecation

Neurobiological aspects of the organization of pelvic floor musculature are reviewed. Evolutionary considerations on the origin of these muscles indicate that they develop with specific attachments and function, i.e., do not derive from preexisting muscles such as the ones from the tail. Anatomically, pelvic floor muscles can be divided into 1) true sphincters and related muscles and 2) muscles which flank the visceral outlets. While in quadrupedal mammals the EAS behaves as a fast twitch muscle, in man this muscle has slow twitch characteristics. Like some epaxial muscles the EAS has a strong connectivity with its surrounding skin. In further analogy with some epaxial muscle the EAS, although endowed with muscle spindles, is devoid of the phasic, monosynaptic component of the stretch reflex. Onuf's nucleus which innervates pelvic floor muscles receives an important group of suprasegmental afferents including, probably, direct corticospinal fibers. Pelvic floor muscles play a fundamental role in signaling arrival of feces to the perineum. While sphincteric activity is important for continence, other mechanisms such as the anorectal angle and anal cushions are also of relevance. Although emphasis has been put on motor factors, fecal incontinence can also result from impairments in sensory mechanisms of the anorectal system. In diseases like amyotrophic lateral sclerosis, Werdnig Hoffman's and others there is selective sparing of neuropathology in Onuf's nucleus. In contrast, the nucleus is affected in some autonomic visceromotor neuronal disorders, e.g., Shy Drager syndrome, Fabry's disease. It has been suggested that Onuf's nucleus occupies an intermediate position between visceral and somatic nuclei.