Comparative study of the periurethral and perianal parts of the human levator ani muscle.

The present study has used histochemical and electron microscopic techniques to provide a quantitative comparison of the striated muscle fibre populations in the periurethral and perianal regions of the human levator ani muscle. Both regions consist predominantly of type I (slow twitch) fibres together with a small number of type II (fast twitch) fibres. However, marked differences have been demonstrated between the two regions with regard to proportions of the two fibre types, diameters of the constituent fibres and the distribution of muscle spindles. These observations are discussed in relation to the differing functional activity of the periurethral and perianal regions of the levator ani muscle.