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Specimens from the human male and female external urethral sphincter and the periurethral levator ani muscle have been examined using histochemical and electron microscopic techniques. In both sexes the external sphincter consists of a single population of type I (slow twitch) fibres with a mean diameter of 17.47 +/- 0.7 micrometers in the absence of muscle spindles. In contrast, the periurethral levator ani possesses muscle spindles and the constituent fibres form a heterogeneous population of type I and type II (fast twitch) fibres, with mean diameters of 45.5 +/- 0.8 micrometer and 59.5 +/- 3.4 micrometers respectively. These findings indicate that the external urethral sphincter is functionally adapted to maintain tone over prolonged periods and may be of considerable importance in producing active urethral closure during continence. The anatomical location and fibre characteristics of the levator ani muscle suggest that these fibres actively assist in urethral closure, particularly during events which cause elevation of intra-abdominal pressure. In view of the differences in fibre characteristics between the external urethral sphincter and the levator ani, EMG activity recorded from a single site in the levator ani may not be representative of the functional status either of other levator ani muscle fibres or of the external urethral sphincter.
Gosling et al. (1981) studied this question.