[Organization and innervation of striated muscle fibers of the striated sphincter in the rat].

Objectives: To determine the organization of striated muscle fibre (FMS) of the rat striated sphincter (SS) and to characterize the motor endplates (MEP).

Material And Methods: The urethras of 30 male and female rats were studied. Two thousand hematein-eosin stained serial sections and 800 sections simultaneously stained for MEPs and nerve endings were studied.

Results: The SMFs of the SS are organized in the urethral wall in four symmetrical bands. MEPs are predominantly found in the cranial third of the SS (22 MEPs per section). Sixty sixteen per cent of MEPs were situated in the lateral regions. Motor units were composed of a maximum of five SMFs.

Conclusion: The SMFs of the SS are organized in four symmetrical bands. Myoconnective insertions of SMFs emphasize the importance of the connective tissue in their contractile action. They are innervated by a single MEP, in the same way as skeletal striated muscle fibres. In the SS, MEPs are predominantly located laterally to the right and to the left. The specific organization of motor units suggests an original mechanism of reinforcement of muscle contraction of the SS.