Anatomic, physiologic, and pharmacologic properties of the feline bulbospongiosus muscle.

Histologic examination was used to determine the anatomic organization of the feline bulbospongiosus muscle (BSM). Bilaterally symmetrical cranial, middle, and caudal muscle fiber groups were identified. The caudal group was the largest of the 3 groups. Mean cross-sectional diameter of nonfixed BSM fibers was 79 micron. Muscle spindles were not observed in serial histologic sections. The innervation of the BSM was provided by the deep perineal nerve. Contractile properties of the BSM were determined in vitro, using direct muscle stimulation. Mean time-to-peak tension values were 71.4 ms and 62.7 ms at 30 C and 37 C, respectively, and were longer than that for the feline periurethral striated muscle and external anal sphincter muscle, but slightly shorter than that for the slow-twitch soleus muscle. Stimulation frequency at peak tetanic tension was 40 Hz, twitch-to-tetanus ratio was 0.30, and stimulus interval at peak tension for double response was 44 ms. Electrophysiologic properties examined were resting potentials and miniature end-plate potentials (MEPP). Mean resting potential was 68 mV. The MEPP had low mean amplitude (0.57 mV), fast mean rise time (0.99 ms), and low maximal frequency (0.06 Hz). Only focally recordable MEPP were observed. Pharmacologic properties were investigated by alternating direct and indirect stimulation. The latter was accomplished by electric field stimulation of intramuscular nerve fibers. A high sensitivity of the BSM to d-tubocurarine blockade was present. The results of this study support classification of the BSM as a slow-twitch muscle.