Abstract
Histochemical characteristics of the external anal sphincter muscle (EAS) of the rabbit were compared with those of the extensor digitorum longus muscle (EDL) and soleus muscle (Sol).
EAS was composed of 0.2% type 1, 62.5% type 2A and 37.3% type 2B fibers, while EDL was composed of 7.6% type 1, 23.2% type 2A and 69.2% type 2B fibers, and Sol almost solely of type 1 fibers. Fiber diameters of each fiber type in EAS were from one half to two thirds those of corresponding ones in EDL or Sol.
In recent years combined studies on the physiological and histochemical properties of each fiber type have demonstrated that type 1 fibers are slow twitch and fatigue-resistant, type 2A fibers, fast-twitch and fatigue-resistant, and type 2B fibers, fast-twitch and fast-fatigue. EAS as well as EDL therefore composed of fast-twitch fibers, though the former is composed predominantly of type 2A fibers, and the latter predominantly of type 2B fibers. The predominance of type 2A fibers in EAS may be qualified for the unique function of keeping the anal canal closed to fecal urgency.
