Comparison of the motor discharge to the voluntary sphincters of continence in the rat

Background The rat external anal sphincter (EAS) and external urethral sphincter (EUS) are voluntary muscles of continence that can display similar synchronous electromyographic (EMG) activity patterns. However, the two sphincters are quite different in structure and function. The EUS is a fast twitch muscle and contains fibers expressing type 2B myosin. In contrast, the EAS exhibits slower kinetics and lacks type 2B fibers. This striking contrast in kinetics and fiber type profiles may be shaped by differences in the basal motor drive that each sphincter receives. Methods A double EMG approach was used to obtain spontaneously active single motor unit action potentials from the EUS and EAS simultaneously and compare their basal discharge frequencies in urethane anaesthetized rats. Key Results The basal firing rates of motor units of the EUS and EAS were not significantly different (3.9±0.9Hz vs. 3.1±1.6Hz, respectively, n=7 animals, P=0.32, paired Student's t-test). However, auto-correlogram analysis showed that EUS is driven by neurons with faster instantaneous firing frequencies: 30.5±2.4Hz vs 14.3±0.9Hz in the EAS (P=0.03, paired Student's t-test). Conclusions & Inferences The oscillator(s) driving the EUS operate(s) at a frequency twice that of the EAS. This may explain the presence of type 2B fibers in the EUS. In the inter-micturition periods no cross correlation was found in motor discharge to the sphincters suggesting that the two muscles do not share a common central drive to sustain the continent tonus of the two outlet tracts.