Anatomical and Physiological Characteristics of the Ferret Lateral Rectus Muscle and Abducena Nucleus

摘要

The ferret has become a popular model for neurodevelopmental and physiological research in the visual system. Accordingly, we studied single motor units in the ferret extraocular system. Using extracellular stimulation, 62 individual motor units in the ferret abducens nucleus were evaluated for their contractile characteristics. Of these motor units, 56 innervated the lateral rectus (LR) muscle alone while 6 were split between the LR and retractor bulbi (RB) muscle slips. In addition to individual motor units, the whole LR muscle was evaluated for twitch, tetanic peak force and fatigue. Current pulses of 0.2ms duration for twitch generation were followed by 150 ms tetanic trains ranging from 50 Hz to 220 Hz. The abducens nucleus was examined with CTHRP/TMB histochemistry and found to contain an average of 183 motoneurons. Samples of LR mid muscle belly were found to contain an average of 4687 fibers, indicating an LR innervation ratio of 25.6:1. The abducens nucleus motor units showed a twitch cpntraction time of 15.4 ms, a mean twitch tension of 30.2 mg, and an average fusion frequency of 154 Hz. Single unit fatigue index averaged .634. Whole muscle twitch contraction time was 16.7 ms with a mean twitch tension of 3.32 g. The average fatigue index of whole muscle was .408. Compared to cat and squirrel monkeys, the ferret LR motor units contract more slowly yet more powerfully. The functional visual requirements of the ferret may explain these fundamental differences.