Submitted on May 30, 2007
Accepted on August 23, 2007
Anatomical and Physiological Characteristics of the Ferret Lateral Rectus Muscle and Abducens Nucleus
Keith Neil Bishop1, J. Ross McClung1, Stephen J. Goldberg1, and Mary S. Shall2*
1 Anatomy & Neurobiology, Virginia Commonwealth University, Richmond, Virginia, United States
2 Physical Therapy, Virginia Commonwealth University, Richmond, Virginia, United States
* To whom correspondence should be addressed. E-mail: msshall{at}vcu.edu.
The ferret has become a popular model for physiological and neurodevelopmental research in the visual system. We felt it important, therefore, to study extraocular whole muscle as well as single motor unit physiology in the ferret. 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. The abducens nucleus motor units showed a twitch contraction 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. The abducens nucleus was examined with CTHRP histochemistry and found to contain an average of 183 motoneurons. Samples of LR were found to contain an average of 4687 fibers, indicating an LR innervation ratio of 25.6:1. 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.
