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J Appl Physiol 103: 1706-1714, 2007. First published August 23, 2007; doi:10.1152/japplphysiol.00580.2007
8750-7587/07 $8.00
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Anatomic and physiological characteristics of the ferret lateral rectus muscle and abducens nucleus

Keith N. Bishop,1 J. Ross McClung,1 Stephen J. Goldberg,1 and Mary S. Shall2

Departments of 1Anatomy and Neurobiology, and 2Physical Therapy, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, Virginia

Submitted 30 May 2007 ; accepted in final form 23 August 2007

The ferret has become a popular model for physiological and neurodevelopmental research in the visual system. We believed 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 0.634. 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 0.408. The abducens nucleus was examined with horseradish peroxidase conjugated with the subunit B of cholera toxin histochemistry and found to contain an average of 183 motoneurons. Samples of LR were found to contain an average of 4,687 fibers, indicating an LR innervation ratio of 25.6:1. Compared with 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.

extraocular; motoneuron; eye movement; oculomotor


Address for reprint requests and other correspondence: M. S. Shall, Dept. of Physical Therapy, Medical College of Virginia Campus, Virginia Commonwealth Univ., 1200 East Broad St., PO Box 980224, Richmond, VA 23298-0224 (e-mail: msshall{at}vcu.edu)




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