Motoneurons of the Lateral and Medial Rectus Extraocular Muscles in Squirrel Monkey and Cat

Abstract

The goal of this study was to examine and compare the number and size of motoneurons in the cat and squirrel monkey abducens nucleus. We also examined medial rectus muscle motoneuron compartmentalization in the squirrel monkey oculomotor nucleus and compared those cells to abducens nucleus motoneurons. Retrograde labeling of the motoneurons, using cholera toxin conjugate of horseradish peroxidase (CTHRP) injected into cat and monkey lateral or medial rectus muscles, was observed after 24 h. The CTHRP was histochemically localized with tetramethylbenzidine. The slide-mounted sections were analyzed using a computerized imaging system. Cat abducens nucleus motoneurons showed a wide range of cell sizes (26.0–66.0 µm, mean = 37.2 ± 6.2 µm), four or more dendrites per cell and an average of 1,418 cells within a relatively loosely packed nucleus. Squirrel monkey abducens nucleus motoneurons were significantly smaller than those in the cat with a narrower range of cell sizes (20.0–44.0 µm, mean = 31.7 ± 3.8 µm), four or more dendrites per cell and an average of 2,473 cells densely packed within the nucleus. Squirrel monkey medial rectus muscle motoneurons were organized into MRa, MRb and MRc subgroups. MRa motoneurons comprise the primary innervation for the medial rectus muscle and were similar in size to abducens nucleus motoneurons while the MRc subgroup cells were significantly smaller in size. Similar relationships among medial rectus motoneurons have been seen in rhesus monkeys. The relationship of these anatomical findings to previous physiological results regarding the generation of extraocular muscle force in the squirrel monkey is discussed.