Motoneurons of the lateral and medial rectus extraocular muscles in squirrel monkey and cat

The goal of this study was to examine and compare the number and size of mo toneurons in the cat and squirrel monkey abducens nucleus. We also examined medial rectus muscle motoneuron compartmentalization in the squirrel monke y oculomotor nucleus and compared those cells to abducens nucleus motoneuro ns, Retrograde labeling of the motoneurons, using cholera toxin conjugate o f horseradish peroxidase (CTHRP) injected into cat and monkey lateral or me dial rectus muscles, was observed after 24 h. The CTHRP was histochemically localized with tetramethylbenzidine. The slide-mounted sections were analy zed using a computerized imaging system. Cat abducens nucleus motoneurons s howed a wide range of cell sizes (26.0-66.0 mum, mean = 37.2 +/- 6.2 mum), four or more dendrites per cell and an average of 1,418 cells within a rela tively 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 mum, mean = 31.7 +/- 3.8 mum), 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, M Rb and MRc subgroups. MRa motoneurons comprise the primary innervation for the medial rectus muscle and were similar in size to abducens nucleus moton eurons while the MRc subgroup cells were significantly smaller in size. Sim ilar relationships among medial rectus motoneurons have been seen in rhesus monkeys. The relationship of these anatomical findings to previous physiol ogical results regarding the generation of extraocular muscle force in the squirrel monkey is discussed. Copyright (C) 2001 S. Karger AG, Basel.