INTRINSIC DIFFERENCES IN AXONAL GROWTH FROM CRAYFISH FAST AND SLOW MOTONEURONS

The motoneurons innervating the fast and slow flexor muscles in the ab domen of crayfish form morphologically distinct motor terminals. Axons of the fast flexor (FF) motoneurons, which innervate the large (fast) flexor muscle, produce extensive motor terminal arbors with many bran ches. Axons of the slow flexor (SF) motoneurons, which innervate the t hin (slow) flexor muscle, produce small terminal arbors with many fewe r branches. To determine whether intrinsic factors contribute to these differences in terminal arbors, we compared regenerating axonal arbor s from these two populations of motoneurons. We used an explant of the crayfish nerve cord in which axons from the FF and SF motoneurons reg enerate on a homogeneous substrate. We found that regardless of the su bstrate, FF motor axons produced arbors with a greater total length an d a greater number and density of branches than SF motor axons. These differences in regenerated arbors persisted in defined medium and in t he absence of impulse activity, indicating that they result from intri nsic, neuron-specific factors. The greater branching of the FF motor a xons may be related to differences in growth cones: growth cones of FF axons were significantly larger with more filopodia than growth cones of SF axons. (C) 1995 Academic press, Inc.