Intrinsic differences in axonal growth from crayfish fast and slow motoneurons

The motoneurons innervating the fast and slow flexor muscles in the abdomen 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 branches. Axons of the slow flexor (SF) motoneurons, which innervate the thin (slow) flexor muscle, produce small terminal arbors with many fewer branches. To determine whether intrinsic factors contribute to these differences in terminal arbors, we compared regenerating axonal arbors from these two populations of motoneurons. We used an explant of the crayfish nerve cord in which axons from the FF and SF motoneurons regenerate on a homogeneous substrate. We found that regardless of the substrate, FF motor axons produced arbors with a greater total length and a greater number and density of branches than SF motor axons. These differences in regenerated arbors persisted in defined medium and in the absence of impulse activity, indicating that they result from intrinsic, neuron-specific factors. The greater branching of the FF motor axons 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.