, 2009, Rossignol and Ruxolitinib Dubuc, 1994 and Thompson et al., 2011). Raphespinal axons arise from cells in the midline raphe

(Figure 6) and travel caudally through the spinal cord as dispersed bundles of axons neighboring the central gray matter (Figure 6). Complete lesions of raphespinal axons require extensive bilateral lesions that extend ventrally well below the central canal. Accordingly, the most reliable model for examining regeneration of this system is a complete spinal cord transection or crush (Figure 6C). While there has been some question regarding the existence of intrinsic serotonin-containing neurons with the spinal cord that would complicate the assessment of axonal regeneration even below a complete transection site, routine serotonin immunohistochemistry with an antibody to 5-hydroxytyptamine (5HT) does not detect residual neuronal or axonal labeling below a complete injury (Figure 6C). Although there are few reports of regeneration after complete lesions (Coumans et al.,

2001), the extent of regeneration reported is modest. Many previous studies report treatment-related increases in serotonergic axons below an injury and growth of serotonergic axons into partial spinal cord lesion sites containing cell grafts (Lu et al., 2003). Such growth could result either from regeneration of transected axons or sprouting of neighboring axon terminals that were spared by the lesion. Distinguishing between Dasatinib in vivo these is probably impossible, so “increase in serotonergic axon density” or not “axon growth into the lesion site” is the most appropriate phrases for describing these forms of axon growth. Rubrospinal projections are considered to be rudimentary in humans although this point is not entirely settled (Nathan and Smith, 1982 and ten Donkelaar, 1988). In rodents, rubrospinal axons arise from the magnocellular division of the red nucleus (Figure 7A), cross the midline, and project through the dorsal part of the lateral column of the spinal cord and modulate motor function. (Küchler et al., 2002 and Morris

et al., 2011). Rubrospinal axons can be labeled by making tracer injections into the brainstem (Figures 7D and 7E show the pathway after injections in a mouse). The rubrospinal tract can be completely transected by lateral funicular lesions, which therefore represent an attractive model system for the study of mechanisms underlying motor axon regeneration, albeit with the important caveat that the projection is of limited importance in humans. Rubrospinal axons exhibit a greater capacity to regenerate than CST axons (Liu et al., 1999). This system, like others, is also subject to the caveat that growth into or beyond a lesion site can arise from either sprouting of spared axons or regeneration of transected axons unless it can be confirmed by complete reconstruction of axons extending past the lesion that growth originated from an axon that was unequivocally cut.