An important hallmark of C3-C4 propriospinal
neurons is the establishment of a bifurcating axonal trajectory (Figure 8A). Descending collaterals establish synaptic connections to motor neurons at C6-T1 and interneurons, whereas ascending axon collaterals extend to the lateral reticular nucleus (LRN), which in turn gives rise to mossy fiber inputs to the cerebellum (Figure 8A). A series of lesion studies in the cat proposes an essential role of these relay neurons in target reaching of the MLN0128 concentration forelimb. Both excitatory and inhibitory neurons are contained within C3-C4 propriospinal neurons, but genetic identity of this specialized premotor action reporting system is currently unknown. The spinal cord is the origin of a diverse set of spinocerebellar projection neurons, establishing direct mossy fiber input to cerebellar granule cells (Orlovsky et al., 1999 and Oscarsson, 1965). Details regarding the anatomical and functional diversification of spinocerebellar projection neurons extend beyond the scope of
this Review; however, in considering these issues more broadly, a few important points can be Cell Cycle inhibitor made. Functionally distinct populations of spinocerebellar neurons are generally located at defined rostrocaudal segments in conserved laminar positions and establish projections to stereotyped cerebellar lobules. Ventral spinocerebellar tract (VSCT) neurons reside at lumbar
levels and are active preferentially during the flexion phase of stepping, monitoring intrinsic spinal network activity in the cat (Arshavsky et al., 1978) (Figure 8B). In contrast, Clarke’s column (CC) neurons are located more rostrally, receive direct sensory feedback (Walmsley, 1991), integrate this information with descending corticospinal input, and express the neurotrophic factor GDNF ( Hantman and Jessell, 2010) ( Figure 8B). Spatial and functional diversification of ascending spinal projection neurons highlights the need to understand the developmental and genetic cascades involved in their specification. much At the mechanistic level, neuron diversification along the rostrocaudal axis has been studied most extensively for motor neurons, in which combinatorial expression of different Hox transcription factors plays important roles (Dalla Torre di Sanguinetto et al., 2008, Dasen et al., 2005 and Dasen et al., 2008). Since all spinal neurons arise locally, these include long-distance projection neurons to supraspinal targets. A possible mechanism for generation of required diversity at the molecular level may therefore be an intersection between dorsoventral and Hox transcriptional networks. Recent studies have begun to address how early developmental diversification relates to connectivity and function and have added an important facet to our understanding of motor circuits.