This conclusion was also corroborated by experiments mentioned below. Because a Lonafarnib mw mouse’s muscles and skeleton are growing at the time branches are being removed, it is possible

that, despite the loss of a large number of branches, there is no net loss of axonal material supported by each motor neuron soma. In particular, no net loss might occur if the remaining branches had to elongate to keep pace with the growth of the muscle. To explore this issue, we calculated the length, surface area, and volume of axonal motor units within the sternomastoid muscle at various developmental ages. Our measurements showed that branch pruning did cause the total length of an axon’s branches within the muscle to decrease between birth and 13 days postnatally. However, the total amount of axoplasm in

these arbors actually increased (Figure 2E; see Experimental Procedures for details learn more of analysis). The net increase in axonal material was due to an increase in both the length within the muscle of the remaining axon branches and an increase in their calibers. Thus, despite the profound branch loss, a motor neuron’s total axoplasmic volume once the axon reaches the target muscle is actually increasing over this developmental period. Given that the distance between the muscle target and the neuronal cell body is also increasing due to animal growth, the total increase in axoplasm per neuron is even greater than what we have measured. One potential reason for the pruning is that it restricts the spatial extent of an axonal arbor to focus an initially diffuse projection into a more circumscribed area. In the small clavotrapezius and cleidomastoid muscles, nearly all adult motor units extend throughout Adenosine the entire muscle, so spatial focusing cannot be occurring in these muscles (Lu et al., 2009). We could analyze the possibility of spatial refinement of motor axons in the sternomastoid muscle because in maturity, each motor axon was confined to a small subregion of the muscle (Figure 3A) (see also Keller-Peck et al., 2001). We found that relative to the area of the

muscle, there was no significant change in the extent of motor arbors between the young ages and later (compare Figures 3A and 3B). The fact that motor axon arbors do not become more limited in extent implies that the impetus for branch removal at early stages is not based on the position of the branch within the muscle. This result is also consistent with the data mentioned above arguing against proximal branch trimming, because each proximal branch typically projects to nonoverlapping regions of the muscle’s endplate band (see also Lu et al., 2009); therefore loss of a proximal branch would have been expected to focus an axon’s projection to a smaller territory. The results already described indicate that axons innervate more postsynaptic target cells at birth than later.