FIGURE 3-1 Size discrepancy between the rat and the human spinal cords.

The human spinal cord is more than four times as long as the entire CNS of the rat. (A) A caudal segment of the human spinal cord, including the cauda equina. The human cauda equina is approximately the same length as the entire CNS of a rat, which includes its brain. (B) The diameter of the human spinal cord is also much larger than that of the rat spinal cord. Twenty slices of a rat spinal cord can fit inside one slice of a human cord.

SOURCE: Reprinted with permission, from Dobkin and Havton, 2004. Copyright 2004 from Annual Reviews.

times the length of the 1 to 3 millimeters often affected by contusion injuries in rats (Metz et al., 2000). Consequently, regeneration of nerve fibers over a few vertebral segments in a rat—which can result in the restoration of function—is equivalent to only a fraction of the distance that is needed to restore function in humans (Dobkin and Havton, 2004). Furthermore, because neurons from both species demonstrate the same degree of spontaneous sprouting of their axons, approximately 2 millimeters (von Meyenburg et al., 1998), there are added complexities in promoting sufficient axon

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