spinal cord injury
Traumatic spinal cord injury leads to a series of reactive changes.
First, the injury could directly damage the cell bodies and/or processes of neurons. The cells that are damaged might die and, as far as is known, they are not replaced (although the adult spinal cord does contain stem cells). However, the functional consequences of this neuronal loss are typically modest.
Often, the damage to spinal axons is much more important. The spinal cord is segmentally arranged and the sensory, motor and autonomic functions of each segment depend crucially on connections with supraspinal sites for all conscious or voluntary actions.
Damage to these connections leaves spinal segments caudal to the lesion site partially or totally isolated from the brain, which has debilitating consequences.
The distal axon segment (the part isolated from the neuronal cell body) retracts from postsynaptic neurons and undergoes Wallerian degeneration, and although the proximal segment typically survives, it does not spontaneously regenerate.
The injured axon also encounters a series of inhibitory cues within the injured CNS neuropil, which further prevent a successful regenerative response.