This is because when an axon is severed, the portion of the axon distal to the point of injury dies (i.e. the part that is no longer connected to the neuronal cell body). This death precipitates a number of changes that involve other cells, notably the Schwann cells. The overall process is termed Wallerian degeneration. The only way a particular axon can become functional again is through growth of the proximal part out to the periphery following the original route. The success of this regeneration is critically dependent on the cell's ability to navigate over this route. Whether it succeeds is determined most by the degree of integrity of the connective tissue components of the nerve. Thus a crush injury which severs axons but spares the connective tissue is associated with more successful regeneration, whereas a completely severed nerve will almost certainly not regenerate without surgical intervention.
The first degree involves temporary malfunction in a portion of the axon (black) such as can occur with pressure, poor circulation or cold.
The second and subsequent degrees include the severance of the axon, and consequent Wallerian degeneration. The prognosis for a second degree injury is generally for good regeneration once the insult is withdrawn, but very long periods of time can be involved. Regrowth of axons to the periphery rarely exceeds 1mm/day. Consider how long axons would take to extend from an upper arm injury to the hand.
The prognosis is poor for degrees 3 onwards. The loss of the endoneurium (3) and the perineurium (4) restrict axonal growth and navigation.
The fifth degree, complete nerve section, is likely to be permanent unless the cut ends of the nerve can be brought into apposition, usually through microsurgery. If part of the nerve is absent, as can occur following removal of a tumour, or in severe accidents, the gap must be bridged with something through which the axons can grow. In practice this means a piece of nerve from elsewhere must be microsurgically placed into the gap. This requires sacrifice of a length of nerve from elsewhere in the body, so is only done if the cut nerve's function is very important.
Some neurological terminology related to Sunderland's scale.
Department of Physiology,
University of Sydney
Last updated 10 April 2002