sprouting axons from the proximal segment toward the target organ. The
injury causes Wallerian degeneration distal to the level of injury and prox-
imal axonal degeneration to at least the next node of Ranvier. In more
severe traumatic injuries, the proximal degeneration may extend beyond
the next node of Ranvier. Electrodiagnostic studies demonstrate denerva-
tion changes in the affected muscles, and in cases of reinnervation, motor
unit potentials (MUPs) are present (answer a).Axonal regeneration occurs
at the rate of 1 mm/day (answer b)or 1 in./month and can be monitored
with an advancing Tinel’s sign. The Tinel’s sign is observed when tapping
over nerve trunk that has been damaged or is regenerating following
trauma causes a sensation of tingling and pins in its distribution up to the
site of regeneration. A nerve trunk will regenerate about 1 mm/day (see
transport rates discussed in the next paragraph). If this sign is absent, there
is a poor prognosis. The endoneurial tubes remain intact (answer d),and,
therefore, recovery is complete, with axons reinnervating their original
motor and sensory targets.
The process of response to injury is referred to as Wallerian degener-
ation. Axonal regeneration occurs in neurons if the perikarya survive fol-
lowing damage. The segment distal (answer e)to the wound, including
the myelin, is phagocytosed and removed by macrophages. The proxi-
mal segment is capable of regeneration because it remains in continuity
with the perikaryon. Chromatolysis is the first step in the regeneration
process, in which there is breakdown of the Nissl substance (RER, ribo-
somes), swelling of the perikaryon, and migration of the nucleus periph-
erally. Degeneration of perikarya and neuronal processes occurs when
there is extensive neuronal damage. Transneuronal degeneration occurs
only when there are synapses with a single damaged neuron. In the pres-
ence of inputs from multiple neurons, transneuronal degeneration does
not occur.
Axonal transport occurs by several different mechanisms. Slow
axonal transport/dendritic transport (1–5 mm/day) involves the move-
ment of cytoskeletal elements such as actin, tubulin, and neurofilaments
from the perikaryon down the axon. Rapid anterograde (away from the
perikaryon) transport and retrograde (toward the perikaryon) transport
(200–300 mm/day) transports membrane-bound organelles, for example,
newly formed secretory vesicles and mitochondria anterogradely. Receptors,
recycled membranes, and worn-out organelles are transported retrogradely.
244 Anatomy, Histology, and Cell Biology