Question

Question 5 1.5 pts Your patient sustained a clean cut to the forearm that partially lacerated the median nerve. Under what circumstances is axon regeneration more likely (list 3)? 50% Laceration

Answer 1

An injury causing partial nerve injuries; i.e. a neuroma-in-continuity (i.e. all degrees of nerve injury may co-exist). Surgical exploration is necessary, using electrodiagnostic methods intra-operatively, and tentative nerve reconstruction with nerve grafts due to a loss of substance creating a nerve defect.

Intracellular signals after a nerve injury

After severe nerve injuries, such as nerve transection and nerve laceration injuries, there is a cascade of different intra-neuronal signals elicited from the site of lesion, including the ones released from invading macrophages and reacting Schwann cells, that are transported up to the nerve cell body.These injury-induced signals, occurring in different phases, make the neurons able to switch from transmission state to a regenerative state causing changed expressions of many genes involved in survival or death of the neuron as well as in regrowth of the axons.

Wallerian degeneration and cellular reactions after nerve injury

The segment of the nerve distal to the site of a lesion undergoes the process that is known as Wallerian degeneration, which starts immediately after an injury.This process involves degeneration of the axons and breakdown of the myelin, with subsequent proliferation of the Schwann cells, recruitment of immune cells, such as macrophages (eliminating myelin and cell debris), as well as remodelling of the tissues. These events have the purpose to create an optimal milieu for the axonal outgrowth after the injury.

The proliferation of Schwann cells, forming an optimal pathway for the outgrowing axons, is accompanied by upregulation of a large number of different trophic factors in the Schwann cells supporting the axons during the regrowth processes that decline over time, leading to impaired nerve regeneration.Thus, a nerve repair or reconstruction procedure should be done promptly due to the fact that Schwann cells have ‘a best before date’ after a nerve injury.

Axonal outgrowth and re-myelination

Soon after the nerve injury, the distal tip of the proximal axon emits numerous collateral sprouts that cross the injury zone and then the formed axons regenerate into the distal nerve end at a rate of around 1 mm to 2 mm/day in humans if the milieu is optimal. In successful cases, there is a re-myelination of the axons in which the size of the axon as well as the thickness of the myelin increase. The diameter of the regenerated axons does not reach pre-injury levels; thereby the conduction velocity will be below normal. After completion of the nerve regeneration process, the intracellular activities in the nerve cell body go back to a transmitter mode.