Wallerian Degeneration and Regeneration PDF

Neuroscience I Dr. L. Young-Martin WALLERIAN DEGENERATION AND REGENERATION The neurone’s cell body maintains the functional and anatomic integrity of the axon i.e. if the axon is severed, that part distal to the cut degenerates. There are 5 different grades of injury: Grade I injury (e.g. applied pr...

Neuroscience I Dr. L. Young-Martin WALLERIAN DEGENERATION AND REGENERATION The neurone’s cell body maintains the functional and anatomic integrity of the axon i.e. if the axon is severed, that part distal to the cut degenerates. There are 5 different grades of injury: Grade I injury (e.g. applied pressure) is the least harmful, Grade V injury ((e.g. complete severance of the nerve) is the most harmful. Processes of degeneration and regeneration which occur in a fifth degree injury are as follows: I. (a) (b) (c) (d) CHANGES IN THE NERVE CELL BODY – Retrograde Reaction Chromatolysis occurs within 24 hours of injury – i.e. Nissl granules begin to disappear. The process is complete within 15 – 20 days. The Golgi apparatus then starts to disintegrate and neurofibrils also disappear. The cell body imbibes water and swells, assuming a spherical shape. The nucleus is then pushed to one side of the cell body. NB: If the nucleus is extruded – degeneration of the neurone occurs. If the nucleus is retained – regeneration of the neurone occurs. Repair begins ~20 days following injury and is usually complete in ~80 days following injury. Nissl granules and Golgi apparatus gradually reappear, and the cell regains its normal size II. CHANGES IN THE NERVE FIBRE Processes that occur distal to the site of injury and within 24 hours of injury include: (a) The myelin sheath breaks down (macrophages have been implicated in this process). (b) Orthograde or Wallerian degeneration occurs from the point of damage to the terminal endings. Retrograde degeneration occurs to the nearest collateral or axon branch (c) The Schwann cells divide mitotically forming cords of cells lying within the endoneurial tubes. (d) Macrophages from the endoneurium invade the degenerating myelin sheath and axis cylinder and remove debris. As debris is removed, Schwann cell cytoplasm fills the endoneurial tubes. The process takes - 3 months. Processes that occur at the site of injury include: a. Schwann cells elongate and send processes outward to the injured site. This is known as Regenerative Sprouting. b. Neurofibrils from the proximal end begin to grow and are guided by the Schwann cells into the old cylinder, in which they grow and eventually form the new axon. c. Schwann cells then spin new myelin sheaths around the cylinder and gradually the entire neurone is regenerated. REGENERATION 1. 2. 3. 4. 5. Along with the above processes, the central axon elongates and then grows out in all directions. Regeneration fibrils appear to be guided by strands of Schwann cells of the peripheral endoneurial tubes. Schwann cells filling the endoneurial tubes form the medullary sheath around the successful fibril similarly to the developing nerve fibre. The peripheral nerve rarely achieves more than 80% of its original diameter. Regeneration does not normally occur in the CNS. (NB Oligodendrocytes form the myelin sheaths around several nerve cells in the CNS). Neuroscience I Dr. L. Young-Martin Notes to Remember Injury to peripheral nerves often results in Denervation Supersensitivity of the end-organ or muscle that was innervated. Suggestions put forward to explain this phenomenon are: i) The denervated end-organ becomes hyper-responsive to circulating neurotransmitters. ii) Regenerative sprouting to new areas of the end-organ / tissue may have occurred during regeneration of the injured nerve; iii) There is lack of reuptake of secreted neurotransmitters; iv) There is up-regulation of the receptors on the end-organ