The classification of nerve disorders is based on the severity of the injury and its potential for reversibility. The least severe nerve injury is neuropraxia, or first-degree nerve injury. Distortion of the myelin about the nodes of Ranvier caused by ischemia, mechanical compression, or electrolyte imbalance produces temporary loss of nerve conduction. Neuropraxia has the greatest potential for complete recovery, usually within 2 or 3 months. A more extensive injury caused by interruption of the axon with secondary Wallerian degeneration distal to the site of injury is defined as axonotmesis, or second-degree nerve injury. The supporting connective tissue surrounding the axon and Schwann cells, however, is preserved, and the recovery period, ranging from weeks to months, depends on the distance between the site of injury and the end organs. Complete disruption of the nerve and its supporting structures is seen in neurotmesis, or third-degree nerve injury. This is the most severe degree of injury, with no potential for regeneration.
Neurotmesis has been further divided by Sunderland  into three subtypes: (1) third-degree nerve injury, in which the endoneurium is disrupted with intact perineurium and epineurium; (2) fourth-degree nerve injury, in which all neural elements, sparing the epineurium, are disrupted; (3) and fifth-degree nerve injury, in which there is complete transection and discontinuity of the nerve. Surgical end-to-end anastomosis or nerve grafting depends heavily on the chronicity of the process.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.