neurologic disorders
Overview of Peripheral Nervous System Disorders
Pathophysiology
Because sensory and motor cell bodies are in different locations, a nerve cell body disorder typically affects either the sensory or motor component but rarely both. Damage to the myelin sheath ( Overview of Demyelinating Disorders ) slows nerve conduction. Demyelination affects mainly heavily myelinated fibers, causing large-fiber sensory dysfunction (buzzing and tingling sensations), motor weakness, and diminished reflexes. The hallmark of acquired demyelinating polyneuropathy is severe motor weakness with minimal atrophy. Because the vasa nervorum do not reach the center of a nerve, centrally located fascicles are most vulnerable to vascular disorders (eg, vasculitis, ischemia). These disorders result in small-fiber sensory dysfunction (sharp pain and burning sensations, decreased temperature sensation), motor weakness proportional to atrophy, and less severe reflex abnormalities than in other nerve disorders. The distal two thirds of a limb is affected most. Initially, deficits tend to be asymmetric because the vasculitic or ischemic process is random. However, multiple infarcts may later coalesce, causing symmetric deficits ( Multiple Mononeuropathy ). Toxic-metabolic or genetic disorders usually begin symmetrically. Immune-mediated processes may be symmetric or, early in rapidly evolving processes, asymmetric. Damage to the axon transport system for cellular constituents, especially microtubules and microfilaments, causes significant axon dysfunction. First affected are the smaller fibers (because they have greater metabolic requirements) at the most distal part of the nerve. Then, axonal degeneration slowly ascends, producing the characteristic distal-to-proximal pattern of symptoms (stocking-glove sensory loss, followed by weakness). Damage to the myelin sheath (eg, by injury or Guillain-Barré Syndrome (GBS) ) can often be repaired by surviving Schwann cells in about 6 to 12 weeks. After axonal damage, the fiber regrows within the Schwann cell tube at about 1 mm/day once the pathologic process ends. However, regrowth may be misdirected, causing aberrant innervation (eg, of fibers in the wrong muscle, of a touch receptor at the wrong site, or of a temperature instead of a touch receptor). Regeneration is impossible when the cell body dies and is unlikely when the axon is completely lost. 