Imbalances of the PNS (Peripheral Nervous System)

Imbalances of the PNS

Because the sensory and motor cell bodies are in different locations within the PNS, nerve cell body disorders affect either the sensory or motor component, but usually not both. Demyelination or damage to the myelin sheath slows nerve conduction. This affects mostly heavily myelinated fibers, causing buzzing or tingling sensations, motor weakness, and reduced reflexes. Centrally located fascicles are most vulnerable to vas-cular disorders such as vasculitis or ischemia,­ which can cause sharp pain or burning sensations or motor weakness proportional to atrophy. The distal two-thirds of a limb are usually affected. Toxic-­metabolic or genetic disorders are usually symmetrical when they begin. Immune-mediated processes may be symmetric or asymmetric. Significant axon dysfunction is caused by damage to the axon transport system, especially the microtubules and microfilaments. Axonal degenera-tion slowly ascends, producing sensory loss and weak-ness. Damage to the myelin sheath such as by injury or Guillain-Barre syndrome may be repaired by surviving Schwann cells within 6–12 weeks. Regenera-tion is nearly impossible when the cell body dies and is unlikely when axons are totally lost.