The major pathological processes that affect the peripheral nervous system are axonal degeneration (distal axonopathy), segmental demyelination, and neuronopathy. Axonal degeneration or "dying back neuropathy" characteristically has a predilection for large diameter and long fibres. Segmental demyelination refers to primitive alteration of the myelin sheath. Neuronopathy is due to initial damage and death of neuronal cell bodies. Human peripheral nerve lesions result in a severe degeneration of axotomized axons and death of some neuronal cell bodies. In general, sensory neurons are more susceptible to injury induced cell death than motor neurons. Peripheral nerve injuries and neuropathies cause disability because of incomplete recovery of function despite optimal treatment. Therefore, the study of the induction and enhancement of peripheral nerve regeneration has become the research aim of many investigators.
It is widely agreed that thyroid hormones are one of the most important physiological regulators of mammalian nervous system development and maturation. Clinical and experimental studies report that thyroid deficiency in embryonic or early postnatal life has a marked influence on the function of the central and peripheral nervous systems. The major consequence of hypothyroidism in peripheral nerves,of the neonatal rat is a reduction in the number of myelinated and non-myelinated fibers. Therefore, thyroid hormones, which are required for the development and maturation of the nervous system, could be an important factor involved in peripheral nerve regeneration.
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