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The peripheral nerves, neuromuscular junction, and skeletal muscle represent the final component of the lower motor neuron unit and are discussed together for that reason. Diseases involving these structures result in muscle weakness. The peripheral nerves also have sensory and autonomic fibers, and—unlike diseases of muscle, which result in motor dysfunction alone—peripheral nerve diseases are manifested by a combination of motor and sensory loss.
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Peripheral nerves are composed of axons leading to and from sensory, motor, and autonomic neurons. Each individual axon is surrounded by a myelin sheath of varying thickness. The nerves contain Schwann cells, which form the myelin, and supporting connective tissue (endo-, epi-, and perineurium). The blood supply to peripheral nerves is by small arterioles called the vasa nervorum.
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Peripheral Neuropathy
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Peripheral neuropathy is a clinical term that denotes nontraumatic disease of the nerves. Most peripheral neuropathies tend to affect the longest fibers first, producing a typical symmetric “glove and stocking” distribution of sensory loss and involvement of the muscles of the hands and feet. Sensory and mixed neuropathies are more common than pure motor neuropathy. Neuropathy affecting autonomic nerves results in autonomic dysfunction.
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Two basic pathologic lesions occur in peripheral neuropathy: segmental demyelination and axonal degeneration. Both changes result in failure of nerve conduction. In the case of sensory nerves, the nerve damage results in degenerative changes in the neuron within the sensory nerve root ganglion. When neuronal loss occurs, the lesion is irreversible. There are many causes of peripheral neuropathy (Table 66-1). These are classified by etiology rather than by pathogenesis because pathologic examinations of nerve biopsy material are not routinely performed. Nerve biopsy (sural nerve) is indicated in (1) cases where vasculitis, amyloidosis, granuloma, or inflammation is suspected; and in (2) specialized centers for research into nerve diseases. Identification of fine structural changes in the myelin sheath and axons requires teased nerve preparations and electron microscopy. These techniques are not within the scope of the average pathology laboratory.
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