This may be helpful in differentiating weakness primarily due to nerve involvement from that due to muscle involvement. Disorders of the nerves may be demyelinating or axonal. In demyelinating disorders loss of the myelin insulating sheath results in a reduced nerve conduction velocity, whereas axonal loss leads to a reduction in the action potential obtained as a result of stimulating the nerve. Gross loss of muscle mass may also lead to reduction in the action potential detected in the muscle following stimulation of the nerve supplying it. Thus it is possible to differentiate predominantly axonal neuropathies from those due to demyelination and, by examining a wide variety of nerves neurophysiologically, it may be possible to discern whether the neuropathy is generalized or localized. Localized neuropathies are often due to entrapment syndromes such as carpal tunnel syndrome or root lesions. Electrical recordings of muscle activity may also be made. These may be taken from either the surface of the muscle or from needles inserted within the muscle. In myasthenia gravis and muscle relaxant toxicity there is usually a decrement of the response of muscle to repetitive nerve stimulation. In the Eaton-Lambert syndrome associated with bronchial carcinoma there is usually an increment in the muscle response to repetitive stimulation. There are also characteristic features differentiating neuropathic causes of muscle weakness and myopathic causes. A normal muscle is electrically silent, but, following denervation, affected muscle fibers may develop spontaneous contractions known as fibrillations. Surviving axons attempt to reinervate denervated muscle; thus during voluntary muscle movement the motor unit potentials are larger than normal but fewer in number. In myopathic disorders the loss of muscle fibers leads to small and shorter motor unit potentials. Occasionally, myopathic disorders may also be associated with fibrillations which occur particularly in the inflammatory myopathies.

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