Insertion activity is increased in neuropathic and certain myopathic or myotonic disorders and in myositis. Abnormal
Figure 24-17 Motor unit action potentials/, Normal potential.fi, Long-duration polyphasic potential (shown twiceC, Short-duration, low-amplitude, polyphasic potentisfFrom Aminoff MJ: Electromyography in Clinical Practice, 3rd ed. New York, Churchill Livingstone, 1998.)
spontaneous activity is present in various neuromuscular diseases. Fibrillation potentials ( iFig, 24-18 ) are biphasic or triphasic discharges with a positive onset except in the endplate region; they have an amplitude of about 300 pV, have a duration of up to 5 msec, fire up to 20 times per second, and represent action potentials generated in single muscle fibers. They usually fire rhythmically, and this probably reflects oscillations of the resting membrane potential of skeletal muscle fibers.y Positive sharp waves (llFig,ll24-19 ) may be found in association with them and consist of an initial positive deflection followed by a slow deflection in the negative direction. Although similar in size to fibrillation potentials, they often last for 10 msec or more and may discharge at rates up to 100 per second. Fibrillation potentials and positive sharp waves are found in conditions with increased muscle irritability. They occur in denervated muscle but may not appear for 3 to 4 weeks after an acute neuropathic lesion. They are also found in patients with inflammatory disorders of muscle, occasionally in patients with muscular dystrophies, after muscle trauma, in patients with certain metabolic disorders (such as acid maltase deficiency), and in association with botulism.
In patients with chronic partial denervation, as well as
Figure 24-18 Fibrillation potentials recorded in partially denervated muscle. In accordance with convention, an upward deflection in this and subsequent figures indicates that the active electrode is negative with respect to the referefFromAminoff MJ: Electromyography in Clinical Practice, 3rd ed. New York, Churchill Livingstone, 1998.)
Figure 24-19 Positive sharp waves recorded in partially denervated muscfFrom Aminoff MJ: Electromyography in Clinical Practice, 3rd ed. New York, Churchill Livingstone, 1998.)
with certain of the muscular dystrophies or inflammatory disorders of muscle, complex repetitive discharges are found. These have a high discharge frequency, but their amplitude and frequency remain constant (iiFig.i24-20 ). They appear to arise in the muscle itself and are probably initiated by a fibrillating muscle fiber that then depolarizes adjacent fibers by ephaptic transmission. y By contrast, in the myotonic discharges ( lllFig, l24-21 ) that characterize certain myotonic disorders with delayed relaxation after voluntary contraction, high-frequency trains of action potentials occur spontaneously, wax and wane in amplitude and frequency, and relate to a disorder of the muscle fiber membrane.
Fasciculation potentials have the dimensions of motor unit action potentials (see later) and represent the spontaneous activation of all the muscle fibers in individual units. They produce a sudden dull thump over the loudspeaker. They are found most often with disease of the anterior horn cells but may also occur with other neuropathic disorders, occasionally with certain myopathic disorders (as in thyrotoxicosis), and sometimes as an isolated phenomenon without pathological significance. Studies in patients with diverse lower motor neuron lesions have revealed that they may arise at multiple sites along motor axons or the cell bodies of diseased motor neuron,y but in most instances they originate at the distal extremity of the axon. y Myokymic discharges, in which motor unit action potentials discharge spontaneously in characteristic patterns, are discussed below.
The motor unit action potentials may show characteristic abnormalities in various diseases of the motor units. In myopathies, the number of muscle fibers in individual motor units is reduced. In consequence, the mean duration and amplitude of the motor unit action potentials is reduced (see Fig.ii24:17 ) and the incidence of polyphasic potentials is increased. Moreover, because the individual units generate less tension than normal, an increased number is recruited for any given degree of voluntary activity. By contrast, in neuropathic disorders, the number of motor
Figure 24-20 Spontaneous, high-frequency repetitive discharge of action potentials in a partially denervated musfFrom Aminoff MJ: Electromyography in Clinical Practice, 3rd ed. New York, Churchill Livingstone, 1998.)
units--but not their content of muscle fibers--is reduced; indeed, if denervated muscle fibers have been reinnervated by sprouting from the axon terminals of surviving units, the number of fibers per unit may actually be increased. In consequence, motor unit action potentials may be longer in duration and larger in amplitude than normal, with an increased incidence of polyphasic potentials, indicating that reinnervation has occurred (see Fig 24-17 ). During mild voluntary activity, there is an increase in the rates at which individual units begin firing and at which they fire before additional units are recruited; and during maximal effort the interference pattern is reduced.
In disorders with abnormal neuromuscular transmission, such as myasthenia gravis, motor unit action potentials vary in amplitude and area during continued activity and, in addition, there is an excess of small, short-duration potentials in affected muscle. The variability of the potentials reflects the reduced safety factor for neuromuscular transmission, as a consequence of which there is variation in the number of muscle fibers firing with each discharge of the unit. Single-fiber electromyography is a technique in which action potentials are recorded from two or more muscle fibers belonging to the same motor unit by means of a special electrode, and the temporal variability (or jitter) between the two action potentials at consecutive discharges is measured. y This jitter reflects variation in neuromuscular transmission time in the two motor endplates involved. An increased jitter is therefore to be expected in patients with diseases of neuromuscular transmission, and impulse blocking may also occur.
Myokymic discharges consist of a spontaneously occurring grouped pattern of firing of motor units; double, triple, or multiple discharges occur, followed by a period of silence and then by another grouped discharge. They may occur in patients with radiation-induced plexopathy or myelopathy and occasionally with chronic radiculopathies or entrapment neuropathies, Guillain-Barre syndrome, multiple sclerosis, or gold intoxication; they may also be found in facial muscles in patients with brain stem gliomas.y , y
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