Fibrillation potentials and positive sharp waves
Fibrillation potentials and
positive sharp waves. Fibrillation potentials and positive sharp
waves are the action potentials of single muscle fibers that
spontaneously discharge in the absence of innervation. The
potentials typically fire at a constant rate, most often at 1 to 15
Hz. Fibrillation potentials (closed arrow) are biphasic or
triphasic spikes (initial phase is electropositive or downward) and
are usually 20 to 200 μV in amplitude. Positive sharp waves
(open arrow) are biphasic potentials with an initial
positivity followed by a long negative phase. These potentials are
also individual muscle action potentials, like fibrillations, but
are recorded with the needle electromyogram electrode from an
injured portion of a muscle fiber (occasionally, this injury is
caused by the needle itself). Thus, positive sharp waves have the
same significance as fibrillation potentials. Both of these
potentials are seen in disorders that cause loss of muscle fiber
innervation. Neurogenic disorders produce denervated muscle fibers
because of the loss of motor axons. However, fibrillations may also
be seen in myopathic disorders and after muscle trauma. This is
because each muscle fiber is innervated only at the motor endpoint
region, and myopathy can cause segmental fiber necrosis, with the
generation of one or more denervated segments of that myofiber.
These surviving, denervated segments may generate fibrillation
potentials or positive sharp waves.
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