When a needle electrode is inserted into a muscle, a brief burst of activity occurs for 2 to 3 seconds or less. Once this insertion activity has settled, spontaneous activity is not present in normal muscle except in the endplate region, where endplate "noise" can be recorded and corresponds to the nonpropagated miniature endplate potentials generated by the spontaneous release of acetylcholine at the neuromuscular junctions.
Figure 24-16 Motor unit activity during increasing voluntary effort. The number of units activated increases with the force of contraction, and eventually (bottom trace) individual potentials can no longer be recofFnom Am inoff MJ: Electromyography in Clinical Practice, 3rd ed. New York, Churchill Livingstone, 1998.)
When slight voluntary contraction is initiated, a few of the motor units within the muscle are activated and initially fire irregularly and at a low rate. With increasing effort, they fire more rapidly; and, at a certain firing rate, additional units are recruited. With further effort, so many units are active that individual potentials cannot clearly be distinguished, leading to a complete "interference pattern" on the trace of the oscilloscope ( MiFig..24-16 ).
Individual motor unit action potentials are usually biphasic or triphasic in configuration, but up to 10 to 15 percent are polyphasic, that is, they have five or more phases. They normally have a duration of between 2 and 15 msec and an amplitude of between 300 pV and 3 mV ( ..Fig, 2.4.-17.).
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