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Neurogenic and myopathic diseases have different effects on the motor unit. A. A motor unit potential is recorded by inserting a needle electrode into the
muscle. The muscle fibers innervated by a single motor neuron are not usually adjacent to one another, yet the highly effective transmission at the
neuromuscular junction ensures that each muscle fiber innervated by the same neuron will generate an action potential and contract in response to an
action potential in the motor neuron. Activation of one or a few motor neurons produces a simple extracellular voltage signal and a small contraction
(middle trace). A larger contraction activates a greater number of motor neurons and a larger number of muscle fibers, producing a strong contraction
(lower trace). The extracellular voltage signal is more complex. Interference from the extracellular currents through the large number of muscle fibers,
Source: Diseases of the Nerve and Motor Unit, Principles of Neural Science, Fifth Editon
which are activated at slightly different times by the large number of motor neurons.
Citation: Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ, Mack S. Principles of Neural Science, Fifth Editon; 2012 Available
B. When motor neurons are diseased, the number of motor units under voluntary control is reduced. The muscle fibers supplied by the degenerating
at: http://mhmedical.com/ Accessed: May 03, 2017
motor neuron (cell A) become denervated and atrophic. However, the surviving neuron (cell B) sprouts axonal branches that reinnervate some of the
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denervated muscle fibers. The electromyogram shows larger than normal motor unit potentials (middle trace) because the surviving motor neuron
innervates more than the usual number of muscle fibers (it also innervates formerly denervated fibers). Axons of the surviving motor neuron fire