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Mapping Out Motor Weakness

Fadial, Tom, MD

doi: 10.1097/01.EEM.0000557701.94710.8d
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Dr. Fadialis an assistant professor of emergency medicine at McGovern Medical School in Houston. As the educational technology and innovation officer there, he develops unique educational tools, including algorithms that can be found athttps://ddxof.com. His other medical education projects can be found athttp://fadial.com.

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A systematic approach to motor weakness progresses along an anatomic tract from the cerebral cortex to individual sarcomeres. Impulses are generated in the primary motor cortex mapped to the homunculus, then aggregate as they descend through the internal capsule.

Fibers decussate in the medulla and descend in the contralateral lateral corticospinal tract. These upper motor neurons synapse with the lower motor neuron in the anterior horn of the spinal cord. The lower motor neuron is bundled with neighboring fibers into a peripheral nerve and activates the target muscle fibers at the neuromuscular junction.

Recalling these findings can be simplified by understanding the underlying process. Denervation near the target muscle fibers (lower motor neuron disease) results in dampening of the efferent limb of spinal reflexes, resulting in hyporeflexia.

The absence of nourishing stimulation leads to muscle atrophy and disorganized interpretation of proximal activity, which produces fasciculation.

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