What are motor neuron disorders (MNDs)?

Updated: Apr 28, 2014
  • Author: Hidehiro Takei, MD; Chief Editor: Adekunle M Adesina, MD, PhD  more...
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Answer

Answer

Motor neuron disorders (MNDs) are a clinically and pathologically heterogeneous group of neurologic diseases characterized by progressive degeneration of motor neurons; they include both sporadic and hereditary diseases. Either or both of the following 2 sets of motor neurons can be affected:

  • Upper motor neurons (UMNs), which originate from the primary motor cortex of the cerebrum (precentral gyrus) and possess long axons forming corticospinal and corticobulbar tracts

  • Lower motor neurons (LMNs), which originate in the brainstem (cranial nerve [CN] motor nuclei) and spinal cord (anterior horn cells) and directly innervate skeletal muscles

MNDs can be classified into those affecting primarily the UMNs, those affecting primarily the LMNs, and those affecting both, and the nomenclature is used accordingly. The patient’s symptoms vary, depending on which set of motor neurons is involved.

In this review, the following conditions will be discussed:


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