How are motor neurons classified?

Updated: Sep 06, 2019
  • Author: Carmel Armon, MD, MSc, MHS; Chief Editor: Nicholas Lorenzo, MD, MHCM, CPE, FAAPL  more...
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Answer

The cell bodies (soma) of lower motor neurons reside in the spinal cord or the brainstem, and the axons (fibers) are connected directly to muscles at the neuromuscular junctions. These are considered first-order motor neurons, because they are connected directly to the muscles.

The somas of upper motor neurons reside in the brain, where they control the activity of lower motor neurons. Second-order motor neurons can be distinguished from higher-order motor neurons. Second-order motor neurons are upper motor neurons whose cell bodies reside primarily in the precentral gyrus or the primary motor cortex of the frontal lobe. They send fibers that directly connect to lower motor neurons in the brain stem that innervate the muscles of the face, pharynx, and larynx or to lower motor neurons in the spinal cord that innervate the limb, trunk, and respiratory muscles.

Third- and higher-order motor neurons are located in the frontal lobes of the brain anterior to the precentral gyrus (ie, the prefrontal cortex). These neurons are involved in planning and organizing motor activity and direct the second-order motor neurons. The somas of these third- and higher-order motor neurons reside in the brain, and their axons form associative or commissural projections within the brain.


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