THE NERVE fibers from the motor cells of the anterior horns of the spinal cord arborize several times in their distal passage and finally innervate a large number of muscle fibers. A group of muscle fibers thus innervated is generally termed a motor unit (motoneuron) and probably constitutes a functional and trophic unit. For this reason, muscular atrophy due to morbid changes in the lower motor neurons is characterized by the synchronous atrophy of larger or smaller groups of muscle fibers. The most conspicuous example of this type of muscular atrophy is met with in amyotrophic lateral sclerosis, but the same type of atrophy is seen in other diseases.
Darkschewitsch1 asserted that in amyotrophic lateral sclerosis muscle fibers atrophy in groups. Slauck2 corroborated this assertion and attributed the phenomenon to the fact that every motor nerve fiber innervates a group of muscle fibers. S. Wohlfahrt and G. Wohlfart
WOHLFART G. MUSCULAR ATROPHY IN DISEASES OF THE LOWER MOTOR NEURONContribution to the Anatomy of the Motor Units. Arch NeurPsych. 1949;61(6):599-620. doi:10.1001/archneurpsyc.1949.02310120003001