The emergence of the “motoneuron concept”: From the early 19th C to the beginning of the 20th C

This article addresses the emergence of the “motoneuron concept,” i.e., the idea that this cell had properties of particular advantage for its control of muscle activation. The motor function of the ventral roots was established early in the 19th C and the term “motor cell,” (or “motor nerve cell”) was introduced shortly thereafter by Albrecht von Kölliker and some other histologists. They knew that motor cells were among the neurons with the largest soma in vertebrates and for this reason they were, and remained for many decades, the best and most studied neuronal model. The work of clinicians like Guillaume Duchenne de Boulogne and Jean-Martin Charcot on motor degenerative syndromes began before a clear description of motor cells was available, because it was initially more difficult to establish whether the deficits of paralysis and muscle weakness were due to neuronal or muscular lesions. Next, the pioneering physiologist, Charles Sherrington, who was influenced greatly by the anatomical contributions and speculations of Santiago Ramón y Cajal, used the term, “motor neuron,” rather than motor cell for the neuron that he considered was functionally “the final common path” for providing command signals to the musculature. In the early 20th C he proposed that activation of a motor neuron resulted from the sum of its various excitatory and inhibitory CNS inputs. The contraction of motor neuron to “motoneuron(e)” was put into common usage by John Fulton (among possibly others) in 1926. The motoneuron concept is still evolving with new discoveries on the horizon.