The postnatal changes in the performance of the striated muscles of man must depend on concomitant alterations in the efferent structures of the nervous system and muscles. The pyramidal tract, because of its isolation and accessibility in the pyramids, offers an opportunity to compare the development of somatic motor neurons and function subsequent to birth.
It is well known that during most of the first year of infancy movements are involuntary and nonpurposeful. In man the ability to perform or initiate voluntary muscular action (kinetism) commences late in the first year and requires some time for integration. Maximum muscular efficiency is acquired probably toward the end of the second or the first part of the third decade. In senility there is loss in the speed, flexibility and coordination of movement, conditions frequently accompanied by tremor.
Instances of injury to the central nervous system during birth are common, and autopsy statistics
LASSEK AM. THE HUMAN PYRAMIDAL TRACT: V. POSTNATAL CHANGES IN THE AXONS OF THE PYRAMIDS. Arch NeurPsych. 1942;47(3):422–427. doi:10.1001/archneurpsyc.1942.02290030080006
Customize your JAMA Network experience by selecting one or more topics from the list below.