SKELETAL MUSCLE INNERVATION

The establishment of neural contacts with developing skeletal muscle fibers is a critical developmental stage. The contacts enhance muscle development and are important for the complete differentiation and function of the fibers. The motor nerve axons make contact with the masses of myoblasts constituting the developing muscles as early as between the 5th and 6th week if they are trunk muscles. However, it is between this time and the 10th week that the branches of the large somatic (alpha) motor neurons begin to ramify among the developing motor fibers of the muscles and to establish the formation of neuromuscular junctions. Muscle spindles (proprioceptors) can be distinguished at about the 12th week. They become innervated by the small gamma motor nerves.

Movements of the mother, and especially of the uterus, serve as stimuli to induce muscular activity to occur in the fetus before the 4th month, although the mother is not aware of it until the “quickening” at about the 4th month. Long before birth, the diaphragm contracts periodically in response to phrenic nerve activity (hiccups). The fetus begins to swallow amniotic fluid at 12½ weeks; before birth, it may at times suck the fingers. Therefore, the phrenic nerves and the muscular diaphragm used for breathing, and the sensory nerves of the lips, mouth, and throat, as well as the striated muscle with their motor nerves of the lips, tongue, jaws, and throat used for the complicated reflex functions of suckling and swallowing, are functionally well developed at birth. In contrast, the trunk and limb muscles at birth are uniformly slow in contracting.

Voluntary control of the skeletal muscles cannot occur in the neonate because of the lack of dendritic development of the cerebral neurons, especially those of the motor cortex, and the fact that the fibers of the upper motor neurons of the corticobulbar and corticospinal tracts have only begun to be myelinated. It is not until the end of the first year after birth that the myelination of the nerve fibers of the corticospinal tract is nearly completed. This is about the time when the child has sufficient voluntary control over the skeletal muscles to be able to stand and walk.