The development of neonatal neuromuscular system is accomplished by the functional interaction between the spinal neurons and its target cells, skeletal muscle cells, and the intrinsic and extrinsic factors affecting this process. The aim of this study was to identify the effect of suspension unloading (SU) and neuromuscular electrical stimulation (NMES) upon the development of the neonatal spinal cord. For this study, the neonatal rats were randomly divided into three groups: a control group, an experimental group I, and an experimental group II. The SU for experimental group I and II was applied from postnatal day (PD) 5 to PD 30, and the NMES for experimental group II was applied from PD 16 to PD 30 using NMES that gave isometric contraction with 10 Hz for 30 minutes twice a day. In order to observe the effect of SU and NMES, this study observed neutrophin-3 (NT-3) and microtubule associated protein 2 (MAP2) immunoreactivity in the lumbar spinal cord (L4-5) at the PD 15 and PD 30. The results are as follows. At PD 15, lumbar spinal cord of experimental group I and II had significantly lower NT-3 and MAP2 immunoreactivity than control group. It proved that a microgravity condition restricted the spinal development. At PD 30, lumbar spinal cord of control group and experimental group II had significantly higher NT-3 and MAP2 immunoreactivity than experimental group I. It proved that the NMES facilitated the spinal development by spinal cord-skeletal muscle interaction. These results suggest that weight bearing during the neonatal developmental period is essential for the development of neuromuscular development. Also, the NMES on its target skeletal muscle can encourage the development of the spinal cord system with a full supplementation of the effect of weight bearing, which is an essential factor in neonatal developmental process.

The aim of this study was to identify the effect of suspension unloading (SU) and electrical stimulation upon the development of neonatal muscular system. For this study, the neonatal rats were randomly divided into three groups: a control group, an experimental group I, and an experimental group II. The SU for experimental group I and II was applied from postnatal day (PD) 5 to PD 30. The electrical stimulation for soleus muscle of experimental group IIwas applied from PD 16 to PD 30 using neuromuscular electrical stimulation (NMES), which gave isometric contraction with 10 pps for 30 minutes twice a day. In order to observe the effect of SU and ES, this study observed myocyte enhancer factor 2C (MEF2C) and vascular endothelial growth factor (VEGF) immunoreactivity in the soleus muscles at PD 15 and PD 30. In addition, the motor behavior test was performed through footprint analysis at PD 30. The following is the result. At PD 15, the soleus muscles of experimental group Iand II had significantly lower MEF2C, VEGF immunoreactivity than the control group. It proved that microgravity conditions restricted the development of the skeletal muscle cells at PD 15. At PD 30, soleus muscles of the control group and experimental group II had significantly higher MEF2C, VEGF, immunoreactivity than experimental group I. It proved that the NMES facilitated the development of the skeletal muscle cells. At PD 30, it showed that SU caused the decrease in stride length of parameter of gait analysis and an increase in toe-out angle, and that the NMES decreased these variations. These results suggest that weight bearing during neonatal developmental period is essential for muscular development. They also reveal that NMES can encourage the development of muscular systems by fully supplementing the effect of weight bearing, which is an essential factor in the neonatal developmental process.