The structure of muscle spindles in the intercostal muscles and rectus abdominis of the human fetus]

The structure and innervation of muscle fibres were studied in external intercostal muscles and the streight muscle of the abdomen in 24--26-week human fetuses. The diameter of most spindles was shown to be within the range of 50--70 mkm, while in the streight muscle of the abdomen it could reach 100 mkm. In the external intercostal muscles the spindles, as a rule, were longer (300-500 mkm) than in the streight muscle of the abdomen (100--300 mkm) which was likely due to special functioning of the spindles in rhythmically working muscles. According to sensory innervation all the spindles may be divided into 3 main types: simple, intermediate and complex. In the external intercostal muscles there occur 3 types of spindles, while in the streight muscle of the abdomen there are spindles of a complex type.