Innervation

Motor fibres within the anterior rami of the spinal nerves grow to the bases of the limb buds. Their pathway is determined by the local limb mesenchyme. In this way, the formation of plexuses is due to the mesenchymal guiding structures, and not to the displacement of the muscles.
At S17, the principal branches of the arm nerves can be distinguished. They form the brachial plexus which subdivides into dorsal and ventral groups of nerves. The ventral group further subdivides into a medial and lateral fascicle. The branches of these fascicles go mainly towards the flexors. The branches of the posterior fascicle supply the extensors.
The formation of the lumbosacral plexus starts a little later than that of the brachial plexus. The branches of this plexus form an upper group for the abdominal wall, the inguinal and the genital regions, a lateral (later anterior) and a medial group for the extensors of the upper leg, and a posterior group for the flexors of the upper and lower leg and the foot.