The differentiation of muscle stretch receptors in the rat after neonatal de-efferentation.

It has been established previously that muscle stretch receptors of the rat fail to develop after neonatal denervation, but differentiate in full number per muscle after de-efferentation, when sensory innervation remains preserved. Recent immunocytochemical and morphometric studies have revealed, however, that some properties of stretch receptors are altered after motor denervation. In de-efferented muscle spindles, the maturation of myosin of the nuclear bag fibres becomes arrested. In de-efferented tendon organs, the mean length becomes increased and mean width decreased in in extensor digitorum longus muscles which become prolonged after de-efferentation, whereas the opposite changes occur in de-efferentation soleus muscles which become shortened. In spite of these alterations, however, neonatal de-efferentation does not impede the differentiation of ultrastructural characteristics and functional properties of stretch receptors. Thus motor innervation and muscle function are dispensable, whereas sensory innervation is absolutely essential for the development of stretch receptors.