The influence of electrical activity on the development of newborn innervated rat muscles.

Mammalian slow and fast muscles are incompletely differentiated at birth, and their specific characteristics emerge gradually during the early postnatal period. The pattern of activity received by young muscles may be instrumental in bringing about their differentiation. Hind limb muscles of rats were stimulated via the sciatic nerve in order to test the influence of different patterns of activity on their development. Slow frequency stimulation (10 Hz) prevented the development of fast contraction times in "fast" muscles. The slow soleus became slightly faster when stimulated a 25 Hz. Stimulation of muscles of rats between 4 and 12 days at 40 Hz apparently resulted in some denervation in both fast and slow muscles, although stimulation at this frequency in older rats resulted in shortening of slow muscle contractions.