Fig. 4. Neurotrophin 4/5 (NT-4/5) accelerates the fast-to-slow myosin heavy chain (MyHC) isoform switch in neonatal rat soleus and its signaling is required for the normal development of the soleus muscle fiber phenotype. The percentage of fibers expressing fast MyHC only (fast), both fast and slow MyHC simultaneously (hybrid) and slow MyHC only (slow) are shown for 4-week-old (A,C) and 6-week-old (B,D) rat soleus muscles treated with saline, NT-4/5 or TrkB–IgG. Values represent means ± S.E.M. *=P<0.05 vs saline.