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Fig. 8. Hypothetical models of retrograde regulation of soleus (SOL) motoneuron properties by neurotrophin 4/5 (NT-4/5). (A) Following injection in the muscle, NT-4/5 forms or activates a retrograde signal that is transported by primary afferent (Ia) neurons from muscle spindles. This signal influences cellular mechanisms involved in the determination of the properties of the synapses of these afferent neurons onto SOL motoneurons and, via this mechanism, accelerates the acquisition of the repetitive pattern of discharge of SOL motoneurons and, consequently, the fast-to-slow myosin heavy chain (MyHC) transformation in the SOL muscle. (B) Alternatively, following injection in the muscle, NT-4/5 forms or activates a retrograde signal that is transported by SOL motoneurons. This signal influences cellular mechanisms involved in the maturation of intrinsic SOL motoneuron properties, i.e. rehobase, afterhyperpolarization (AHP) or input resistance (IR), and, via this mechanism, accelerates the acquisition of the repetitive pattern of discharge of SOL motoneurons and, consequently, the fast-to-slow MyHC transformation in this muscle. {uparrow} = increase; {downarrow} = decrease.