Acetylcholinesterase activity in motor nerve fibres in correlation to muscle fibre types in rat

Rat muscle nerves were examined histochemically for their activity of acetylcholinesterase (AChE). The corresponding muscles were stained for myofibrillar ATPase and for NADH diaphorase. The nerves to the extensor digitorum longus (EDL) muscle and to the medial head of the gastrocnemius (MG) muscle consist of a motor axons of high AChE activity. Both muscles are characterized by the prevalence of type II muscle fibres. On the other hand, the soleus muscle and the quandratus femoris muscle, both mainly composed of type I muscle fibres, are innervated by a motor axons of low AChE activity. Since it is well established that EDL and MG are typical fast-twitch muscles and that the soleus, and probably also the auadratus femoris, is a typical slow-twitch muscle, it is suggested that, in rat, fast muscles are innervated by motor nerve fibres of high AChE activity and slow muscles are innervated by motor axons of low AChE activity.