Are There Adaptation Processes in α-Motoneurons as in Muscle Fibres?

Abstract

Adaptation processes may be reflected not only in the muscle fibres, but also in the motoneuron of the motoric unit. Signs of load-specific adaptation in motoneurons were found (Pieper 1984). These are for example activation of motoric cells by increased oxidative and glycolytic metabolism and activation of protein synthesis, increase of nerve conduction velocity and increase of the motoric endplate on the adapted fibre. Almeida et al. (1996) correlated neurophysiological, mechanical and histochemical parameters to demonstrate muscle adaptation with the training of rats. They revealed that neuronal and muscle components of motor units are both affected by training. Recent papers focussed on the problem of whether the α-motoneuron adapts its size and metabolism according to adaption processes of the muscle fibres which are innervated by it. Seburn et al. (1994) measured the SDH activity in rat hind limb muscle fibres and innervating motoneurons under conditions of increased activity over a wide range of activity levels. They found changed SDH activity of type I fibres and unchanged SDH activity of corresponding motoneurons, suggesting that the oxidative activity of motoneurons does not change despite clear adaptations in the muscle fibres they innervate. Nakano et al. (1997) observed similar effects. They measured unchanged SDH activity in motoneurons innervating rat soleus and extensor digitorum longus muscles after chronic activity. However, the size of motoneurons increased for the soleus muscle of trained rats, not for the extensor digitorum longus muscle. It was concluded that chronic activity has a stronger impact of motoneurons innervating slow twitch rather than fast twitch muscles.