Various models of chronic muscular activity, as workload training, tendon transfer, or paresis and peripheral nerve damage are examples leading to muscle overload, which may induce measurable effects in motor units. The aim of this study was to investigate whether 5-week overloading of muscles connected with their voluntary activation in a running wheel and by a treadmill training change electrophysiological properties of their motoneurons. Rats were subjected to chronic overload of the medial gastrocnemius (MG) muscle by cutting the lateral gastrocnemius, soleus and plantaris muscles from the Achilles tendon and sewing them directed proximally to the skin. As the result of this operation, only the MG muscle was able to evoke a foot plantar flexion during the daily locomotor activity. After one week of convalescence, rats were subjected to extensive voluntary activity on a running wheel and additionally to a training program on a treadmill (1 hour daily with a speed of 27 cm/s) for 5 weeks, 5 days a week. The acute experiments were carried out on the MG motoneurones in deeply anaesthetized animals. Intracellular recordings were performed from MG motoneurones located in L4-L5 spinal segments using glass micropipettes filled with 2 M potassium citrate solution. The results were compared to the control group of normally active, intact animals. Parameters of antidromic action potentials were measured and effects of intracellular injection of rectangular pulses of depolarization current were analyzed. The basic electrophysiological properties were considerably modified by the overloading either in fast and slow motoneurones. Moreover, we observed changes in their rhythmic properties, as the increased maximum steady-state frequencies of motoneuronal firing resulting in changes in the course of the steady-state frequency-current curves in the overloaded animals. The results of this study may help understand neuromuscular mechanisms of plasticity of overloaded muscles.
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