Krutki P, Hałuszka A, Mrówczyński W, Gardiner PF, Celichowski J. Adaptations of motoneuron properties to chronic compensatory muscle overload. J Neurophysiol 113: 2769 -2777, 2015. First published February 18, 2015 doi:10.1152/jn.00968.2014.-The aim of the study was to determine whether chronic muscle overload has measurable effect on electrophysiological properties of motoneurons (MNs), and whether duration of this overload influences intensity of adaptations. The compensatory overload was induced in the rat medial gastrocnemius (MG) by bilateral tenotomy of its synergists (lateral gastrocnemius, soleus, and plantaris); as a result, only the MG was able to evoke the foot plantar flexion. To assure regular activation of the MG muscle, rats were placed in wheel-equipped cages and subjected to a low-level treadmill exercise. The intracellular recordings from MG motoneurons were made after 5 or 12 wk of the overload, and in a control group of intact rats. Some of the passive and threshold membrane properties as well as rhythmic firing properties were considerably modified in fast-type MNs, while remaining unaltered in slow-type MNs. The significant changes included a shortening of the spike duration and the spike rise time, an increase of the afterhyperpolarization amplitude, an increase of the input resistance, a decrease of the rheobase, and a decrease of the minimum current necessary to evoke steady-state firing. The data suggest higher excitability of fast-type MNs innervating the overloaded muscle, and a shift towards electrophysiological properties of slow-type MNs. All of the adaptations could be observed after 5 wk of the compensatory overload with no further changes occurring after 12 wk. This indicates that the response to an increased level of chronic activation of MNs is relatively quick and stable. membrane properties; rat MUSCLE OVERLOAD CAN BE ACHIEVED by workload training (Fry 2004;McDonagh and Davies 1984), ablation or tenotomy of synergists (Gardiner et al. 1991;Noble and Pettigrew 1989;Olha et al. 1988), peripheral nerve damage, or denervation of synergistic muscles (Degens et al. 1995). Overload induces measurable adaptive effects in the whole muscle, its motor units (MUs), and individual muscle fibers. The most prominent response is muscle hypertrophy, reflected in the increase of muscle mass, cross-section area, and the increased strength (Allbrook 1981;Seynnes et al. 2006). Some biochemical adaptations related to this hypertrophy have been reported in muscle fibers (Baldwin et al. 1982;Dearth et al. 2013;Ianuzzo and Chen 1979). Several studies have reported changes in contractile properties of MUs in response to chronic muscle overload. They point on increased tetanic forces of all MU types, longer contraction times of fast resistant and slow MUs, decreased fatigue resistance of fast fatigable units, and altered proportions of MUs within an overloaded muscle (Noble and Pettigrew 1989;Olha et al. 1988).It is well documented that motoneurons are highly susceptible to altered levels of physical...

Bączyk M, Hałuszka A, Mrówczyński W, Celichowski J, Krutki P. The influence of a 5-wk whole body vibration on electrophysiological properties of rat hindlimb spinal motoneurons. J Neurophysiol 109: 2705-2711, 2013. First published March 13, 2013 doi:10.1152/jn.00108.2013The study aimed at determining the influence of a whole body vibration (WBV) on electrophysiological properties of spinal motoneurons. The WBV training was performed on adult male Wistar rats, 5 days a week, for 5 wk, and each daily session consisted of four 30-s runs of vibration at 50 Hz. Motoneuron properties were investigated intracellularly during experiments on deeply anesthetized animals. The experimental group subjected to the WBV consisted of seven rats, and the control group of nine rats. The WBV treatment induced no significant changes in the passive membrane properties of motoneurons. However, the WBV-evoked adaptations in excitability and firing properties were observed, and they were limited to fast-type motoneurons. A significant decrease in rheobase current and a decrease in the minimum and the maximum currents required to evoke steady-state firing in motoneurons were revealed. These changes resulted in a leftward shift of the frequency-current relationship, combined with an increase in slope of this curve. The functional relevance of the described adaptive changes is the ability of fast motoneurons of rats subjected to the WBV to produce series of action potentials at higher frequencies in a response to the same intensity of activation. Previous studies proved that WBV induces changes in the contractile parameters predominantly of fast motor units (MUs). The data obtained in our experiment shed a new light to possible explanation of these results, suggesting that neuronal factors also play a substantial role in MU adaptation.