INTRODUCTION: Long-lasting physical exercises evoke adaptive changes in electrophysiological properties of motoneurons (MNs). It remains unknown whether such adaptations also concern peripheral input from muscle receptors, most of all muscle spindles, which provide powerful excitatory input to MNs.

AIM(S): The aim of this study was to investigate whether the strength training based on weight-lifting exercises evokes changes in afferent synaptic transmission to MNs.

METHOD(S): Male Wistar rats (n=15) performed a 5-week voluntary progressive weight-lifting training, and a day after the last training session an acute electrophysiological experiment was performed under general anesthesia. The respective control group was composed of rats restricted to standard cage activity (n=15). Intracellular recordings were made from two pools of MNs: innervating the medial gastrocnemius (MG, n=188) or lateral gastrocnemius and soleus (LG-S, n=187) muscles. Basic membrane properties of MNs were measured and monosynaptic Ia EPSPs were recorded after stimulation of homonymous or heteronymous afferents from synergistic muscles.

RESULTS: EPSP amplitudes were higher in the weight-lifting group in comparison to control. This effect was observed for homonymous EPSPs as well as for heteronymous EPSPs in fast-type MNs. The increase in the input resistance was also observed in fast MNs in response to the training, both in MG and LG-S MNs, and this membrane property positively correlated with the EPSP amplitude.

CONCLUSIONS: The enhancement of synaptic transmission from muscle spindles to MNs in response to weight-lifting training suggests synaptic plasticity, but one should also consider adaptive changes in MN membrane properties or altered levels of presynaptic inhibition of Ia fibers. Adaptive changes were predominant in fast MNs, suggesting their potent recruitment during weight-lifting training.

FINANCIAL SUPPORT: The study was supported by the National Science Centre (NCN) Grant No. 2022/45/B/NZ7/00102