Author(s): KRAUZE, M., KRUTKI, P., Institution: POZNAŃ UNIVERSITY OF PHYSICAL EDUCATION, Country: POLAND, Abstract-ID: 571

INTRODUCTION:
Activity-dependent plasticity involves essentially every region of the central nervous system, including motoneurons (MNs), which form the link between the motor command and excitation of muscle fibers which they innervate. Muscle afferents provide potent excitatory input to MNs, forming a critical substrate for reflex modulation, and the postsynaptic response to synaptic input depends also on the intrinsic electrical properties of the MN. The purpose of this study was to investigate unknown adaptations in monosynaptic afferent transmission from muscle spindles to MNs, coupled with changes in excitability, in rats performing resistance training based on weight-lifting exercises.
METHODS:
The study was performed on adult male Wistar rats. The training was implemented in two daily sessions for 5 weeks. Each training session consisted of repeated squat‑like lifting movements with loads increasing over the training period, from the average of 89-94% of the body weight in the first week up to 163-171% in the final week. Electrophysiological experiments were performed in separate groups of trained and control rats under deep anesthesia. Intracellular recordings were performed from antidromically identified MNs in the L4–L5 spinal segments, innervating the medial gastrocnemius or lateral gastrocnemius and soleus muscles. The passive and threshold properties, and parameters monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulation of Ia afferent fibers from homonymous and heteronymous muscles were recorded intracellularly from MNs.
RESULTS:
The significant increase of Ia EPSP amplitudes (by 42-43%), compared to control, was observed in fast-type MNs only, both for homonymous and heteronymous afferent connections. These changes coincided with the increased input resistance (by 26-32%) and decreased rheobase (by 19%) in fast-type MNs, and these changes strongly correlated with EPSP amplitudes.
CONCLUSION:
The training-induced adaptation to resistance training may be attributable to a greater size and/or number of Ia synapses on MNs or altered tonic modulation of Ia fibers. The enhanced excitatory drive from Ia afferents, accompanied by elevated excitability of MNs, would lead to the more frequent recruitment of fast motor units and, in this way, the generation of greater forces. This is critical for explosive movements, and enhanced Ia drive may facilitate rapid adjustments in muscle stiffness and joint stability. Stronger excitation of MNs in a monosynaptic reflex loop from Ia afferents may also be responsible for the commonly observed enhancement of muscle force during early stages of weight-lifting training, without significant alterations in the content of myosin heavy chains
The study was supported by the Polish National Science Centre (NCN) Grant No. 2022/45/B/NZ7/00102.