Author(s): KRUTKI, P., KRAUZE, M., Institution: POZNAN UNIVERSITY OF PHYSICAL EDUCATION, Country: POLAND, Abstract-ID: 570

INTRODUCTION:
Neural plasticity within the motor system is a fundamental determinant of how animals adapt to increases or decreases of chronic physical activity levels. Motor unit and muscle activation are shaped by a synaptic drive to motoneurons (MNs) and their intrinsic properties. 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 locomotor endurance training on a treadmill.
METHODS:
The study was performed on adult male Wistar rats. The training was implemented as a 5‑week progressive treadmill running program, with daily sessions ranging from 20 min. to 80 min., and speeds increasing every week from 14 to 33 m/min. The protocol included periodic accelerations to mimic natural variations in locomotor demand. 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 37-39%), compared to control, was observed in slow-type MNs only, both for homonymous and heteronymous afferent connections. No additional changes in input resistance and rheobase were observed in slow nor fast-type MNs. However, strong positive correlations of EPSP amplitudes with input resistance were observed for all types of MNs.
CONCLUSION:
The training-induced adaptation to endurance training may be attributable to a greater size and/or number of Ia synapses on MNs or altered levels of presynaptic inhibition of Ia fibers. Since retrograde signals transported from muscles to the spinal cord seem to be associated with activity-dependent plasticity of MNs, the adaptations observed in slow MNs suggest more frequent recruitment of low-threshold motor units during endurance training. Moreover, repetitive activation of homonymous and synergistic MNs through feedback reflex loops from contracting muscles plays a role in controlling synaptic function during endurance exercises.
The study was supported by the Polish National Science Centre (NCN) Grant No. 2022/45/B/NZ7/00102.