INTRODUCTION: Membrane and firing properties of motoneurons (MNs) are modified due to increased motor activity. However, it remains unknown whether adaptations also concern peripheral input from muscle receptors, most of all muscle spindles, which are a potent source of excitatory connections to MNs.

AIM(S): The aim of this study in a rat model was to investigate whether physical exercises during treadmill endurance training evoke adaptive changes in Ia afferent synaptic transmission to MNs.

METHOD(S): Male Wistar rats (n=15) were exposed to a 5-week endurance running on a treadmill, and a day after the last training session an acute electrophysiological experiment was performed on each rat under general anesthesia. The respective control group of untrained rats (n=15) was assigned. Lumbar spinal MNs innervating the medial gastrocnemius (MG) or lateral gastrocnemius and soleus (LG-S) muscles were investigated intracellularly. The passive membrane properties and parameters of monosynaptic Ia EPSPs evoked from homonymous or heteronymous afferents from synergistic muscles were compared between the experimental and control groups.

RESULTS: The potentiation of Ia synaptic excitation of motoneurons, expressed by higher heteronymous EPSP amplitudes was observed in slow-type MNs, both in MG and LG-S MNs, and positive correlation of EPSP amplitudes with input resistance was observed. No significant changes in EPSP parameters were found for homonymous EPSPs.

CONCLUSIONS: The demonstrated adaptations to endurance training might be attributed to a greater size and/or number of Ia synapses on MNs, changes in MN membrane properties or altered levels of presynaptic inhibition of Ia fibers. Selective enhancement of synaptic transmission to slow MNs is likely related to the greater recruitment of slow motor units during treadmill exercises.

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