Non-technical summary Voluntary limb movements are initiated in the brain but the neurones responsible for activating the muscles (motoneurones and interneurones) are located in the spinal cord. The spinal cord also contains neurones that provide the brain, and especially the cerebellum, with continuous information on effects of the descending commands. We show that one population of such neurones provide the cerebellum with information on how likely the brain's commands (mediated by descending reticulospinal neurones) are to be executed as planned, depending on the degree of inhibition of motoneurones. They may therefore play an important role in preventing errors in activation of motoneurones and thereby help the brain to correct its signals to the spinal cord before such errors have been committed. AbstractThe study aimed to investigate the role that four populations of spinocerebellar neurones play in forwarding information on descending commands relayed by feline reticulospinal neurones. Both intracellular and extracellular recording was used from medially located ventral spinocerebellar tract (VSCT) neurones as well as from spinal border (SB) subpopulation of VSCT neurones and from dorsal spinocerebellar tract neurones located in Clarke's column (CC DSCT) and in the dorsal horn (dh DSCT) in the lumbosacral enlargement. Axons of reticulospinal neurones were stimulated within the ipsilateral and contralateral medial longitudinal fascicle (MLF). We found striking differences in synaptic input from reticulospinal neurones to these four populations of spinocerebellar neurones. Both monosynaptic and disynaptic excitatory input was found in VSCT and SB neurones, only disynaptic in CC DSCT neurones and none in dh DSCT neurones. Discharges of VSCT and SB neurones were potently modulated by inhibitory actions of group I and II afferents. Following application of single stimuli to peripheral nerves these neurones ceased to respond for about 5 ms and thereafter discharged at a lower incidence rate. As inhibition of spinocerebellar neurones and of α-motoneurones is evoked by the same premotor interneurones, VSCT neurones may provide the cerebellum with information on the likely outcome of reticulospinal actions on motoneurones depending on the degree to which they are inhibited. They may thereby enable the cerebellum to adjust descending commands relayed by reticulospinal neurones to the requirements of a given situation and thus prevent errors in the centrally initiated movements.

Spinal polarization evoked by direct current stimulation [trans-spinal direct current stimulation (tsDCS)] is a novel method for altering spinal network excitability; however, it remains not well understood. The aim of this study was to determine whether tsDCS influences spinal motoneuron activity. Twenty Wistar rats under general pentobarbital anesthesia were subjected to 15 min anodal ( n = 10) or cathodal ( n = 10) tsDCS of 0.1 mA intensity, and the electrophysiological properties of their motoneurons were intracellularly measured before, during, and after direct current application. The major effects of anodal intervention included increased minimum firing frequency and the slope of the frequency-current ( f-I) relationship, as well as decreased rheobase and currents evoking steady-state firing (SSF). The effects of cathodal polarization included decreased maximum SSF frequency, decreased f-I slope, and decreased current evoking the maximum SSF. Notably, the majority of observed effects appeared immediately after the current onset, developed during polarization, and outlasted it for at least 15 min. Moreover, the effects of anodal polarization were generally more pronounced and uniform than those evoked by cathodal polarization. Our study is the first to present polarity-dependent, long-lasting changes in spinal motoneuron firing following tsDCS, which may aid in the development of more safe and accurate application protocols in medicine and sport. NEW & NOTEWORTHY Trans-spinal direct current stimulation induces significant polarity-dependent, long-lasting changes in the threshold and firing properties of spinal motoneurons. Anodal polarization potentiates motoneuron firing whereas cathodal polarization acts mainly toward firing inhibition. The alterations in rheobase and rhythmic firing properties are not restricted to the period of current application and can be observed long after the current offset.

Previous studies of motor unit contractile properties in the rat medial gastrocnemius revealed that these units generate higher forces in males than in females. Therefore, in the present study the number and morphometric parameters of muscle fibres and the innervation ratio of motor units in the medial gastrocnemius muscle were studied in male and female Wistar rats. The study additionally aimed at determining reasons of gender differences in motor unit force parameters, i.e. the number and diameter of muscle fibres, and mean values of the motor unit innervation ratios. Following staining of reticuline fibres by silver impregnation, the number, diameter and cross-section area of muscle fibres were determined on microscopic images of transverse muscle sections. In males, the muscles were approximately 1.5 times larger by mass and contained about 11 800 muscle fibres, whereas in females the muscles contained around 8000 fibres. In addition, the mean diameter and mean cross-section area of muscle fibres were 14 and 29% larger in males, respectively. Based on previously determined numbers of motoneurons innervating the medial gastrocnemius muscle in male and female rats, the mean innervation ratio, i.e. the number of muscle fibres innervated by one motoneuron, was estimated. This ratio was approximately 26% greater in males compared to females, with values of 207 and 153 fibres per motoneuron, respectively. Therefore, the differences in muscle fibre morphometric parameters and in the innervation ratio are responsible for higher forces of motor units in male muscles.

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Compensatory muscle overload leads to substantial modifications in MU force development mechanisms, which are MU-type-specific and influence whole muscle force regulation.

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Recently, it has been shown that spinal cord polarization considerably modulates motoneuron activity, with certain observed changes in motoneuron membrane and firing properties outlasting the duration of polarization. The purpose of this study was to determine whether repeated sessions of transcutaneous transspinal direct current stimulation (tsDCS) induce adaptive changes in motoneuron properties. In this study, adult male Wistar rats under isoflurane anesthesia were subjected to anodal (n=6) or cathodal (n=6) tsDCS (100 uA, 15 min) 5 days per week for 5 weeks. Sham control group rats (n=6) served as a reference. Intracellular recordings from lumbar spinal motoneurons were performed under pentobarbital anesthesia one day after the final tsDCS session to analyze membrane and firing properties. Anodal polarization appeared to be effective in evoking significant adaptive changes toward the facilitation of motoneuron firing. When compared to the sham polarization group, these adaptations were expressed by the increased input resistance (P=0.0077), decreased voltage threshold for spike generation (P=0.0248) and doublet threshold (P=0.0311), increased maximum SSF frequency (P=0.0073), SSF frequency range (P=0.0075), and slope of the frequency-current relationship (P=0.0111). However, the effects of cathodal polarization were modest and generally not significant in regard to the sham control. These novel findings support the existing knowledge on alterations in spinal neuronal network excitability in response to polarization and provide the direct evidence of adaptive neuroplasticity of spinal motoneurons in response to chronically applied tsDCS.

Trans-spinal direct current stimulation (tsDCS) is a novel neuromodulatory technique that has been used during neurological rehabilitation and sports to modulate muscle activation. However, the physiological mechanisms that underly the longlasting functional effects of polarization are not yet fully understood, nor are their relationships with specific neuronal populations. The acute facilitatory and depressive effects of anodal and cathodal polarization on motoneurons have been recently demonstrated, and the aim of this study was to determine whether tsDCS-evoked modulations of motoneuron properties are able to persist over several hours. Intracellular recordings from multiple antidromically identified rat motoneurons were performed both before and after the application of tsDCS (0.1 mA for 15 min), at various time points up to 180 min after the offset of anodal or cathodal tsDCS. The examined effects of anodal polarization included decreased rheobase, voltage threshold, the minimum and maximum currents necessary for rhythmic firing, increased rhythmic firing frequencies and the slope of the f-I relationship. The majority of these facilitatory changes to threshold and firing properties were sustained for 30-60 min after polarization. In contrast, the significant effects of cathodal polarization were absent, except the short-lasting decreased ability for motoneurons to induce rhythmic activity. This study provides direct evidence that a single polarization session can alter the electrophysiological properties of motoneurons for at least one hour and provides a basis for the further use of tsDCS techniques under conditions where the sustained modification of motoneuron firing is desired.