The temperature sensitivity of motor units in rat soleus.

Temperature has a significant impact on the performance of the neuromuscular system and motor control processes. The smallest functional components of these systems are motor units (MUs), which may differ significantly between different muscles. The influence of temperature on the contractile properties of slow-twitch (S) MUs from soleus (SOL) muscles in rats was investigated under hypothermia (25 °C), normothermia (37 °C), and hyperthermia (41 °C). Hypothermia prolonged the twitch time parameters, decreased the rate of force development, increased the twitch-to-tetanus ratio, enhanced twitch force, and abolished post-tetanic depression. In contrast, hyperthermia did not alter twitch time parameters. Moreover, there was no effect on force despite the noted increase in post-tetanic depression and the twitch-to-tetanus ratio. Therefore, hypothermia induced more profound changes in S MUs compared with hyperthermia. The temperature effects in SOL MUs were compared to the effects previously reported for S MUs in the medial gastrocnemius (MG). The major differences between the S MUs of both muscles were the effects of temperature on twitch force, post-tetanic force modulation, twitch-to-tetanus ratio, and the slope of the force-frequency curve under hypothermia. Hyperthermia shortened twitch time parameters solely in the MG. In contrast, post-tetanic depression, twitch-to-tetanus ratio, and the slope of the force-frequency curve were influenced by hyperthermia only in SOL MUs. The different temperature effects of S MUs probably corresponded to differences in muscle architecture and their diverse functional tasks and enzyme activity. In summary, S MUs in SOL are more thermal-sensitive than their counterparts in MG.

Exercise Training Decreases Nitrite Concentration in the Heart and Locomotory Muscles of Rats Without Changing the Muscle Nitrate Content.

BACKGROUND: Skeletal muscles are postulated to be a potent regulator of systemic nitric oxide homeostasis. In this study, we aimed to evaluate the impact of physical training on the heart and skeletal muscle nitric oxide bioavailability (judged on the basis of intramuscular nitrite and nitrate) in rats. METHODS AND RESULTS: Rats were trained on a treadmill for 8 weeks, performing mainly endurance running sessions with some sprinting runs. Muscle nitrite (NO2-) and nitrate (NO3-) concentrations were measured using a high-performance liquid chromatography-based method, while amino acids, pyruvate, lactate, and reduced and oxidized glutathione were determined using a liquid chromatography coupled with tandem mass spectrometry technique. The content of muscle nitrite reductases (electron transport chain proteins, myoglobin, and xanthine oxidase) was assessed by western immunoblotting. We found that 8 weeks of endurance training decreased basal NO2- in the locomotory muscles and in the heart, without changes in the basal NO3-. In the slow-twitch oxidative soleus muscle, the decrease in NO2- was already present after the first week of training, and the content of nitrite reductases remained unchanged throughout the entire period of training, except for the electron transport chain protein content, which increased no sooner than after 8 weeks of training. CONCLUSIONS: Muscle NO2- level, opposed to NO3-, decreases in the time course of training. This effect is rapid and already visible in the slow-oxidative soleus after the first week of training. The underlying mechanisms of training-induced muscle NO2- decrease may involve an increase in the oxidative stress, as well as metabolite changes related to an increased muscle anaerobic glycolytic activity contributing to (1) direct chemical reduction of NO2- or (2) activation of muscle nitrite reductases.

Differences in Ia proprioceptive input to spinal motoneurons between male and female rats.

Male and female rats differ in muscle fibre composition, related motor unit contractile properties, and muscle spindle density but not number. On the other hand, their motoneurons' intrinsic properties, excitability and firing properties are similar. The aim of this study was to investigate whether apparent sex differences in body mass and muscle force influence the proprioceptive input from muscle spindles to motoneurons. Medial gastrocnemius motoneurons were investigated intracellularly in deeply anaesthetised male and female rats. Monosynaptic Ia excitatory postsynaptic potentials (EPSPs) were evoked using electrical stimulation of primary afferents from homonymous muscle. Data were analysed using a mixed linear model. The central latencies of EPSPs were 0.38-0.80 ms, with no differences in means between males and females. The maximum EPSP amplitude varied between 2.03 and 8.09 mV in males and 1.24 and 6.79 mV in females. The mean maximum EPSP amplitude was 26% higher in males than in females. The mean EPSP rise time, half-decay time and total duration did not differ between the sexes. EPSP amplitudes correlated with the resting membrane potential, input resistance and EPSP rise time in both sexes. The observed sex differences in the Ia proprioceptive input may be related either to mechanical loading differences in males and females associated with their different body mass or hormonal differences influencing the levels of neuromodulation in spinal circuits. The results highlight the importance of taking sex into consideration in the studies on the influence of afferent inputs on MN excitability.

Skeletal muscle models composed of motor units: A review.

The mathematical muscle models should include several aspects of muscle structure and physiology. First, muscle force is the sum of forces of multiple motor units (MUs), which have different contractile properties and play different roles in generating muscle force. Second, whole muscle activity is an effect of net excitatory inputs to a pool of motoneurons innervating the muscle, which have different excitability, influencing MU recruitment. In this review, we compare various methods for modeling MU twitch and tetanic forces and then discuss muscle models composed of different MU types and number. We first present four different analytical functions used for twitch modeling and show limitations related to the number of twitch describing parameters. We also show that a nonlinear summation of twitches should be considered in modeling tetanic contractions. We then compare different muscle models, most of which are variations of Fuglevand's model, adopting a common drive hypothesis and the size principle. We pay attention to integrating previously developed models into a consensus model based on physiological data from in vivo experiments on the rat medial gastrocnemius muscle and its respective motoneurons. Finally, we discuss the shortcomings of existing models and potential applications for studying MU synchronization, potentiation, and fatigue.

The influence of temperature on contractile properties of motor units in rat medial gastrocnemius.

The effects of hypothermia and hyperthermia on mammalian skeletal muscle function have previously been reported. However, their effects on the contractile properties of different motor unit (MU) types were not described. This study aimed to explore the effect of temperature on contractile properties of MUs in rat medial gastrocnemius kept at 25 °C (hypothermia), 37 °C (normothermia), and 41 °C (hyperthermia). Hypothermia prolonged the twitch time parameters of all MU types, shifting the steep part of the force-frequency curve towards lower frequencies and increasing its steepness. In addition, it reduced the rate of force development but not the twitch and tetanus forces of slow-twitch (S) MUs. Moreover, it reduced the tetanic force of fast-twitch fatigable (FF) MUs and increased the twitch force of fast-twitch fatigue-resistant (FR) MUs. In contrast, hyperthermia had opposite effects on twitch time properties and the force-frequency relationship. The twitch-to-tetanus ratio decreased for FF and FR MUs, and the steep part of the force-frequency curve shifted towards higher frequencies and decreased in steepness. Our findings indicate that FF MUs are the most sensitive and S MUs are the least sensitive to temperature. Furthermore, force control processes involving changes in motoneuronal firing frequency were radically modified for fast MUs, especially FF MUs.

The electrophysiological properties of hindlimb motoneurons do not differ between male and female rats.

Several studies have reported differences in the morphological characteristics of motoneurons and the contractile properties of motor units of male and female rats. However, differences in spinal motoneuron activity between the sexes are not well understood. This study investigates the electrophysiological properties of spinal α-motoneurons in male and female Wistar rats under pentobarbital anaesthesia. Fast and slow types of tibial motoneurons were recorded intracellularly in 15 male and 15 female rats, and the measured parameters were compared statistically using two-way ANOVA and Tukey post hoc tests. The membrane properties, action potential parameters and firing characteristics were not different between sexes, though significant differences were observed in the properties of fast and slow motoneuron types within both sex groups. We conclude that the sex-related differences observed in motor performance between male and female rats are largely due to differences in muscle mass, the proportion of muscle fibre types and the related motor unit contractile properties, while the mechanisms of motor control dependent on the electrophysiological activity of motoneurons are similar between the sexes. These findings are significant, as they indicate that results of experiments investigating electrophysiological properties can be reliably compared between sexes.

Nitrite Concentration in the Striated Muscles Is Reversely Related to Myoglobin and Mitochondrial Proteins Content in Rats.

Skeletal muscles are an important reservoir of nitric oxide (NO•) stored in the form of nitrite [NO2-] and nitrate [NO3-] (NOx). Nitrite, which can be reduced to NO• under hypoxic and acidotic conditions, is considered a physiologically relevant, direct source of bioactive NO•. The aim of the present study was to determine the basal levels of NOx in striated muscles (including rat heart and locomotory muscles) with varied contents of tissue nitrite reductases, such as myoglobin and mitochondrial electron transport chain proteins (ETC-proteins). Muscle NOx was determined using a high-performance liquid chromatography-based method. Muscle proteins were evaluated using western-immunoblotting. We found that oxidative muscles with a higher content of ETC-proteins and myoglobin (such as the heart and slow-twitch locomotory muscles) have lower [NO2-] compared to fast-twitch muscles with a lower content of those proteins. The muscle type had no observed effect on the [NO3-]. Our results demonstrated that fast-twitch muscles possess greater potential to generate NO• via nitrite reduction than slow-twitch muscles and the heart. This property might be of special importance for fast skeletal muscles during strenuous exercise and/or hypoxia since it might support muscle blood flow via additional NO• provision (acidic/hypoxic vasodilation) and delay muscle fatigue.

Motor Unit Fatigability following Chronic Carnosine Supplementation in Aged Rats.

Studies suggest that carnosine (beta-alanyl-L-histidine) is effective in treating neuromuscular diseases associated with aging, but there is still a need to clarify its role in motor units (MUs) function during aging. In this study, 40 male Wistar rats aged 15 months were randomly assigned to a control or to two experimental groups in which 0.1% carnosine supplementation was performed for 10 or 34 weeks. After 34 weeks, we examined fast fatigable (FF), fast fatigue-resistant (FR) and slow (S) MUs' force properties and fatigability, as well as antioxidant potential, advanced glycation end products, activity of enzymes, and histidyl dipeptides content in the medial gastrocnemius muscle. Short- and long-term carnosine supplementation maintained the force of FF MUs at a higher level during its rapid decline seen from the initial 10 to 70 s of the fatigue test. In FF, especially long-term, and in FR MUs, especially short-term, carnosine supplementation resulted in less rapid force decline during the initial 70 s of the second fatigue protocol. Carnosine supplementation did not change muscle antioxidant potential and mortality rate (~35% in all groups), nor muscle mass with aging. Moreover, instead of the expected increase, a decrease in histidyl dipeptides by ~30% in the red portion of medial gastrocnemius muscle after long-term supplementation was found. After chronic carnosine supplementation, the specific changes in fatigue resistance were observed in FF and FR units, but not in S MU types that were not accompanied by an improvement of antioxidant potential and activity of glycolytic or oxidative enzymes in aged rats. These observations indicate that carnosine supplementation during aging may generate different physiological adaptations which should be considered as an important factor when planning treatment strategies.

Ia EPSPs in rat spinal motoneurons are potentiated after a 5-week whole body vibration.

Whole body vibration (WBV) is often applied as an alternative method for strength training or to prevent muscle force decrease. In this study, we evaluated the influence of WBV on Ia monosynaptic input from muscle spindles because the tonic vibration reflex is responsible for the enhancement of muscle activity observed after WBV. The aim was to investigate whether repeated activation of muscle spindles during WBV may result in altered synaptic excitation of motoneurons. WBV was performed on adult male Wistar rats, 5 days/wk, for 5 wk, and each daily session consisted of four 30-s runs of vibration at 50 Hz. Fast-type medial gastrocnemius motoneurons were investigated intracellularly in deeply anesthetized animals in the experimental (n = 7, 34 motoneurons) and control (n = 7, 32 motoneurons) groups. Monosynaptic Ia excitatory postsynaptic potentials (EPSPs) were evoked by electrical stimulation of afferent fibers from the synergistic lateral gastrocnemius and soleus muscles. Data were analyzed using a mixed linear model. The central latencies of EPSPs were 0.45-0.9 ms with no differences in the mean values between the analyzed groups (P = 0.291). WBV induced an increase of the mean EPSP amplitude by 28% (P = 0.025), correlated with the resting membrane potential and input resistance, and a shortening of the mean EPSP rise time by 11% (P = 0.012). The potentiation of synaptic excitation of motoneurons was not accompanied by changes of passive membrane properties, pointing to synaptic plasticity. This indicates that WBV may support rehabilitation or training processes aimed at increasing muscle strength on the basis of increased motoneuronal drive.NEW & NOTEWORTHY The study provides new information on neuronal plasticity following repeatedly exerted mechanical loading. We demonstrate in electrophysiological experiments on rat lumbar motoneurons that low-volume whole body vibration applied systematically for 5 wk potentiates synaptic excitation from primary muscle afferents. The adaptive changes are expressed by higher amplitudes and shorter rise times of monosynaptic EPSPs evoked in motoneurons of the vibrated group compared with the control.

Evaluating Sexual Dimorphism of the Muscle Spindles and Intrafusal Muscle Fibers in the Medial Gastrocnemius of Male and Female Rats.

This study sought to investigate the sexual dimorphism of muscle spindles in rat medial gastrocnemius muscle. The muscles were cut transversely into 5-10 and 20 µm thick serial sections and the number, density, and morphometric properties of the muscle spindles were determined. There was no significant difference (p > 0.05) in the number of muscle spindles of male (14.45 ± 2.77) and female (15.00 ± 3.13) rats. Muscle mass was 38.89% higher in males (1.08 vs. 0.66 g in females), making the density of these receptors significantly higher (p < 0.01) in females (approximately one spindle per 51.14 mg muscle mass vs. one per 79.91 mg in males). There were no significant differences between the morphometric properties of intrafusal muscle fibers or muscle spindles in male and female rats (p > 0.05): 5.16 ± 2.43 and 5.37 ± 2.27 µm for male and female intrafusal muscle fiber diameter, respectively; 5.57 ± 2.20 and 5.60 ± 2.16 µm for male and female intrafusal muscle fiber number, respectively; 25.85 ± 10.04 and 25.30 ± 9.96 µm for male and female shorter muscle spindle diameter, respectively; and 48.99 ± 20.73 and 43.97 ± 16.96 µm for male and female longer muscle spindle diameter, respectively. These findings suggest that sexual dimorphism in the muscle spindles of rat medial gastrocnemius is limited to density, which contrasts previous findings reporting differences in extrafusal fibers diameter.

Two weeks of moderate intensity locomotor training increased corticosterone concentrations but did not alter the number of adropin-immunoreactive cells in the hippocampus of diabetic type 2 and control rats.

Adropin (ADR) plays a role in metabolism regulation and its alterations in obesity and diabetes have been found. Treatment with ADR was beneficial in metabolic diseases, and physical exercise increased ADR concentrations in obese patients. However, data on the distribution of ADR in the brain are sparse. The role of metabolic status and physical exercise on its expression in the brain is undiscovered. We hypothesized that diabetes type 2 (DM2) and/or exercise will alter number of ADR-immunoractive (-ir) cells in the rat brain. Animals were divided into groups: diabetes type 2 (receiving high-fat diet and injections of streptozotocin) and control (fed laboratory chow diet; C). Rats were further divided into: running group (2 weeks of forced exercise on a treadmill) and non-running group. Body mass, metabolic and hormonal profiles were assessed. Immunohistochemistry was run to study ADR-ir cells in the brain. We found that: 1) in DM2 animals, running decreased insulin and increased glucose concentrations; 2) in C rats, running decreased insulin concentrations and had no effect on glucose concentration in blood; 3) running increased corticosterone (CORT) concentrations in DM2 and C rats; 4) ADR-ir cells were detected in the hippocampus and ADR-ir fibers in the arcuate nucleus of the hypothalamus, which is a novel location; 5) metabolic status and running, however, did not change number of these cells. We concluded that 2 weeks of forced moderate intensity locomotor training induced stress response present as increased concentration of CORT and did not influence number of ADR-ir cells in the brain.

Motor Unit Force Potentiation and Calcium Handling Protein Concentration in Rat Fast Muscle After Resistance Training.

Post-tetanic potentiation (PTP) of force depends on intramuscular Ca2+ levels and sensitivity and may be affected by fatigue. The aim of this study was to determine the ability of isolated fast fatigue-resistant (FR) and fast-fatigable (FF) motor units (MUs) to potentiate force evoked with single and 40-Hz electrical stimulation after 5 weeks of voluntary weight-lifting training. Tetanic contractions evoked by gradually increasing (10-150 Hz) stimulation frequency served as conditioning stimulation. Additionally, the concentration of myosin light chain kinase and proteins engaged in calcium handling was measured in rat fast medial gastrocnemius muscle. After the training, the potentiation of twitch force and peak rate of force development was increased in FF but not FR MUs. Force potentiation of 40-Hz tetanic contractions was increased in both fast MU types. After the training, the twitch duration of FR MUs was decreased, and FF MUs were less prone to high-frequency fatigue during conditioning stimulation. Muscle concentration of triadin was increased, whereas concentrations of ryanodine receptor 1, junctin, FKBP12, sarcoplasmic reticulum calcium ATPase 1, parvalbumin, myosin light chain kinase, and actomyosin adenosine triphosphatase content were not modified. After short-term resistance training, the twitch contraction time and twitch:tetanus force ratio of FR MUs are decreased, and PTP ability is not changed. However, PTP capacity is increased in response to submaximal activation. In FF MUs increase in PTP ability coexists with lesser fatigability. Further work is required to find out if the increase in triadin concentration has any impact on the observed contractile response.

Effect of synchronization of firings of different motor unit types on the force variability in a model of the rat medial gastrocnemius muscle.

The synchronized firings of active motor units (MUs) increase the oscillations of muscle force, observed as physiological tremor. This study aimed to investigate the effects of synchronizing the firings within three types of MUs (slow-S, fast resistant to fatigue-FR, and fast fatigable-FF) on the muscle force production using a mathematical model of the rat medial gastrocnemius muscle. The model was designed based on the actual proportion and physiological properties of MUs and motoneurons innervating the muscle. The isometric muscle and MU forces were simulated by a model predicting non-synchronized firing of a pool of 57 MUs (including 8 S, 23 FR, and 26 FF) to ascertain a maximum excitatory signal when all MUs were recruited into the contraction. The mean firing frequency of each MU depended upon the twitch contraction time, whereas the recruitment order was determined according to increasing forces (the size principle). The synchronization of firings of individual MUs was simulated using four different modes and inducing the synchronization of firings within three time windows (± 2, ± 4, and ± 6 ms) for four different combinations of MUs. The synchronization was estimated using two parameters, the correlation coefficient and the cross-interval synchronization index. The four scenarios of synchronization increased the values of the root-mean-square, range, and maximum force in correlation with the increase of the time window. Greater synchronization index values resulted in higher root-mean-square, range, and maximum of force outcomes for all MU types as well as for the whole muscle output; however, the mean spectral frequency of the forces decreased, whereas the mean force remained nearly unchanged. The range of variability and the root-mean-square of forces were higher for fast MUs than for slow MUs; meanwhile, the relative values of these parameters were highest for slow MUs, indicating their important contribution to muscle tremor, especially during weak contractions.

Contractile history affects sag and boost properties of unfused tetanic contractions in human quadriceps muscles.

PURPOSE: A period of extra-efficient force production ("boost") followed by a decline in force ("sag") is often observed at the onset of unfused tetanic contractions. We tested the hypothesis that in human muscle boost and sag are diminished in repeated contractions separated by short rest periods and are re-established or enhanced following long rest periods. METHODS: Two sets of 3 unfused tetanic contractions were evoked in the right quadriceps muscle group of 29 participants via percutaneous stimulation of the femoral nerve. Contractions consisted of 20 pulses evoked at inter-pulse intervals of 1.25 × twitch time to peak torque. Contractions were evoked 5 s apart and sets were evoked 5 min apart. RESULTS: The ratio of the angular impulse of pulses 1-10 to the angular impulse of pulses 11-20 was used as the boost indicator. By this metric, boost was higher (P < 0.05) in the first relative to the second and third contractions within a set, but did not differ between sets (Set 1: 1.31 ± 0.15, 1.18 ± 0.12, 1.14 ± 0.12 vs Set 2: 1.34 ± 0.17, 1.17 ± 0.13, 1.14 ± 0.13). Sag (the percent decline in torque within each contraction) was also higher (P < 0.05) in the first relative to the second and third contractions within a set, but did not differ between sets (Set 1: 40.8 ± 7.5%, 35.4 ± 6.8%, 33.2 ± 7.8% vs Set 2: 42.1 ± 8.0%, 35.5 ± 6.8%, 33.9 ± 7.2%). Participants' sex and resistance training background did not influence boost or sag. CONCLUSION: Boost and sag are sensitive to contractile history in whole human quadriceps. Optimizing boost may have application in strength and power sports.

Biomechanical conditioning of the motor unit transitory force decrease following a reduction in stimulation rate.

BACKGROUND: The biomechanical background of the transitory force decrease following a sudden reduction in the stimulation frequency under selected experimental conditions was studied on fast resistant motor units (MUs) of rat medial gastrocnemius in order to better understand the mechanisms of changes in force transmission. METHODS: Firstly, MUs were stimulated with three-phase trains of stimuli (low-high-low frequency pattern) to identify patterns when the strongest force decrease (3-36.5%) following the middle high frequency stimulation was observed. Then, in the second part of experiments, the MUs which presented the largest force decrease in the last low-frequency phase were alternatively tested under one of five conditions to analyse the influence of biomechanical factors of the force decrease: (1) determine the influence of muscle stretch on amplitude of the force decrease, (2) determine the numbers of interpulse intervals necessary to evoke the studied phenomenon, (3) study the influence of coactivation of other MUs on the studied force decrease, (4) test the presence of the transitory force decrease at progressive changes in stimulation frequency, (5) and perform mathematical analysis of changes in twitch-shape responses to individual stimuli within a tetanus phase with the studied force decrease. RESULTS: Results indicated that (1) the force decrease was highest when the muscle passive stretch was optimal for the MU twitch (100 mN); (2) the middle high-frequency burst of stimuli composed of at least several pulses was able to evoke the force decrease; (3) the force decrease was eliminated by a coactivation of 10% or more MUs in the examined muscle; (4) the transitory force decrease occured also at the progressive decrease in stimulation frequency; and (5) a mathematical decomposition of contractions with the transitory force decrease into twitch-shape responses to individual stimuli revealed that the force decrease in question results from the decrease of twitch forces and a shortening in contraction time whereas further force restitution is related to the prolongation of relaxation. CONCLUSIONS: High sensitivity to biomechanical conditioning indicates that the transitory force decrease is dependent on disturbances in the force transmission predominantly by collagen surrounding active muscle fibres.

Initial force production before sag is enhanced by prior contraction followed by a 3-minute rest period in fast motor units of the rat medial gastrocnemius.

Unfused tetanic contractions evoked in fast motor units exhibit extra-efficient force production at the onset of contraction, an effect called "boost". Boost is diminished in subsequent contractions if there is a short rest period between contractions, but can be re-established with a longer period of rest. We tested the hypothesis that contractile activity and rest could enhance boost-related metrics. Two sets of 3 unfused tetani were evoked 3 min apart in fast fatigable (FF) and fast fatigue-resistant (FR) motor units of the rat medial gastrocnemius. The greatest changes occurred in the first unfused tetanic contractions. Relative to the first contraction in the first set, the first contraction in the second set exhibited higher peak force during boost in a subset of motor units (76% of FF and 48% of FR). Enhanced force during boost was influenced by interaction of slowing of twitch contraction time (up to 20% and 25%, for FF and FR motor units, respectively), half-relaxation time (up to 37% and 49% for FF and FR motor units, respectively), and potentiation of the first twitch (up to 13% and 5% for FF and FR motor units, respectively). Examination of twitches evoked between sets suggested opportunity for greater enhancement of boost with shorter intervening rest periods. The phenomenon of enhanced boost following motor unit activity may interest sports scientists.

Functional Isolation of Single Motor Units of Rat Medial Gastrocnemius Muscle.

This work outlines functional isolation of motor units (MUs), a standard electrophysiological method for determining characteristics of motor units in hindlimb muscles (such as the medial gastrocnemius, soleus, or plantaris muscle) in experimental rats. A crucial element of the method is the application of electrical stimuli delivered to a motor axon isolated from the ventral root. The stimuli may be delivered at constant or variable inter-pulse intervals. This method is suitable for experiments on animals at varying stages of maturity (young, adult or old). Moreover, this protocol can be used in experiments studying variability and plasticity of motor units evoked by a large spectrum of interventions. The results of these experiments may both augment basic knowledge in muscle physiology and be translated into practical applications. This procedure focuses on the surgical preparation for the recording and stimulation of MUs, with an emphasis on the necessary steps to achieve preparation stability and reproducibility of results.

Transitory force decrease following a sudden reduction in stimulation frequency in motor units of rat medial gastrocnemius.

Effects of a sudden decrease in the stimulation frequency for motor unit force were studied in rat medial gastrocnemius. For 161 functionally isolated single motor units of three types (S, FR, FF), unfused tetanic contractions were evoked by three-phase trains of stimuli (low-high-low frequency). The course of the tetanus at the onset of the third phase of the force recording was analyzed in tetani with variable fusion degree. For 78 units within the third phase of tetanus, a transitory force decrease to a level lower than in the first phase (identical frequency), was observed. This phenomenon was more frequent for fast fatigue resistant (65.9%) than for fast fatigable and slow motor units (27.1% and 35.5%, respectively). Moreover, the force decrease was strongest for fast resistant motor units (up to 36.5%) and when contractions evoked at variable frequencies of stimulation were compared, the highest amplitudes of the studied force decrease were noted for middle-fused tetani (0.50-0.90). A new phenomenon of transitory force decrease in tetanic contractions of motor units with a decrease in stimulation frequency was found. Most probably, the phenomenon is dependent on disturbances in the force transmission by collagen surrounding active muscles fibers.

The differences in the structure of the motor nucleus of the medial gastrocnemius muscle in male and female rats.

There are many reports describing sexual dissimilarities in the CNS, particularly in the brain and cortical regions. However, knowledge regarding sexual dissimilarities in the spinal cord and in particular in the hindlimb muscle-motoneuron connectivity controlling locomotion is limited. In order to recognise sex differences in the architecture of the medial gastrocnemius (MG) motor nucleus in rats of the same age, retrograde-labelled motoneurons were identified following a bath of the proximal stump of the transected MG nerve in horseradish peroxidase. The rostrocaudal distribution of motoneurons along the spinal cord and on transverse sections as well as the size and density of motoneurons in the motor nucleus were determined from serial microscopic images. It was shown that the MG pool extended throughout the L4-L6 segments, with a length that was 32% greater in males. The position of the moto pool within the ventral horn of the spinal cord transversal sections was also different in both sexes: the pool was located more dorsally and laterally in males. Three size categories of motoneurons with different soma diameters were distinguished: α1 (27.5-40.0µm), α2 (>40.0µm) and γ (<27.5µm). The density of α (α1 and α2) motoneurons as well as γ motoneurons was higher in females, by about 13% and 23%, respectively. The number of α1 motoneurons was 8% higher in females, whereas there were 46% more α2 motoneurons in males. The most significant differences in the distribution concerned α2 motoneurons, which revealed a lower density in the rostral parts of the MG motor nucleus in females. It was therefore concluded that the length of the MG motor nucleus was shorter, whereas the mean density of α and γ motoneurons was higher in females.

Adaptation of motor unit contractile properties in rat medial gastrocnemius to treadmill endurance training: Relationship to muscle mitochondrial biogenesis.

This study aimed at investigating the effects of 2, 4 and 8 weeks of endurance training on the contractile properties of slow (S), fast fatigue resistant (FR) and fast fatigable (FF) motor units (MUs) in rat medial gastrocnemius (MG) in relation to the changes in muscle mitochondrial biogenesis. The properties of functionally isolated MUs were examined in vivo. Mitochondrial biogenesis was judged based on the changes in mitochondrial DNA copy number (mtDNA), the content of the electron transport chain (ETC) proteins and PGC-1α in the MG. Moreover, the markers of mitochondria remodeling mitofusins (Mfn1, Mfn2) and dynamin-like protein (Opa1) were studied using qPCR. A proportion of FR MUs increased from 37.9% to 50.8% and a proportion of FF units decreased from 44.7% to 26.6% after 8 weeks of training. The increased fatigue resistance, shortened twitch duration, and increased ability to potentiate force were found as early as after 2 weeks of endurance training, predominantly in FR MUs. Moreover, just after 2 weeks of the training an enhancement of the mitochondrial network remodeling was present as judged by an increase in expression of Mfn1, Opa1 and an increase in PGC-1α in the slow part of MG. Interestingly, no signs of intensification of mitochondrial biogenesis assessed by ETC proteins content and mtDNA in slow and fast parts of gastrocnemius were found at this stage of the training. Nevertheless, after 8 weeks of training an increase in the ETC protein content was observed, but mainly in the slow part of gastrocnemius. Concluding, the functional changes in MUs' contractile properties leading to the enhancement of muscle performance accompanied by an activation of signalling that controls the muscle mitochondrial network reorganisation and mitochondrial biogenesis belong to an early muscle adaptive responses that precede an increase in mitochondrial ETC protein content.