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).

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.

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.

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).

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.

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 [NO] and nitrate [NO] (NO). 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 NO 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).

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.

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.

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.

Polarity-dependent adaptations of motoneuron electrophysiological properties after 5-wk transcutaneous spinal direct current stimulation in rats.

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 ( = 6) or cathodal ( = 6) tsDCS (100 μA, 15 min) 5 days per week for 5 wk.

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.

Long-lasting modifications of motoneuron firing properties by trans-spinal direct current stimulation in rats.

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 long-lasting 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.

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.

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.

Morphometric properties and innervation of muscle compartments in rat medial gastrocnemius.

The rat medial gastrocnemius (MG) muscle is composed of the proximal and distal compartments. In this study, morphometric properties of the compartments and their muscle fibres at five levels of the muscle length and the innervation pattern of these compartments from lumbar segments were investigated. The size and number of muscle fibres in the compartments were different.

Prolonged activity evokes potentiation and the "sag" phenomenon in slow motor units of rat soleus.

Slow motor units (MUs) have no sag in their unfused tetani. This study in anesthetized rats shows that the sag can be observed in slow soleus MUs after prolonged activity. Twitches and unfused tetanic contractions were recorded from male (n=35) and female (n=39) MUs before and after the four minutes of the fatigue test (trains of 13 pulses at 40 Hz repeated every second).

Summation of slow motor unit forces at constant and variable interpulse intervals in rat soleus muscle.

Effects of the summation of forces generated by functionally isolated slow-twitch motor units (MU) of the rat soleus muscle were examined in this study. Initially, the twitch, fused tetanic and unfused tetanic contractions evoked by trains of stimuli at variable interpulse intervals were recorded for each MU. Then, two, three or four MUs were co-activated, and the recorded forces were compared to the algebraic sum of the forces of individual MUs.

Decomposition of motor unit tetanic contractions of rat soleus muscle: Differences between males and females.

Mathematical decomposition of tetanic contractions of slow motor units (MUs) of the rat heterogeneous medial gastrocnemius muscle revealed immense variability of twitch-shape responses to successive pulses, contrary to results obtained for fast MUs. The aim of this study in rat soleus muscle, almost exclusively composed of slow MUs, was to reveal whether such variability of twitch-shape decomposed components was a common property of slow MUs in the two studied muscles, and whether ranges of the force amplitude or time parameters of these decomposed twitches showed sex differences. Unfused tetanic contractions evoked by stimulation at variable interpulse intervals were analyzed for 10 MUs of males and 10 MUs of females.

Force regulation and electrical properties of motor units in overloaded muscle.

Introduction: The influence of long-term muscle overload on force regulation and electrical properties of motor units (MUs) was investigated in rats.

Methods: Compensatory overload of the medial gastrocnemius was induced by tenotomy of its synergists. Electrophysiological experiments were performed on functionally isolated MUs 3 months after the surgery.

Slow motor units in female rat soleus are slower and weaker than their male counterparts.

The aim of the study was to investigate sex-related differences in contractile properties, parameters of action potentials, and mechanisms of force regulation of motor units in the rat soleus muscle, which is a frequent experimental model in animal research. It was revealed that the mean mass of the muscle in males was bigger than in females, by approximately 80%. However, the relation of the muscle mass to the body mass was not significantly different.

A novel method to attain sinusoidal mechanical responses from single motor units.

Introduction: The relationship between output force and motor command depends on the intrinsic dynamic responses of motor units (MUs), which can be characterized by evoking accurate sinusoidal force responses at different frequencies. In this study we sought to determine whether sinusoidal modulation of the stimulation rate of single MUs results in reliable sinusoidal force changes.

Methods: Single axons of rat ventral roots were stimulated electrically by changing the pulse rate sinusoidally at different frequency modulation (0.