Spinoreticular neurons that receive group III input are inhibited by MLR stimulation

Immediate crossover fatigue after unilateral submaximal eccentric contractions of the knee flexors involves peripheral alterations and increased global perceived fatigue

After a unilateral muscle exercise, the performance of the non-exercised contralateral limb muscle can be also impaired. This crossover fatigue phenomenon is still debated in the literature and very few studies have investigated the influence of eccentric contractions. This study was designed to assess neuromuscular adaptations involved in the crossover fatigue of the non-exercised contralateral knee flexor muscles. Seventeen healthy young men performed a unilateral submaximal eccentric exercise of the right knee flexors until a 20% reduction in maximal voluntary isometric contraction torque was attained in the exercised limb. Before (PRE), immediately after exercise cessation (POST) and 24 hours later (POST24), neuromuscular function and perceived muscle soreness were measured in both the exercised limb and non-exercised limb. In addition, global perceived fatigue was assessed at each measurement time. At POST, significant reductions in maximal voluntary isometric contraction were observed in the exercised limb (-28.1%, p < 0.001) and in the non-exercised limb (-8.5%, p < 0.05), evidencing crossover fatigue. At POST, voluntary activation decreased in the exercised limb only (-6.0%, p < 0.001), while electrically evoked potentiated doublet torque was impaired in both the exercised limb and the non-exercised limb (-11.6%, p = 0.001). In addition, global perceived fatigue significantly increased at POST (p < 0.001). At POST24, all measured variables returned to PRE values, except for perceived muscle soreness scores exhibiting greater values than PRE (p < 0.05). A possible cumulative interaction between peripheral alterations and global perceived fatigue may account for the immediate crossover fatigue observed in the non-exercised limb.

The Solitary Nucleus Connectivity to Key Autonomic Regions in Humans MRI and Literature based Considerations

The nucleus tractus solitarius (NTS), is a key brainstem structure relaying interoceptive peripheral information to the interrelated brain centers for eliciting rapid autonomic responses and for shaping longer-term neuroendocrine and motor patterns. Structural and functional NTS' connectivity has been extensively investigated in laboratory animals. But there is limited information about NTS' connectome in humans. Using MRI, we examined diffusion and resting state data from 20 healthy participants in the Human Connectome Project. The regions within the brainstem (n=8), subcortical (n=6), cerebellar (n=2) and cortical (n=5) parts of the brain were selected via a systematic review of the literature and their white matter NTS connections were evaluated via probabilistic tractography along with functional and directional (i.e., Granger-causality) analyses. The underlying study confirms previous results from animal models and provides novel aspects on NTS integration in humans. Two key findings can be summarized: (i) the NTS predominantly processes afferent input and (ii) a lateralization towards a predominantly left-sided NTS processing. Our results lay the foundations for future investigations into the NTS' tripartite role comprised of interoreceptors' input integration, the resultant neurochemical outflow and cognitive/affective processing. The implications of these data add to the understanding of NTS' role in specific aspects of autonomic functions.

The physiology and pathophysiology of exercise hyperpnea

In health, the near-eucapnic, highly efficient hyperpnea during mild-to-moderate intensity exercise is driven by three obligatory contributions, namely, feedforward central command from supra-medullary locomotor centers, feedback from limb muscle afferents, and respiratory CO₂ exchange (V̇CO₂). Inhibiting each of these stimuli during exercise elicits a reduction in hyperpnea even in the continuing presence of the other major stimuli. However, the relative contribution of each stimulus to the hyperpnea remains unknown as does the means by which V̇CO2 is sensed. Mediation of the hyperventilatory response to exercise in health is attributed to the multiple feedback and feedforward stimuli resulting from muscle fatigue. In patients with COPD, diaphragm EMG amplitude and its relation to ventilatory output are used to decipher mechanisms underlying the patients' abnormal ventilatory responses, dynamic lung hyperinflation and dyspnea during exercise. Key contributions to these exercise-limiting responses across the spectrum of COPD severity include high dead space ventilation, an excessive neural drive to breathe and highly fatigable limb muscles, together with mechanical constraints on ventilation. Major controversies concerning control of exercise hyperpnea are discussed along with the need for innovative research to uncover the link of metabolism to breathing in health and disease.

Deciphering the Neural Control of Sympathetic Nerve Activity: Status Report and Directions for Future Research

Sympathetic nerve activity (SNA) contributes appreciably to the control of physiological function, such that pathological alterations in SNA can lead to a variety of diseases. The goal of this review is to discuss the characteristics of SNA, briefly review the methodology that has been used to assess SNA and its control, and to describe the essential role of neurophysiological studies in conscious animals to provide additional insights into the regulation of SNA. Studies in both humans and animals have shown that SNA is rhythmic or organized into bursts whose frequency varies depending on experimental conditions and the species. These rhythms are generated by brainstem neurons, and conveyed to sympathetic preganglionic neurons through several pathways, including those emanating from the rostral ventrolateral medulla. Although rhythmic SNA is present in decerebrate animals (indicating that neurons in the brainstem and spinal cord are adequate to generate this activity), there is considerable evidence that a variety of supratentorial structures including the insular and prefrontal cortices, amygdala, and hypothalamic subnuclei provide inputs to the brainstem regions that regulate SNA. It is also known that the characteristics of SNA are altered during stress and particular behaviors such as the defense response and exercise. While it is a certainty that supratentorial structures contribute to changes in SNA during these behaviors, the neural underpinnings of the responses are yet to be established. Understanding how SNA is modified during affective responses and particular behaviors will require neurophysiological studies in awake, behaving animals, including those that entail recording activity from neurons that generate SNA. Recent studies have shown that responses of neurons in the central nervous system to most sensory inputs are context-specific. Future neurophysiological studies in conscious animals should also ascertain whether this general rule also applies to sensory signals that modify SNA.

Neural regulation of cardiovascular response to exercise: role of central command and peripheral afferents

During dynamic exercise, mechanisms controlling the cardiovascular apparatus operate to provide adequate oxygen to fulfill metabolic demand of exercising muscles and to guarantee metabolic end-products washout. Moreover, arterial blood pressure is regulated to maintain adequate perfusion of the vital organs without excessive pressure variations. The autonomic nervous system adjustments are characterized by a parasympathetic withdrawal and a sympathetic activation. In this review, we briefly summarize neural reflexes operating during dynamic exercise. The main focus of the present review will be on the central command, the arterial baroreflex and chemoreflex, and the exercise pressure reflex. The regulation and integration of these reflexes operating during dynamic exercise and their possible role in the pathophysiology of some cardiovascular diseases are also discussed.

Cortical inhibition is reduced following short-term training in young and older adults

The purpose of this study was to investigate age-related differences in short-term training adaptations in cortical excitability and inhibition. Thirty young (21.9 ± 3.1 years) and 30 older (72.9 ± 4.6 years) individuals participated in the study. Each participant was randomly assigned to a control (n = 30) or a resistance training (n = 30) group, with equal numbers of young and older subjects in each group. Participants completed 2 days of testing, separated by 2 weeks during which time the training group participated in resistance training of the ankle dorsiflexor muscles three times per week. During each testing session, transcranial magnetic stimulation was used to generate motor evoked potentials (MEPs) and silent periods in the tibialis anterior. Hoffmann reflexes (H-reflexes) and compound muscle action potentials (M-waves) were also evoked via electrical stimulation of the peroneal nerve. At baseline, young subjects had higher maximum voluntary contraction (MVC) force (p = 0.002), larger M-wave amplitude (p < 0.001), and longer duration silent periods (p = 0.01) than older individuals, with no differences in the maximal amplitude of the MEP (p = 0.23) or H-reflex (p = 0.57). In the trained group, MVC increased in both young (17.4 %) and older (19.8 %) participants (p < 0.001), and the duration of the silent period decreased by ~15 and 12 ms, respectively (p < 0.001). Training did not significantly impact MEP (p = 0.69) or H-reflex amplitudes (p = 0.38). There were no significant changes in any measures in the control group (p ≥ 0.19) across the two testing sessions. These results indicate that a reduction in cortical inhibition may be an important neural adaptation in response to training in both young and older adults.

The mechanisms of the widespread production of phosphorylated HSP25 after fatiguing muscle stimulation

We previously showed that a widespread heat shock protein (HSP) response to fatigue of a single hindlimb muscle was responsible for a global adaptive response to an acute localized stress. We also demonstrated that the HSP response resulted from the activation of nerve afferents from the stimulated muscle. However, we did not examine the role played by the different muscle afferents or the efferent arm of HSP response. In the present study we measured the changes in phosphorylated HSP25 (pHSP25) levels in resting hindlimb muscles and the diaphragm, kidney and brain in response to a fatiguing stimulation of one tibialis anterior muscle that was repeated in five series of experiments: (1) intact muscle innervation, (2) during the selective procaine block of conduction in group IV muscle afferents, (3) after muscle nerve transection to suppress all the sensory messages, and under pharmacological blockade of the (4) alpha-adrenergic or (5) glutamatergic neurotransmission. The data showed that: (1) the pHSP25 response in hindlimb muscles resulted from the stimulation of both group III and IV muscle afferents while the pHSP25 response in the diaphragm, kidney and brain resulted from the sole activation of the group IV fibres, and (2) the blockade of alpha-adrenergic, but not glutamatergic, neurotransmission suppressed the pHSP25 response in all explored tissues except the brain. The present study highlights the role played by the group III and IV muscle afferents in the fatigue-induced pHSP25 response and shows that the sympathetic nerve supply to the muscles and kidney represents the efferent arm of the pHSP25 activation. However, the pHSP25 changes in the brain cannot be explained by the pathways investigated here.

Changes in stationary upright standing and proprioceptive reflex control of foot muscles after fatiguing static foot inversion

We searched for the consequences of a maximal static foot inversion sustained until exhaustion on the post-exercise stationary upright standing and the proprioceptive control of the foot muscles. Twelve healthy subjects executed an unilateral maximal static foot inversion during which continuous power spectrum analyses of surface electromyograms of the tibialis anterior (TA), peroneus longus (PL), and gastrocnemius medialis (GM) muscles were performed. Superimposed pulse trains (twitch interpolation) were delivered to the TA muscle to identify "central" or "peripheral" fatigue. Before and after the fatiguing task, we measured (1) the repartition of the plantar and barycentre surfaces with a computerized stationary platform, (2) the peak contractile TA response to electrical stimulation (TA twitch), (3) the tonic vibratory response (TVR) of TA and GM muscles, and (4) the Hoffman reflex. During static exercise, "central" fatigue was diagnosed in 5/12 subjects whereas in the 7 others "peripheral" TA fatigue was deduced from the absence of response to twitch interpolation and the post-exercise decrease in twitch amplitude. The sustained foot inversion was associated with reduced median frequency in TA but not in PL and GM muscles. After static exercise, in all subjects both the mean plantar and rearfoot surfaces increased, indicating a foot eversion, the TVR amplitude decreased in TA but did not vary in GM, and the Hoffman reflex remained unchanged. Whatever was the mechanism of fatigue during the maximal foot inversion task, the facilitating myotatic reflex was constantly altered in foot invertor muscles. This could explain the prevailing action of the antagonistic evertor muscles.

Fatiguing stimulation of one skeletal muscle triggers heat shock protein activation in several rat organs: the role of muscle innervation

We hypothesised that activation of muscle afferents by fatigue triggers a widespread activation of heat shock proteins (HSPs) in resting muscles and different organs. In anaesthetised rats, HSP25 and HSP70 levels were determined in both tibialis anterior (TA) and extensor digitorum longus (EDL) muscles and in the diaphragm, kidney and brain by ELISA, which mostly identifies phosphorylated HSP, and western blotting. One TA muscle was electrically stimulated and tissues were sampled 10 or 60 min after the stimulation had ended. The nerve supply to the stimulated TA or its counterpart in the contralateral limb was left intact or suppressed. In control rats, no muscle stimulation was performed and tissues were sampled at the same time points (10 or 60 min). After TA stimulation, ELISA showed an increased HSP25 content in the contralateral TA, EDL and diaphragm at 10 min but not at 60 min, and HSP70 increased in all sampled tissues at 60 min. Western blotting did not show any changes in HSP25 and HSP70 at 10 min, while at 60 min HSP25 increased in all sampled tissues except the brain and HSP70 was elevated in all tissues. Denervation of the contralateral non-stimulated limb suppressed HSP changes in TA and after denervation of the stimulated TA the widespread activation of HSPs in other organs was absent. Our data suggest that fatigue-induced activation of skeletal muscle afferents triggers an early increase in phosphorylated HSP25 in muscles and a delayed elevation of non-phosphorylated HSP25 and HSP70 in skeletal and respiratory muscles, kidney and brain.

New perspectives concerning feedback influences on cardiorespiratory control during rhythmic exercise and on exercise performance

The cardioaccelerator and ventilatory responses to rhythmic exercise in the human are commonly viewed as being mediated predominantly via feedforward 'central command' mechanisms, with contributions from locomotor muscle afferents to the sympathetically mediated pressor response. We have assessed the relative contributions of three types of feedback afferents on the cardiorespiratory response to voluntary, rhythmic exercise by inhibiting their normal 'tonic' activity in healthy animals and humans and in chronic heart failure. Transient inhibition of the carotid chemoreceptors during moderate intensity exercise reduced muscle sympathetic nerve activity (MSNA) and increased limb vascular conductance and blood flow; and reducing the normal level of respiratory muscle work during heavier intensity exercise increased limb vascular conductance and blood flow. These cardiorespiratory effects were prevented via ganglionic blockade and were enhanced in chronic heart failure and in hypoxia. Blockade of μ opioid sensitive locomotor muscle afferents, with preservation of central motor output via intrathecal fentanyl: (a) reduced the mean arterial blood pressure (MAP), heart rate and ventilatory responses to all steady state exercise intensities; and (b) during sustained high intensity exercise, reduced O(2) transport, increased central motor output and end-exercise muscle fatigue and reduced endurance performance. We propose that these three afferent reflexes - probably acting in concert with feedforward central command - contribute significantly to preserving O(2) transport to locomotor and to respiratory muscles during exercise. Locomotor muscle afferents also appear to provide feedback concerning the metabolic state of the muscle to influence central motor output, thereby limiting peripheral fatigue development.

Sympathetic responses to exercise in myocardial infarction rats: a role of central command

In congestive heart failure (CHF), exaggerated sympathetic activation is observed during exercise, which elicits excess peripheral vasoconstriction. The mechanisms causing this abnormality are not fully understood. Central command is a central neural process that induces parallel activation of motor and cardiovascular systems. This study was undertaken to determine whether central command serves as a mechanism that contributes to the exaggerated sympathetic response to exercise in CHF. In decerebrated rats, renal and lumbar sympathetic nerve responses (RSNA and LSNA, respectively) to 30 s of fictive locomotion were examined. The fictive locomotion was induced by electrical stimulation of the mesencephalic locomotor region (MLR). The study was performed in control animals (fractional shortening > 40%) and animals with myocardial infarctions (MI; fractional shortening < 30%). With low stimulation of the MLR (current intensity = 20 microA), the sympathetic responses were not significantly different in the control (RSNA: +18 +/- 4%; LSNA: +3 +/- 2%) and MI rats (RSNA: +16 +/- 5%; LSNA: +8 +/- 3%). With intense stimulation of the MLR (50 microA), the responses were significantly greater in MI rats (RSNA: +127 +/- 15%; LSNA: +57 +/- 10%) than in the control rats (RSNA: +62 +/- 5%; LSNA: +21 +/- 6%). In this study, the data demonstrate that RSNA and LSNA responses to intense stimulation of the MLR are exaggerated in MI rats. We suggest that intense activation of central command may play a role in evoking exaggerated sympathetic activation and inducing excessive peripheral vasoconstriction during exercise in CHF.

Differential sympathetic outflow and vasoconstriction responses at kidney and skeletal muscles during fictive locomotion

We compared sympathetic and circulatory responses between kidney and skeletal muscles during fictive locomotion evoked by electrical stimulation of the mesencephalic locomotor region (MLR) in decerebrate and paralyzed rats ( n = 8). Stimulation of the MLR for 30 s at 40-μA current intensity significantly increased arterial pressure (+38 ± 6 mmHg), triceps surae muscle blood flow (+17 ± 3%), and both renal and lumbar sympathetic nerve activities (RSNA +113 ± 16%, LSNA +31 ± 7%). The stimulation also significantly decreased renal cortical blood flow (−18 ± 6%) and both renal cortical and triceps surae muscle vascular conductances (RCVC −38 ± 5%, TSMVC −17 ± 3%). The sympathetic and vascular conductance changes were significantly dependent on current intensity for stimulation at 20, 30, and 40 μA. The changes in LSNA and TSMVC were significantly less than those in RSNA and RCVC, respectively, at all current intensities. At the early stage of stimulation (0–10 s), decreases in RCVC and TSMVC were significantly correlated with increases in RSNA and LSNA, respectively. These data demonstrate that fictive locomotion induces less vasoconstriction in skeletal muscles than in kidney because of less sympathetic activation. This suggests that a neural mechanism mediated by central command contributes to blood flow distribution by evoking differential sympathetic outflow during exercise.

Discharge patterns of somatosensitive neurons in the nucleus tractus solitarius of the cat

Encoding of sensory information by nucleus of the solitary tract (NTS) neurons is incompletely understood. Using extracellular single-unit recording in alpha-chloralose-urethane anesthetized cats, we have examined the discharge characteristics of NTS neurons to activation of somatic Adelta and C fiber afferents by skeletal muscle contraction evoked by electrical stimulation of lower lumbar/upper sacral ventral roots. Generally, somatic afferent stimulation evoked two distinct firing patterns. The first population (36/43 cells) increased their firing rate to brief somatic stimuli. A subset (21/27 cells) exhibited a rapid decay of their firing rate during sustained somatic stimulation. Peak instantaneous firing frequency (F(p)) increased proportionally with the intensity of somatic stimulation (105+/-4 vs. 119+/-4 vs. 139+/-4 Hz, 10, 20 and 40 Hz, respectively, P<0.0001), whereas steady-state firing frequency (F(ss)) was not altered (25+/-2 vs. 27+/-2 vs. 27+/-2 Hz, 10, 20 and 40 Hz, respectively, P=0.72). Two indices were derived to quantify the decay properties. The decay rate constant (obtained from exponential curve fitting) was not altered by stimulation frequency (461+/-10 vs. 442+/-14 vs. 429+/-26 ms, 10, 20 and 40 Hz, respectively, P=0.415), nor was the decay index (derived to express the percent reduction in firing rate with respect to the initial peak firing rate; 76+/-2 vs. 77+/-2 vs. 81+/-2%, 10, 20 and 40 Hz, respectively, P=0.187). In contrast, the second population (seven of 43 cells) decreased their firing rate to stimulation. Of the NTS neurons tested for barosensitivity (29/36), none responded to pressure stimulation. These results have identified a population of somatosensitive NTS neurons that exhibit rapid firing rate decay properties during sustained stimulation. However, this population could faithfully encode phasic excitation during rhythmic somatosensory input. These results are discussed in relation to the role of somatosensory input on baroreflex function.