Respiratory sensations, cardiovascular control, kinaesthesia and transcranial stimulation during paralysis in humans

Diencephalic and brainstem circuit mechanisms underlying autonomic cardiovascular adjustments to exercise: Recent insights from rodent studies

Autonomic cardiovascular adjustments to exercise, essential for meeting the increased metabolic demands of exercising skeletal muscle, are regulated by motor volition-driven neural activation, i.e., central command. The contribution of brain mechanisms to these adjustments has been suggested for more than a century, yet the functional brain architecture remains incompletely understood. This article discusses recent findings primarily obtained from rodent studies utilizing advanced experimental tools, particularly those enabled by genetic engineering, such as optogenetics and viral neural tracing, to elucidate the diencephalic and brainstem circuits responsible for autonomic cardiovascular adjustments during voluntary exercise. Particular attention is paid to the central neural pathways and specific neuronal populations involved in transmitting central command signals, that drive not only somatic muscular activity but also autonomic cardiovascular responses. The uncovered diencephalic and brainstem circuits are relevant to understanding the brain substrate of central command, which is essential for maintaining cellular homeostasis and enhancing physical performance. Future studies and potential subjects for further investigation to deepen our understanding of the brain mechanisms underlying autonomic cardiovascular regulation are also discussed.

Dyspnea in young subjects with congenital central hypoventilation syndrome

BACKGROUND

It has been stated that patients with congenital central hypoventilation syndrome (CCHS) do not perceive dyspnea, which could be related to defective CO2 chemosensitivity.

METHODS

We retrospectively selected the data of six-minute walk tests (6-MWT, n = 30), cardiopulmonary exercise test (CPET, n = 5) of 30 subjects with CCHS (median age, 9.3 years, 17 females) who had both peripheral (controller loop gain, CG0) and central CO2 chemosensitivity (hyperoxic, hypercapnic response test [HHRT]) measurement.

MAIN RESULTS

Ten subjects had no symptom during the HHRT, as compared to the 20 subjects exhibiting symptoms, their median ages were 14.7 versus 8.8 years (p = 0.006), their maximal PETCO2 were 71.6 versus 66.7 mmHg (p = 0.007), their median CO2 response slopes were 0.28 versus 0.30 L/min/mmHg (p = 0.533) and their CG0 values were 0.75 versus 0.50 L/min/mmHg (p = 0.567). Median dyspnea Borg score at the end of the 6-MWT was 1/10 (17/30 subjects >0), while at the end of the CPET it was 3/10 (sensation: effort). This Borg score positively correlated with arterial desaturation at walk (R = 0.43; p = 0.016) and did not independently correlate with CO2 chemosensitivities.

CONCLUSION

About half of young subjects with CCHS do exhibit mild dyspnea at walk, which is not related to hypercapnia or residual CO2 chemosensitivity.

IMPACT

Young subjects with CCHS exhibit some degree of dyspnea under CO2 exposure and on exercise that is not related to residual CO2 chemosensitivity. It has been stated that patients with CCHS do not perceive sensations of dyspnea, which must be tempered. The mild degree of exertional dyspnea can serve as an indicator for the necessity of breaks.

Deep brain stimulation of the motor thalamus relieves experimentally induced air hunger

RESEARCH QUESTION

We previously reported that deep brain stimulation (DBS) of the motor thalamus, in a patient with post-stroke tremor, relieved breathlessness associated with COPD. This raised the question of whether motor thalamus DBS mitigates the ascending dyspnoea signal. We therefore sought to conduct a fully powered cohort study of experimentally induced air hunger, an uncomfortable urge to breathe in patients with motor thalamus DBS "ON" and "OFF".

METHODS

16 patients (three females) with DBS of the ventral intermediate nucleus (VIM) as treatment for tremor underwent hypercapnic air hunger tests, with DBS ON and OFF. Patients rated air hunger on a visual analogue scale (VAS) every 15 s. Hypercapnia and ventilation were matched for ON and OFF states (end-tidal carbon dioxide tension mean±sd 43±4 and 43±4 mmHg, respectively; ventilation 13.7 and 13.4 L·min-1, respectively). Participants' ventilation was constrained to baseline levels by breathing from a 3-L inspiratory reservoir with fixed flow of fresh gas while targeting their resting breathing frequency to a metronome.

RESULTS

Overall steady-state air hunger was 52±28%VAS for ON and 67±20%VAS for OFF (p=0.002; two-tailed paired t-test). The mean reduction in air hunger during VIM DBS was -14.4%VAS. DBS of the motor thalamus relieved air hunger in 13 patients, heightened air hunger in two and caused no change in one.

CONCLUSION

DBS of the motor thalamus for tremor relief also mitigates the air hunger component of dyspnoea. We posit that DBS of the motor thalamus heightens the gating control of the thalamus modulating the ascending air hunger signal. Extent of relief suggests that thalamic DBS may prove to be a viable therapy for intractable dyspnoea.

The functional role of conscious sensation of movement

This paper proposes a new framework for investigating neural signals sufficient for a conscious sensation of movement and their role in motor control. We focus on signals sufficient for proprioceptive awareness, particularly from muscle spindle activation and from primary motor cortex (M1). Our review of muscle vibration studies reveals that afferent signals alone can induce conscious sensations of movement. Similarly, studies employing peripheral nerve blocks suggest that efferent signals from M1 are sufficient for sensations of movement. On this basis, we show that competing theories of motor control assign different roles to sensation of movement. According to motor command theories, sensation of movement corresponds to an estimation of the current state based on afferent signals, efferent signals, and predictions. In contrast, within active inference architectures, sensations correspond to proprioceptive predictions driven by efferent signals from M1. The focus on sensation of movement provides a way to critically compare and evaluate the two theories. Our analysis offers new insights into the functional roles of movement sensations in motor control and consciousness.

Increasing Sweep Gas Flow Reduces Respiratory Drive and Dyspnea in Nonintubated Venoarterial Extracorporeal Membrane Oxygenation Patients: A Pilot Study

BACKGROUND

Data on assessment and management of dyspnea in patients on venoarterial extracorporeal membrane oxygenation (ECMO) for cardiogenic shock are lacking. The hypothesis was that increasing sweep gas flow through the venoarterial extracorporeal membrane oxygenator may decrease dyspnea in nonintubated venoarterial ECMO patients exhibiting clinically significant dyspnea, with a parallel reduction in respiratory drive.

METHODS

Nonintubated, spontaneously breathing, supine patients on venoarterial ECMO for cardiogenic shock who presented with a dyspnea visual analog scale (VAS) score of greater than or equal to 40/100 mm were included. Sweep gas flow was increased up to +6 l/min by three steps of +2 l/min each. Dyspnea was assessed with the dyspnea-VAS and the Multidimensional Dyspnea Profile. The respiratory drive was assessed by the electromyographic activity of the alae nasi and parasternal muscles.

RESULTS

A total of 21 patients were included in the study. Upon inclusion, median dyspnea-VAS was 50 (interquartile range, 45 to 60) mm, and sweep gas flow was 1.0 l/min (0.5 to 2.0). An increase in sweep gas flow significantly decreased dyspnea-VAS (50 [45 to 60] at baseline vs. 20 [10 to 30] at 6 l/min; P < 0.001). The decrease in dyspnea was greater for the sensory component of dyspnea (-50% [-43 to -75]) than for the affective and emotional components (-17% [-0 to -25] and -12% [-0 to -17]; P < 0.001). An increase in sweep gas flow significantly decreased electromyographic activity of the alae nasi and parasternal muscles (-23% [-36 to -10] and -20 [-41 to -0]; P < 0.001). There was a significant correlation between the sweep gas flow and the dyspnea-VAS (r = -0.91; 95% CI, -0.94 to -0.87), between the respiratory drive and the sensory component of dyspnea (r = 0.29; 95% CI, 0.13 to 0.44) between the respiratory drive and the affective component of dyspnea (r = 0.29; 95% CI, 0.02 to 0.54) and between the sweep gas flow and the alae nasi and parasternal (r = -0.31; 95% CI, -0.44 to -0.22; and r = -0.25; 95% CI, -0.44 to -0.16).

CONCLUSIONS

In critically ill patients with venoarterial ECMO, an increase in sweep gas flow through the oxygenation membrane decreases dyspnea, possibly mediated by a decrease in respiratory drive.

EDITOR’S PERSPECTIVE

Spinal projecting neurons in rostral ventromedial medulla co-regulate motor and sympathetic tone

Many behaviors require the coordinated actions of somatic and autonomic functions. However, the underlying mechanisms remain elusive. By opto-stimulating different populations of descending spinal projecting neurons (SPNs) in anesthetized mice, we show that stimulation of excitatory SPNs in the rostral ventromedial medulla (rVMM) resulted in a simultaneous increase in somatomotor and sympathetic activities. Conversely, opto-stimulation of rVMM inhibitory SPNs decreased both activities. Anatomically, these SPNs innervate both sympathetic preganglionic neurons and motor-related regions in the spinal cord. Fiber-photometry recording indicated that the activities of rVMM SPNs correlate with different levels of muscle and sympathetic tone during distinct arousal states. Inhibiting rVMM excitatory SPNs reduced basal muscle and sympathetic tone, impairing locomotion initiation and high-speed performance. In contrast, silencing the inhibitory population abolished muscle atonia and sympathetic hypoactivity during rapid eye movement (REM) sleep. Together, these results identify rVMM SPNs as descending spinal projecting pathways controlling the tone of both the somatomotor and sympathetic systems.

Fat-free mass is associated with exercise pressor responses, but not cold pressor responses, in humans: influence of maximal voluntary contraction

Objective

This study examined the contributions of fat mass (FM) and fat-free mass (FFM) to the magnitude of exercise pressor responses in humans.

Methods

The cumulative blood pressure responses (blood pressure index; BPI) to handgrip exercise (BPIhg), post-exercise-circulatory-occlusion (BPIpeco), and cold-pressor activation (BPIcpt) were collected from 67 individuals grouped by BMI (27.8 ± 7.3 kg/m2), FFM index (FFMi, 29.1 ± 3.8 kg/m2), and FM index (FMi 12.5 ± 4.8 kg/m2) quartiles. BPI responses to HG were also normalized to the time-tension index of HG, providing a relative index of exercise pressor response magnitude (BPInorm).

Results

BPIhg and BPIpeco were significantly elevated in the third FFMi quartile (p ≤ 0.034), while BPInorm significantly decreased in the second and fourth quartiles (p ≤ 0.029). In contrast, no differences in BPIcpt were observed across any FFMi, BMI, or FMi quartiles (p ≥ 0.268). FFM was independently associated with BPIhg, BPI-peco, and BPInorm (all p ≤ 0.049), however, FFM was eliminated as an independent predictor when maximal voluntary contraction (MVC) was included in these regression models (all p ≥ 0.495). Neither FFM nor MVC was associated with BPIcpt (p ≥ 0.229).

Conclusions

These findings indicate that exercise pressor responses, but not cold-pressor responses, are significantly associated with FFM in humans, and that this association is driven by FFM related differences in MVC.

Altered locomotor muscle metaboreflex control of ventilation in patients with COPD

We investigated the locomotor muscle metaboreflex control of ventilation, circulation, and dyspnea in patients with chronic obstructive pulmonary disease (COPD). Ten patients [forced expiratory volume in 1 second (FEV1; means ± SD) = 43 ± 17% predicted] and nine age- and sex-matched controls underwent 1) cycling exercise followed by postexercise circulatory occlusion (PECO) to activate the metaboreflex or free circulatory flow to inactivate it, 2) cold pressor test to interpret whether any altered reflex response was specific to the metaboreflex arc, and 3) muscle biopsy to explore the metaboreflex arc afferent side. We measured airflow, dyspnea, heart rate, arterial pressure, muscle blood flow, and vascular conductance during reflexes activation. In addition, we measured fiber types, glutathione redox balance, and metaboreceptor-related mRNAs in the vastus lateralis. Metaboreflex activation increased ventilation versus free flow in patients (∼15%, P < 0.020) but not in controls (P > 0.450). In contrast, metaboreflex activation did not change dyspnea in patients (P = 1.000) but increased it in controls (∼100%, P < 0.001). Other metaboreflex-induced responses were similar between groups. Cold receptor activation increased ventilation similarly in both groups (P = 0.46). Patients had greater type II skeletal myocyte percentage (14%, P = 0.010), lower glutathione ratio (-34%, P = 0.015), and lower nerve growth factor (NGF) mRNA expression (-60%, P = 0.031) than controls. Therefore, COPD altered the locomotor muscle metaboreflex control of ventilation. It increased type II myocyte percentage and elicited redox imbalance, potentially producing more muscle metaboreceptor stimuli. Moreover, it decreased NGF expression, suggesting a downregulation of metabolically sensitive muscle afferents.NEW & NOTEWORTHY This study's integrative physiology approach provides evidence for a specific alteration in locomotor muscle metaboreflex control of ventilation in patients with COPD. Furthermore, molecular analyses of a skeletal muscle biopsy suggest that the amount of muscle metaboreceptor stimuli derived from type II skeletal myocytes and redox imbalance overcame a downregulation of metabolically sensitive muscle afferents.

Physical Exercise to Redynamize Interoception in Substance use Disorders

Physical exercise is considered a promising medication-free and cost-effective adjunct treatment for substance use disorders (SUD). Nevertheless, evidence regarding the effectiveness of these interventions is currently limited, thereby signaling the need to better understand the mechanisms underlying their impact on SUD, in order to reframe and optimize them. Here we advance that physical exercise could be re-conceptualized as an "interoception booster", namely as a way to help people with SUD to better decode and interpret bodily-related signals associated with transient states of homeostatic imbalances that usually trigger consumption. We first discuss how mismatches between current and desired bodily states influence the formation of reward-seeking states in SUD, in light of the insular cortex brain networks. Next, we detail effort perception during physical exercise and discuss how it can be used as a relevant framework for re-dynamizing interoception in SUD. We conclude by providing perspectives and methodological considerations for applying the proposed approach to mixed-design neurocognitive research on SUD.

Mechanosensitive channels in the mechanical component of the exercise pressor reflex

The cardiovascular response is appropriately regulated during exercise to meet the metabolic demands of the active muscles. The exercise pressor reflex is a neural feedback mechanism through thin-fiber muscle afferents activated by mechanical and metabolic stimuli in the active skeletal muscles. The mechanical component of this reflex is referred to as skeletal muscle mechanoreflex. Its initial step requires mechanotransduction mediated by mechanosensors, which convert mechanical stimuli into biological signals. Recently, various mechanosensors have been identified, and their contributions to muscle mechanoreflex have been actively investigated. Nevertheless, the mechanosensitive channels responsible for this muscular reflex remain largely unknown. This review discusses progress in our understanding of muscle mechanoreflex under healthy conditions, focusing on mechanosensitive channels.

Accuracy of Force Generation and Preparatory Prefrontal Oxygenation in Ballistic Hand Power and Precision Grips

It remains unclear whether accurate motor performance and cortical activation differ among grasping forms across several force levels. In the present study, a ballistic target force matching task (20%, 40%, 60%, and 80% of maximum voluntary force) with power grip, side pinch, and pulp pinch was utilized to explore the accuracy of the forces generated as well as the muscular activity of intrinsic and extrinsic hand muscles. By using near-infrared spectroscopy, we also examined bilateral dorsolateral prefrontal cortex (DLPFC) activation during the preparatory phase (initial 10 s) of the task. The accuracy of the power grip and pulp pinch was relatively higher than that of the side pinch, and the electromyographic activity of intrinsic hand muscles exhibited a similar trend for power grip and side pinch, while the opposite muscle recruitment pattern was observed for pulp pinch. The increment of DLPFC oxygenation across force levels differed among grasping forms, with greater activity at relatively higher levels in the power grip and side pinch, and at relatively lower levels in the pulp pinch. Taken together, the differential contribution of the DLPFC may be responsible for force generation depending on different grasping forms and force levels.

Cardiovascular response to anticipatory and reactionary postural perturbations in young adults

NEW FINDINGS

What is the central question of this study? It has been suggested that the cardiovascular responses to a postural perturbation are centrally mediated and reflex mediated. We wanted to know the extent to which the cardiovascular responses to external perturbations could be executed in a feedforward manner, in anticipation of the perturbation. What is the main finding and its importance? We found no anticipatory component driving heart rate and systolic blood pressure responses, suggesting that reflexive mechanisms dominate cardiovascular regulation after a postural perturbation in young adults.

ABSTRACT

Cardiovascular responses to postural perturbations have been reported, but whether the cardiovascular responses to external perturbations could be executed in anticipation of the perturbation is unknown. The purpose of this study was to determine the effect of anticipated and reactionary perturbations on heart rate (HR) and systolic blood pressure (SBP) responses in healthy young adults. A secondary aim was to determine whether perceived state anxiety scores were correlated with the change in HR response during postural perturbation. Twenty healthy young adults stood on a treadmill and experienced two perturbation conditions (anticipatory vs. reactionary), each with two intensity levels (Step vs. No Step). The HR and SBP were collected continuously. Two-way repeated-measures statistical non-parametric mapping tests were used to compare HR and SBP responses to the perturbations over time (from -3 to +8 s). The results indicated that HR was significantly elevated in the higher intensity perturbations [Step vs. No Step, at 0.56-1.32 s (P < 0.0001) and 1.92-3.44 s (P < 0.0001) post-perturbation], while there were no differences in HR between perturbation types (anticipatory vs. reactionary) or in SBP between perturbation types and intensity levels. The perceived state anxiety scores did not differ between perturbation types and intensity levels but were correlated with the change in HR post-perturbation (P = 0.013). We suggest that reflexive mechanisms dominate cardiovascular regulation after anticipatory and reactionary perturbations. The data highlight the cardiovascular mechanism(s) associated with perturbations that should be considered when assessing postural stability in populations with poor balance performance.

Lumbar V3 interneurons provide direct excitatory synaptic input onto thoracic sympathetic preganglionic neurons, linking locomotor, and autonomic spinal systems

Although sympathetic autonomic systems are activated in parallel with locomotion, the neural mechanisms mediating this coordination are incompletely understood. Sympathetic preganglionic neurons (SPNs), primarily located in the intermediate laminae of thoracic and upper lumbar segments (T1-L2), increase activation of tissues and organs that provide homeostatic and metabolic support during movement and exercise. Recent evidence suggests integration between locomotor and autonomic nuclei occurs within the brainstem, initiating both descending locomotor and sympathetic activation commands. However, both locomotor and sympathetic autonomic spinal systems can be activated independent of supraspinal input, in part due to a distributed network involving propriospinal neurons. Whether an intraspinal mechanism exists to coordinate activation of these systems is unknown. We hypothesized that ascending spinal neurons located in the lumbar region provide synaptic input to thoracic SPNs. Here, we demonstrate that synaptic contacts from locomotor-related V3 interneurons (INs) are present in all thoracic laminae. Injection of an anterograde tracer into lumbar segments demonstrated that 8-20% of glutamatergic input onto SPNs originated from lumbar V3 INs and displayed a somatotopographical organization of synaptic input. Whole cell patch clamp recording in SPNs demonstrated prolonged depolarizations or action potentials in response to optical activation of either lumbar V3 INs in spinal cord preparations or in response to optical activation of V3 terminals in thoracic slice preparations. This work demonstrates a direct intraspinal connection between lumbar locomotor and thoracic sympathetic networks and suggests communication between motor and autonomic systems may be a general function of the spinal cord.

Hemodynamic responses to handgrip and metaboreflex activation are exaggerated in individuals with metabolic syndrome independent of resting blood pressure, waist circumference, and fasting blood glucose

Introduction: Prior studies report conflicting evidence regarding exercise pressor and metaboreflex responses in individuals with metabolic syndrome (MetS). Purpose: To test the hypotheses that 1) exercise pressor and metaboreflex responses are exaggerated in MetS and 2) these differences may be explained by elevated resting blood pressure. Methods: Blood pressure and heart rate (HR) were evaluated in 26 participants (13 MetS) during 2 min of handgrip exercise followed by 3 min of post-exercise circulatory occlusion (PECO). Systolic (SBP), diastolic (DBP), and mean arterial pressure (MAP), along with HR and a cumulative blood pressure index (BPI), were compared between groups using independent samples t-tests, and analyses of covariance were used to adjust for differences in resting blood pressure, fasting blood glucose (FBG), and waist circumference (WC). Results: ΔSBP (∼78% and ∼54%), ΔMAP (∼67% and ∼55%), and BPI (∼16% and ∼20%) responses were significantly exaggerated in individuals with MetS during handgrip and PECO, respectively (all p ≤ 0.04). ΔDBP, ΔMAP, and BPI responses during handgrip remained significantly different between groups after independently covarying for resting blood pressure (p < 0.01), and after simultaneously covarying for resting blood pressure, FBG, and WC (p ≤ 0.03). Likewise, peak SBP, DBP, MAP, and BPI responses during PECO remained significantly different between groups after adjusting for resting blood pressure (p ≤ 0.03), with peak SBP, MAP, and BPI response remaining different between groups after adjusting for all three covariates simultaneously (p ≤ 0.04). Conclusion: These data suggest that exercise pressor and metaboreflex responses are significantly exaggerated in MetS independent of differences in resting blood pressure, FBG, or WC.

Impact of trigeminal and/or olfactory nerve stimulation on measures of inspiratory neural drive: Implications for breathlessness

The perception of breathlessness is mechanistically linked to the awareness of increased inspiratory neural drive (IND). Stimulation of upper airway cold receptors on the trigeminal nerve (TGN) with TGN agonists such as menthol or cool air to the face/nose has been hypothesized to reduce breathlessness by decreasing IND. The aim of this systematic scoping review was to identify and summarize the results of studies in animals and humans reporting on the impact of TGN stimulation or blockade on measures of IND. Thirty-one studies were identified, including 19 in laboratory animals and 12 in human participants. Studies in laboratory animals consistently reported that as TGN activity increased, measures of IND decreased (e.g., phrenic nerve activity). In humans, stimulation of the TGN with a stream of cool air to the face/nose decreased the sensitivity of the ventilatory chemoreflex response to hypercapnia. Otherwise, TGN stimulation with menthol or cool air to the face/note had no effect on measures of IND in humans. This review provides new insight into a potential neural mechanism of breathlessness relief with selected TGN agonists.

Perceived Exertion: Revisiting the History and Updating the Neurophysiology and the Practical Applications

The perceived exertion construct creation is a landmark in exercise physiology and sport science. Obtaining perceived exertion is relatively easy, but practitioners often neglect some critical methodological issues in its assessment. Furthermore, the perceived exertion definition, neurophysiological basis, and practical applications have evolved since the perceived exertion construct's inception. Therefore, we revisit the careful work devoted by Gunnar Borg with psychophysical methods to develop the perceived exertion construct, which resulted in the creation of two scales: the rating of perceived exertion (RPE) and the category-ratio 10 (CR10). We discuss a contemporary definition that considers perceived exertion as a conscious perception of how hard, heavy, and strenuous the exercise is, according to the sense of effort to command the limbs and the feeling of heavy breathing (respiratory effort). Thus, other exercise-evoked sensations would not hinder the reported perceived exertion. We then describe the neurophysiological mechanisms involved in the perceived exertion genesis during exercise, including the influence of the peripheral feedback from the skeletal muscles and the cardiorespiratory system (i.e., afferent feedback) and the influence of efferent copies from the motor command and respiratory drive (i.e., corollary discharges), as well as the interaction between them. We highlight essential details practitioners should consider when using the RPE and CR10 scales, such as the perceived exertion definition, the original scales utilization, and the descriptors anchoring process. Finally, we present how practitioners can use perceived exertion to assess cardiorespiratory fitness, individualize exercise intensity prescription, predict endurance exercise performance, and monitor athletes' responses to physical training.

The detection and sensory perception of inspiratory resistive loads in people with chronic tetraplegia

This study investigated sensations of breathing following tetraplegia. Fifteen people with chronic tetraplegia and fifteen healthy able-bodied controls matched for age, sex, height, and weight participated. Sensations of breathing were quantified by determining the threshold for detecting an added resistance during inspiration. In a separate task, the perceived magnitudes of six suprathreshold resistive loads were determined with a modified Borg scale. The detection threshold of 0.34 cmH₂O/L/s [standard deviation (SD) 0.14] in the tetraplegia group was higher than the 0.23 cmH₂O/L/s (SD 0.10) threshold for able-bodied controls (P = 0.004). Both participant groups perceived larger loads to be more effortful, with the Borg effort rating increasing linearly with the peak inspiratory pressure generated at each load. The relationship between Borg effort rating and peak inspiratory pressure was steeper in participants with tetraplegia than in able-bodied controls (P = 0.001), but there was no difference when pressure was divided by maximal inspiratory pressure (P = 0.95). Despite a higher detection threshold, the findings suggest that the perceived magnitude of a suprathreshold inspiratory load is not impaired in chronic tetraplegia and that load magnitude perception is related to the maximal, and not absolute, inspiratory muscle force.NEW & NOTEWORTHY Sensations of breathing are thought to be impaired following chronic tetraplegia. The detection threshold for an added resistive load during inspiration was higher in people with tetraplegia than in healthy able-bodied participants. However, for inspiratory loads above the detection threshold, the perceived magnitude of a resistive load as a function of the peak inspiratory pressure was greater in tetraplegia. Load magnitude perception was comparable between participant groups when peak pressure was divided by maximal inspiratory pressure.

A brainstem monosynaptic excitatory pathway that drives locomotor activities and sympathetic cardiovascular responses

Exercise including locomotion requires appropriate autonomic cardiovascular adjustments to meet the metabolic demands of contracting muscles, yet the functional brain architecture underlying these adjustments remains unknown. Here, we demonstrate brainstem circuitry that plays an essential role in relaying volitional motor signals, i.e., central command, to drive locomotor activities and sympathetic cardiovascular responses. Mesencephalic locomotor neurons in rats transmit central command-driven excitatory signals onto the rostral ventrolateral medulla at least partially via glutamatergic processes, to activate both somatomotor and sympathetic nervous systems. Optogenetic excitation of this monosynaptic pathway elicits locomotor and cardiovascular responses as seen during running exercise, whereas pathway inhibition suppresses the locomotor activities and blood pressure elevation during voluntary running without affecting basal cardiovascular homeostasis. These results demonstrate an important subcortical pathway that transmits central command signals, providing a key insight into the central circuit mechanism required for the physiological conditioning essential to maximize exercise performance.

Central modulation of cardiac baroreflex moment-to-moment sensitivity during treadmill exercise in conscious cats

It remains undetermined whether the cardiac component of the entire arterial baroreflex is blunted even at the onset of low-intensity exercise. We sought to examine the moment-to-moment sensitivity of the cardiac baroreflex during walking at different speeds and the presumed mechanisms responsible for baroreflex modulation in conscious cats. Arterial baroreflex sensitivity for heart rate was estimated from the baroreflex ratio between changes in systolic arterial blood pressure and heart rate and from the slope of the baroreflex curve between the cardiovascular responses to brief occlusion of the abdominal aorta. Treadmill walking was performed for 1 min at three levels of speed (low: 20-30 m/min, moderate: 40 m/min, and high: 50-60 m/min) or for 3 min at the stepwise change of speed (low to high to low transition). Cardiac baroreflex sensitivity was blunted at the onset of walking, irrespective of speed. Thereafter, the blunted cardiac baroreflex sensitivity was restored around 15 s of walking at any speed, while the blunting occurred again at 45 s of high-speed walking. The inhibition of cardiac baroreflex sensitivity also occurred (1) during the speed transition from low to high and (2) at 45 s of high-speed exercise of the stepwise exercise. The blunted cardiac baroreflex sensitivity was restored immediately to the resting level during the speed transition from high to low, despite sustained pressor and tachycardiac responses. Therefore, moment-to-moment modulation of the cardiac baroreflex during exercise would occur in association with motor intention (i.e., exercise onset) and effort (i.e., treadmill speed).

Muscle stiffening is associated with muscle mechanoreflex-mediated cardioacceleration

PURPOSE

Although the muscle mechanoreflex is an important mediator to cardiovascular regulation during exercise, its modulation factors remain relatively unknown. Therefore, the purpose of this study was to investigate the effect of muscle stiffness on the muscle mechanoreflex.

METHODS

Participants were divided based on their median muscle stiffness (2.00 Nm/mm) into a low group (n = 15) and a high group (n = 15), and the muscle mechanoreflex was compared between the groups. After a 15-min rest in the supine position, heart rate (HR), blood pressure (BP), stroke volume (SV), and cardiac output (CO) were measured at rest for 3 min and during static passive dorsiflexion (SPD) at 20° for 1 min. Following a 15-min re-rest, muscle stiffness and passive resistive torque were evaluated in the distal end of the muscle belly of the medial gastrocnemius.

RESULTS

Peak relative changes in HR (low group: 6 ± 4% and high group: 12 ± 4%) and CO (low group: 8 ± 10% and high group: 13 ± 9%) were greater in the high group than in the low group (both, P < 0.05). A significant positive correlation was found between resistive torque during SPD and muscle stiffness and peak relative changes in HR (r = 0.51 and 0.61, both P < 0.05). However, there was no correlation between muscle elongation during SPD and peak relative changes in HR (r = - 0.23, P = 0.20).

CONCLUSION

These findings suggest that muscle stiffness may be modulatory factor of muscle mechanoreflex.

Critical roles for breathing in the genesis and modulation of emotional states

Breathing can be classified into metabolic and behavioral categories. Metabolic breathing and voluntary behavioral breathing are controlled in the brainstem and in the cerebral motor cortex, respectively. This chapter places special emphasis on the reciprocal influences between breathing and emotional processes. As is the case with neural control of breathing, emotions are generated by multiple control networks, located primarily in the forebrain. For several decades, a respiratory rhythm generator has been investigated in the limbic system. The amygdala receives respiratory-related input from the piriform cortex. Excitatory recurrent branches are located in the piriform cortex and have tight reciprocal synaptic connections, which produce periodic oscillations, similar to those recorded in the hippocampus during slow-wave sleep. The relationship between olfactory breathing rhythm and emotion is seen as the gateway to interpreting the relationship between breathing and emotion. In this chapter, we describe roles of breathing in the genesis of emotion, neural structures common to breathing and emotion, and mutual importance of breathing and emotion. We also describe the central roles of conscious awareness and voluntary control of breathing, as effective methods for stabilizing attention and the contents in the stream of consciousness. Voluntary control of breathing is seen as an essential practice for achieving emotional well-being.

Dyspnea

The clinical term dyspnea (a.k.a. breathlessness or shortness of breath) encompasses at least three qualitatively distinct sensations that warn of threats to breathing: air hunger, effort to breathe, and chest tightness. Air hunger is a primal homeostatic warning signal of insufficient alveolar ventilation that can produce fear and anxiety and severely impacts the lives of patients with cardiopulmonary, neuromuscular, psychological, and end-stage disease. The sense of effort to breathe informs of increased respiratory muscle activity and warns of potential impediments to breathing. Most frequently associated with bronchoconstriction, chest tightness may warn of airway inflammation and constriction through activation of airway sensory nerves. This chapter reviews human and functional brain imaging studies with comparison to pertinent neurorespiratory studies in animals to propose the interoceptive networks underlying each sensation. The neural origins of their distinct sensory and affective dimensions are discussed, and areas for future research are proposed. Despite dyspnea's clinical prevalence and impact, management of dyspnea languishes decades behind the treatment of pain. The neurophysiological bases of current therapeutic approaches are reviewed; however, a better understanding of the neural mechanisms of dyspnea may lead to development of novel therapies and improved patient care.

Enhancement of self-sustained muscle activity through external dead space ventilation appears to be associated with hypercapnia

We reported that external dead space ventilation (EDSV) enhanced self-sustained muscle activity (SSMA) of the human soleus muscle, which is an indirect observation of plateau potentials. However, the main factor for EDSV to enhance SSMA remains unclear. The purpose of the present study was to examine the effects of EDSV-induced hypercapnia, hypoxia, and hyperventilation on SSMA. In Experiment 1 (n = 11; normal breathing [NB], EDSV, hypoxia, and voluntary hyperventilation conditions) and Experiment 2 (n = 9; NB and normoxic hypercapnia [NH] conditions), SSMA was evoked by electrical train stimulations of the right tibial nerve and measured using surface electromyography under each respiratory condition. In Experiment 1, SSMA was significantly higher than that in the NB condition only in the EDSV condition (P < 0.05). In Experiment 2, SSMA was higher in the NH condition than in the NB condition (P < 0.05). These results suggest that the EDSV-enhanced SSMA is due to hypercapnia, not hypoxia or increased ventilation.

Central command-related increases in blood velocity of anterior cerebral artery and prefrontal oxygenation at the onset of voluntary tapping

The anterior cerebral artery (ACA) supplies blood predominantly to the frontal lobe including the prefrontal cortex. Our laboratory reported that prefrontal oxygenated-hemoglobin concentration (Oxy-Hb) increased before and at exercise onset, as long as exercise is arbitrarily started. Moreover, the increased prefrontal oxygenation seems independent of both exercise intensity and muscle mass. If so, mean blood velocity of the ACA (ACA_BV) should increase with "very light motor effort," concomitantly with the preexercise and initial increase in prefrontal Oxy-Hb. This study aimed to examine the responses in ACA_BV and vascular conductance index (ACA_VCI) of the ACA as well as prefrontal Oxy-Hb during arbitrary or cued finger tapping in 12 subjects, an activity with a Borg scale perceived exertion rating of 7 (median). With arbitrary start, ACA_BV increased at tapping onset (14 ± 9%) via an elevation in ACA_VCI. Likewise, prefrontal Oxy-Hb increased at the onset of tapping with a time course resembling that of ACA_BV. A positive cross correlation between the initial changes in ACA_BV and prefrontal Oxy-Hb was found significant in 67% of subjects, having a time lag of 2 s, whereas a positive linear regression between them was significant in 75% of subjects. When tapping was forced to start by cue, the initial increases in ACA_BV, ACA_VCI, and prefrontal Oxy-Hb were delayed and blunted as compared with an arbitrary start. Thus, active vasodilatation of the ACA vascular bed occurs at tapping onset, as long as tapping is arbitrarily started, and contributes to immediate increases in blood flow and prefrontal oxygenation.NEW & NOTEWORTHY Anterior cerebral artery blood velocity and vascular conductance index along with prefrontal oxygenated-hemoglobin concentration all increased at the onset of finger tapping, peaking immediately after tapping onset, as long as tapping was arbitrarily started. Positive cross correlation and linear regression between the increases in ACA_BV and prefrontal Oxy-Hb were significant in 67%-75% of subjects. Active vasodilatation of the ACA vascular bed occurs with arbitrary tapping onset and contributes to increased ACA_BV and prefrontal oxygenation.

Sense of effort is distorted in people with chronic low back pain

BACKGROUND

Proprioceptive deficits in people with low back pain (LBP) have traditionally been attributed to altered paraspinal muscle spindle afference and its central processing. Studies conducted in the upper limb demonstrated that sense of effort is also an important source of kinaesthetic information.

OBJECTIVES

To better understand proprioceptive deficits in people with chronic LBP (cLBP), this study aimed to test whether sense of effort is affected in people with cLBP.

DESIGN

Cross-sectional study.

METHOD

Fourteen participants with cLBP and fourteen healthy participants performed a 120 s force matching task with their trunk extensor muscles at a low intensity.

RESULTS

When visual feedback of the generated force was provided, both groups performed the task accurately. Removal of visual feedback resulted in an increase in error for both groups (p < 0.0001), but the increase in error was significantly larger for the cLBP group (p = 0.023). This larger error could be attributed to undershooting of the target force (p = 0.020). The control group did not consistently undershoot or overshoot the target force (p = 0.93). Furthermore, the amount of undershooting for the cLBP group increased as the task progressed (p = 0.016), which was not observed for the control group (p = 0.80).

CONCLUSIONS

The results of this study revealed that sense of effort is affected in cLBP. People with cLBP overestimated the trunk extension force they generated, and the error increased as the trial progressed. With visual feedback however, people with cLBP were able to compensate and perform the task as accurately as people without cLBP.

Insular functional organization during handgrip in females and males with obstructive sleep apnea

STUDY OBJECTIVES

Brain regulation of autonomic function in obstructive sleep apnea (OSA) is disrupted in a sex-specific manner, including in the insula, which may contribute to several comorbidities. The insular gyri have anatomically distinct functions with respect to autonomic nervous system regulation; yet, OSA exerts little effect on the organization of insular gyral responses to sympathetic components of an autonomic challenge, the Valsalva. We further assessed neural responses of insular gyri in people with OSA to a static handgrip task, which principally involves parasympathetic withdrawal.

METHODS

We measured insular function with blood oxygen level dependent functional MRI. We studied 48 newly-diagnosed OSA (age mean±std:46.5±9 years; AHI±std:32.6±21.1 events/hour; 36 male) and 63 healthy (47.2±8.8 years;40 male) participants. Subjects performed four 16s handgrips (1 min intervals, 80% subjective maximum strength) during scanning. fMRI time trends from five insular gyri-anterior short (ASG); mid short (MSG); posterior short (PSG); anterior long (ALG); and posterior long (PLG)-were assessed for within-group responses and between-group differences with repeated measures ANOVA (p<0.05) in combined and separate female-male models; age and resting heart-rate (HR) influences were also assessed.

RESULTS

Females showed greater right anterior dominance at the ASG, but no differences emerged between OSA and controls in relation to functional organization of the insula in response to handgrip. Males showed greater left anterior dominance at the ASG, but there were also no differences between OSA and controls. The males showed a group difference between OSA and controls only in the ALG. OSA males had lower left activation at the ALG compared to control males. Responses were mostly influenced by HR and age; however, age did not impact the response for right anterior dominance in females.

CONCLUSIONS

Insular gyri functional responses to handgrip differ in OSA vs controls in a sex-based manner, but only in laterality of one gyrus, suggesting anterior and right-side insular dominance during sympathetic activation but parasympathetic withdrawal is largely intact, despite morphologic injury to the overall structure.

Air Hunger: A Primal Sensation and a Primary Element of Dyspnea

The sensation that develops as a long breath hold continues is what this article is about. We term this sensation of an urge to breathe "air hunger." Air hunger, a primal sensation, alerts us to a failure to meet an urgent homeostatic need maintaining gas exchange. Anxiety, frustration, and fear evoked by air hunger motivate behavioral actions to address the failure. The unpleasantness and emotional consequences of air hunger make it the most debilitating component of clinical dyspnea, a symptom associated with respiratory, cardiovascular, and metabolic diseases. In most clinical populations studied, air hunger is the predominant form of dyspnea (colloquially, shortness of breath). Most experimental subjects can reliably quantify air hunger using rating scales, that is, there is a consistent relationship between stimulus and rating. Stimuli that increase air hunger include hypercapnia, hypoxia, exercise, and acidosis; tidal expansion of the lungs reduces air hunger. Thus, the defining experimental paradigm to evoke air hunger is to elevate the drive to breathe while mechanically restricting ventilation. Functional brain imaging studies have shown that air hunger activates the insular cortex (an integration center for perceptions related to homeostasis, including pain, food hunger, and thirst), as well as limbic structures involved with anxiety and fear. Although much has been learned about air hunger in the past few decades, much remains to be discovered, such as an accepted method to quantify air hunger in nonhuman animals, fundamental questions about neural mechanisms, and adequate and safe methods to mitigate air hunger in clinical situations. © 2021 American Physiological Society. Compr Physiol 11:1449-1483, 2021.

Responses of Neurons in the Medullary Lateral Tegmental Field and Nucleus Tractus Solitarius to Vestibular Stimuli in Conscious Felines

Considerable evidence shows that the vestibular system contributes to adjusting sympathetic nervous system activity to maintain adequate blood pressure during movement and changes in posture. However, only a few prior experiments entailed recordings in conscious animals from brainstem neurons presumed to convey baroreceptor and vestibular inputs to neurons in the rostral ventrolateral medulla (RVLM) that provide inputs to sympathetic preganglionic neurons in the spinal cord. In this study, recordings were made in conscious felines from neurons in the medullary lateral tegmental field (LTF) and nucleus tractus solitarius (NTS) identified as regulating sympathetic nervous system activity by exhibiting changes in firing rate related to the cardiac cycle, or cardiac-related activity (CRA). Approximately 38% of LTF and NTS neurons responded to static 40° head up tilts with a change in firing rate (increase for 60% of the neurons, decrease for 40%) of ~50%. However, few of these neurons responded to 10° sinusoidal rotations in the pitch plane, in contrast to prior findings in decerebrate animals that the firing rates of both NTS and LTF neurons are modulated by small-amplitude body rotations. Thus, as previously demonstrated for RVLM neurons, in conscious animals NTS and LTF neurons only respond to large rotations that lead to changes in sympathetic nervous system activity. The similar responses to head-up rotations of LTF and NTS neurons with those documented for RVLM neurons suggest that LTF and NTS neurons are components of the vestibulo-sympathetic reflex pathway. However, a difference between NTS/LTF and RVLM neurons was variability in CRA over time. This variability was significantly greater for RVLM neurons, raising the hypothesis that the responsiveness of these neurons to baroreceptor input is adjusted based on the animal's vigilance and alertness.

Increased prefrontal oxygenation prior to and at the onset of over-ground locomotion in humans

We found using wireless near-infrared spectroscopy that prefrontal oxygenation increased before the onset of arbitrary over-ground walking, whereas the preexercise increase was absent when walking was suddenly started by cue. The difference in prefrontal oxygenation between start modes (considered related to central command) preceded heart rate response variances and demonstrated a positive relationship with the difference in heart rate. The central command-related prefrontal activity may contribute to cardiac adjustment, synchronized with the beginning of over-ground walking.

Evidence of Impaired Cerebellar Connectivity at Rest and During Autonomic Maneuvers in Patients with Autonomic Failure

The objective of the current study was to investigate whether patients with neurogenic orthostatic hypotension (NOH) secondary to autonomic failure have impaired functional connectivity between the cerebellum and central autonomic structures during autonomic challenges. Fifteen healthy controls (61 ± 14 years) and 15 NOH patients (67 ± 6 years; p = 0.12) completed the following tasks during a functional brain MRI: (1) 5 min of rest, (2) 5 min of lower-body negative pressure (LBNP) performed at - 35 mmHg, and (3) Three, 15-s Valsalva maneuvers (VM) at 40 mmHg. Functional connectivity (Conn Toolbox V18) between central autonomic structures and discrete cerebellar regions involved in cardiovascular autonomic control, including the vermis and posterior cerebellum, was assessed using a regions-of-interest approach during rest, LBNP and VM. Functional connectivity was contrasted between controls and patients with autonomic failure. At rest, controls had significantly more intra-cerebellar connectivity and more connectivity between cerebellar lobule 9 and key central autonomic structures, including: bilateral anterior insula (T_R-value: 4.84; T_L-value: 4.51), anterior cingulate cortex (T-value: 3.41) and bilateral thalamus (T_R-value: 3.95; T_L-value: 4.51). During autonomic maneuvers, controls showed significantly more connectivity between cardiovascular cerebellar regions (lobule 9 and anterior vermis) and important autonomic regulatory sites, including the brainstem, hippocampus and cingulate: vermis-brainstem (T-value: 4.31), lobule 9-brainstem (T_R-value, 5.29; T_L-value, 4.53), vermis-hippocampus (T-value, 4.63), and vermis-cingulate (T-value, 4.18). Anatomical and functional studies in animals and humans substantiate a significant role for the cerebellum in cardiovascular autonomic control during postural adjustments. In the current study, patients with NOH related to autonomic failure showed evidence of reduced connectivity between cardiovascular cerebellar regions and several important central autonomic structures, including the brainstem. The cerebellum is an established structure in cardiovascular autonomic control; therefore, evidence of impaired cerebellar connectivity to other autonomic structures may further contribute to the inability to properly regulate blood pressure during postural changes in NOH patients.

Dyspnea, Acute Respiratory Failure, Psychological Trauma, and Post-ICU Mental Health: A Caution and a Call for Research

Dyspnea is an uncomfortable sensation with the potential to cause psychological trauma. Patients presenting with acute respiratory failure, particularly when tidal volume is restricted during mechanical ventilation, may experience the most distressing form of dyspnea known as air hunger. Air hunger activates brain pathways known to be involved in posttraumatic stress disorder (PTSD), anxiety, and depression. These conditions are considered part of the post-intensive care syndrome. These sequelae may be even more prevalent among patients with ARDS. Low tidal volume, a mainstay of modern therapy for ARDS, is difficult to avoid and is likely to cause air hunger despite sedation. Adjunctive neuromuscular blockade does not prevent or relieve air hunger, but it does prevent the patient from communicating discomfort to caregivers. Consequently, paralysis may also contribute to the development of PTSD. Although research has identified post-ARDS PTSD as a cause for concern, and investigators have taken steps to quantify the burden of disease, there is little information to guide mechanical ventilation strategies designed to reduce its occurrence. We suggest such efforts will be more successful if they are directed at the known mechanisms of air hunger. Investigation of the antidyspnea effects of sedative and analgesic drugs commonly used in the ICU and their impact on post-ARDS PTSD symptoms is a logical next step. Although in practice we often accept negative consequences of life-saving therapies as unavoidable, we must understand the negative sequelae of our therapies and work to minimize them under our primary directive to "first, do no harm" to patients.

The reliability of inspiratory resistive load magnitude and detection testing

OBJECTIVES

To assess the test-retest reliability of inspiratory load detection and load magnitude perception tests in healthy volunteers.

DESIGN

Cohort of convenience.

SETTING

Respiratory physiology laboratory.

PARTICIPANTS

Twenty healthy adults.

INTERVENTIONS

On two separate occasions participants performed tests of inspiratory loading. Participants breathed through custom made resistive tubing and were asked to indicate when they detected a different resistance during inspiration. In a second test participants rated the magnitude of presented inspiratory loads using the modified Borg score.

MAIN OUTCOME MEASURES

Intra-class Correlation Coefficient (ICC_2,1) values for repeated tests of inspiratory load detection threshold and load magnitude rating.

RESULTS

ICC_2,1 values ranged from 0.657-0.786 for load detection testing and 0.739 to 0.969 for rating of load magnitude.

CONCLUSIONS

The tests are simple and reliable measures of inspiratory load detection and magnitude rating. They can be used in future research to determine the effectiveness of interventions to reduce the effort of breathing in health and disease.

The pathophysiology of 'happy' hypoxemia in COVID-19

The novel coronavirus disease 2019 (COVID-19) pandemic is a global crisis, challenging healthcare systems worldwide. Many patients present with a remarkable disconnect in rest between profound hypoxemia yet without proportional signs of respiratory distress (i.e. happy hypoxemia) and rapid deterioration can occur. This particular clinical presentation in COVID-19 patients contrasts with the experience of physicians usually treating critically ill patients in respiratory failure and ensuring timely referral to the intensive care unit can, therefore, be challenging. A thorough understanding of the pathophysiological determinants of respiratory drive and hypoxemia may promote a more complete comprehension of a patient's clinical presentation and management. Preserved oxygen saturation despite low partial pressure of oxygen in arterial blood samples occur, due to leftward shift of the oxyhemoglobin dissociation curve induced by hypoxemia-driven hyperventilation as well as possible direct viral interactions with hemoglobin. Ventilation-perfusion mismatch, ranging from shunts to alveolar dead space ventilation, is the central hallmark and offers various therapeutic targets.

Effect of depth of anesthesia on the phase lag entropy in patients undergoing general anesthesia by propofol: A STROBE-compliant study

The PLEM100 (Inbody Co., Ltd., Seoul, Korea) is a device for measuring phase lag entropy (PLE), a recently developed index for the quantification of consciousness during sedation and general anesthesia. In the present study, we assessed changes in PLE along with the level of consciousness during the induction of general anesthesia using propofol. PLE was compared with the bispectral index (BIS), which is currently the most commonly used index of consciousness.After obtaining Institutional Review Board approval and written informed consent, we enrolled 15 patients (8 men, 7 women; mean age: 37 ± 9 years; mean height: 168 ± 8 cm; mean weight; 68 ± 11 kg) undergoing nasal bone reduction. PLE and BIS sensors were attached simultaneously, and general anesthesia was induced via target-controlled infusion (TCI) of propofol. PLE and BIS scores were recorded when the calculated effect site concentration shown on the TCI pump was equal to the target concentrations of 1.5, 2.0, 2.5, 2.8, 3.0, 3.2, 3.4, and 3.5 μg/mL (and at each 0.1 μg/mL increase, thereafter). Observer's Assessment of Alertness/Sedation (OAA/S) scores were also recorded until unconsciousness was achieved. Throughout the anesthesia period, all pairs of PLE and BIS data were collected using data acquisition software.The partial correlation coefficients between OAA/S scores and PLE, and between OAA/S scores and BIS were 0.778 (P < .001) and 0.846 (P < .001), respectively. Throughout the period of anesthesia, PLE and BIS exhibited a significant positive correlation. The partial correlation coefficient prior to the loss of consciousness was 0.838 (P < .001), and 0.669 (P < .001) following the loss of consciousness. Intra-class correlation between the 2 indices was 0.889 (P < .001) and 0.791 (P < .001) prior and following the loss of consciousness, respectively.PLE exhibited a strong and predictable correlation with both BIS and OAA/S scores. These results suggest that PLE is reliable for assessing the level of consciousness during sedation and general anesthesia.

Countdown before voluntary exercise induces muscle vasodilation with baroreflex-mediated decrease in muscle sympathetic nerve activity in humans

We examined whether a countdown (CD) before voluntary cycling exercise induced prospective vascular adjustment for the exercise and, if so, whether and how muscle sympathetic nerve activity (MSNA) was involved in the responses. Young men performed voluntary cycling in a semirecumbent position (n = 14) while middle cerebral artery blood flow velocity (V_MCA; Doppler ultrasonography), heart rate (HR), arterial pressure (AP; finger photoplethysmography), oxygen consumption rate (V̇o₂), oxygen saturation in the thigh muscle (StO2; near-infrared spectrometry), cardiac output (CO; Modelflow method), and total peripheral resistance (TPR) were measured (experiment 1). Another group underwent the same exercise protocol but used only the right leg (n = 10) while MSNA (microneurography) was measured in the peroneal nerve of the left leg (experiment 2). All subjects performed eight trials with a ≥5-min rest between trials. In four trials randomly selected from the eight trials, exercise onset was signaled by a 30-s CD, whereas in the remaining four trials, exercise was started without CD. We found that CD first increased V_MCA, HR, CO, and mean AP, and then decreased TPR and increased StO2 and V̇o₂ (experiment 1; all P < 0.021). Furthermore, the CD-induced increase in mean AP decreased total MSNA and burst frequency (experiment 2; both P < 0.048) through the baroreflex, with decreased TPR and increased StO2 (experiment 2; both P < 0.001). The vasodilation and increased V̇o₂ continued after the start of exercise. Thus CD before starting exercise induced the muscle vasodilatory response with a concomitant reduction in MSNA through the baroreflex to accelerate aerobic energy production after the start of exercise.NEW & NOTEWORTHY Prospective cardiovascular adjustment occurs before starting voluntary exercise, increasing heart rate and arterial pressure followed by muscle vasodilation; however, the precise mechanisms and significance for this vasodilation remain unknown. We found that during the countdown before starting exercise cerebral blood flow velocity increased, followed by increases in heart rate and arterial pressure, which suppressed MSNA through baroreflex, resulting in thigh muscle vasodilation to increase oxygen consumption rate, which might make it easier to start exercise.

Responses of neurons in the rostral ventrolateral medulla of conscious cats to anticipated and passive movements

The vestibular system contributes to regulating sympathetic nerve activity and blood pressure. Initial studies in decerebrate animals showed that neurons in the rostral ventrolateral medulla (RVLM) respond to small-amplitude (<10°) rotations of the body, as in other brain areas that process vestibular signals, although such movements do not affect blood distribution in the body. However, a subsequent experiment in conscious animals showed that few RVLM neurons respond to small-amplitude movements. This study tested the hypothesis that RVLM neurons in conscious animals respond to signals from the vestibular otolith organs elicited by large-amplitude static tilts. The activity of approximately one-third of RVLM neurons whose firing rate was related to the cardiac cycle, and thus likely received baroreceptor inputs, was modulated by vestibular inputs elicited by 40° head-up tilts in conscious cats, but not during 10° sinusoidal rotations in the pitch plane that affected the activity of neurons in brain regions providing inputs to the RVLM. These data suggest the existence of brain circuitry that suppresses vestibular influences on the activity of RVLM neurons and the sympathetic nervous system unless these inputs are physiologically warranted. We also determined that RVLM neurons failed to respond to a light cue signaling the movement, suggesting that feedforward cardiovascular responses do not occur before passive movements that require cardiovascular adjustments.

Dyspnea in COPD: New Mechanistic Insights and Management Implications

Dyspnea is the most common symptom experienced by patients with chronic obstructive pulmonary disease (COPD). To avoid exertional dyspnea, many patients adopt a sedentary lifestyle which predictably leads to extensive skeletal muscle deconditioning, social isolation, and its negative psychological sequalae. This "dyspnea spiral" is well documented and it is no surprise that alleviation of this distressing symptom has become a key objective highlighted across COPD guidelines. In reality, this important goal is often difficult to achieve, and successful symptom management awaits a clearer understanding of the underlying mechanisms of dyspnea and how these can be therapeutically manipulated for the patients' benefit. Current theoretical constructs of the origins of activity-related dyspnea generally endorse the classical demand-capacity imbalance theory. Thus, it is believed that disruption of the normally harmonious relationship between inspiratory neural drive (IND) to breathe and the simultaneous dynamic response of the respiratory system fundamentally shapes the expression of respiratory discomfort in COPD. Sadly, the symptom of dyspnea cannot be eliminated in patients with advanced COPD with relatively fixed pathophysiological impairment. However, there is evidence that effective symptom palliation is possible for many. Interventions that reduce IND, without compromising alveolar ventilation (VA), or that improve respiratory mechanics and muscle function, or that address the affective dimension, achieve measurable benefits. A common final pathway of dyspnea relief and improved exercise tolerance across the range of therapeutic interventions (bronchodilators, exercise training, ambulatory oxygen, inspiratory muscle training, and opiate medications) is reduced neuromechanical dissociation of the respiratory system. These interventions, singly and in combination, partially restore more harmonious matching of excessive IND to ventilatory output achieved. In this review we propose, on the basis of a thorough review of the recent literature, that effective dyspnea amelioration requires combined interventions and a structured multidisciplinary approach, carefully tailored to meet the specific needs of the individual.

A new conceptual framework for the integrated neural control of locomotor and sympathetic function: implications for exercise after spinal cord injury

All mammals, including humans, are designed to produce sustained locomotor movements. Many higher centres are involved in movement, but ultimately these centres act upon a core "rhythm-generating" network within the brainstem-spinal cord. In addition, endurance-based locomotor exercise requires sympathetic neural support to maintain homeostasis and to provide needed metabolic resources. This review focuses on the roles and integration of these 2 neural systems. Part I reviews the cardiovascular, thermoregulatory, and metabolic functions under spinal sympathetic control as revealed by spinal cord injury at different levels. Part II examines the integration between brainstem-spinal sympathetic pathways and the neural circuitry producing motor rhythms. In particular, the rostroventral medulla (RVM) contains the neural circuitry that (i) integrates heart rate, contractility, and blood flow in response to postural changes; (ii) initiates and maintains cardiovascular adaptations for exercise; (iii) provides direct descending innervation to preganglionic neurons innervating the adrenal glands, white adipose tissue, and tissues responsible for cooling the body; (iv) integrates descending sympathetic drive for energy substrate mobilization (lipolysis); and (v) is the relay for descending locomotor commands arising from higher brain centres. A unifying conceptual framework is presented, in which the RVM serves as the final descending supraspinal "exercise integration centre" linking the descending locomotor command signal with the metabolic and homeostatic support needed to produce prolonged rhythmic activities. The role and rationale for an ascending sympathetic and locomotor drive from the lower to upper limbs within this framework is presented. Examples of new research directions based on this unifying framework are discussed.

The Pressor Response to Concurrent Stimulation of the Mesencephalic Locomotor Region and Peripheral Sensory Afferents Is Attenuated in Normotensive but Not Hypertensive Rats

Central command (CC) and the exercise pressor reflex (EPR) regulate blood pressure during exercise. We previously demonstrated that experimental stimulation of the CC and EPR pathways independently contribute to the exaggerated pressor response to exercise in hypertension. It is known that CC and EPR modify one another functionally. Whether their interactive relationship is altered in hypertension, contributing to the generation of this potentiated blood pressure response, remains unknown. To address this issue, the pressor response to activation of the CC pathway with and without concurrent stimulation of the EPR pathway, and vice versa, was examined in normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. In decerebrated, paralyzed animals, activation of the CC pathway was evoked by electrical stimulation of the mesencephalic locomotor region (MLR; 20–50 μA in 10-μA steps). Electrical stimulation of the sciatic nerve (SN, 3, 5, and 10 × motor threshold; MT) was used to activate hindlimb afferents known to carry EPR sensory information. In both WKY and SHR, the algebraic sum of the pressor responses to individual stimulation of the MLR and SN were greater than when both inputs were stimulated simultaneously. Although the blood pressure response to a constant level of SN stimulation was not significantly affected by concurrent MLR stimulation at variable intensities, the pressor response to a constant level of MLR simulation was significantly attenuated by concurrent SN stimulation in WKY but not in SHR. These findings suggest the interactive relationship between CC and the EPR is inhibitory in nature in both WKY and SHR. However, the neural occlusion between these central and peripheral pressor mechanisms is attenuated in hypertension.

The impact of environmental temperature deception on perceived exertion during fixed-intensity exercise in the heat in trained-cyclists

PURPOSE

This study examined the effect of environmental temperature deception on the rating of perceived exertion (RPE) during 30 min of fixed-intensity cycling in the heat.

METHODS

Eleven trained male cyclists completed an incremental cycling test and four experimental trials. Trials consisted of 30 min cycling at 50% P_max, once in 24 °C (CON) and three times in 33 °C. In the hot trials, participants were provided with accurate temperature feedback (HOT), or were deceived to believe the temperature was 28 °C (DEC_LOW) or 38 °C (DEC_HIGH). During cycling, RPE was recorded every 5 min. Rectal and skin temperature, heart rate and oxygen uptake were continuously measured. Data were analysed using linear mixed model methods in a Bayesian framework, magnitude-based inferences (Cohens d), and the probability that d exceeded the smallest worthwhile change.

RESULTS

RPE was higher in the heat compared to CON, but not statistically different between the hot conditions (mean [95% credible interval]; DEC_LOW: 13.0 [11.9, 14.1]; HOT: 13.0 [11.9, 14.1]; DEC_HIGH: 13.1 [12.0, 14.2]). Heart rate was significantly higher in DEC_HIGH (141 b·min^-1 [132, 149]) compared to all other conditions (DEC_LOW: 138 b·min^-1 [129, 146]; HOT: 138 b·min^-1 [129, 145]) after 10 min; however, this did not alter RPE. All other physiological variables did not differ between the hot conditions.

CONCLUSION

Participants were under the impression they were cycling in different environments; however, this did not influence RPE. These data suggest that for trained cyclists, an awareness of environmental temperature does not contribute to the generation of RPE when exercising at a fixed intensity in the heat.

Feedforward- and motor effort-dependent increase in prefrontal oxygenation during voluntary one-armed cranking

KEY POINTS

Some cortical areas are believed to transmit a descending signal in association with motor intention and/or effort that regulates the cardiovascular system during exercise (termed central command). However, there was no evidence for the specific cortical area responding prior to arbitrary motor execution and in proportion to the motor effort. Using a multichannel near-infrared spectroscopy system, we found that the oxygenation of the dorsolateral and ventrolateral prefrontal cortices on the right side increases in a feedforward- and motor effort-dependent manner during voluntary one-armed cranking with the right arm. This finding may suggest a role of the dorsolateral and ventrolateral prefrontal cortices in triggering off central command and may help us to understand impaired regulation of the cardiovascular system in association with lesion of the prefrontal cortex.

ABSTRACT

Output from higher brain centres (termed central command) regulates the cardiovascular system during exercise in a feedforward- and motor effort-dependent manner. This study aimed to determine a cortical area responding prior to arbitrarily started exercise and in proportion to the effort during exercise. The oxygenation responses in the frontal and frontoparietal areas during one-armed cranking with the right arm were measured using multichannel near-infrared spectroscopy, as indexes of regional blood flow responses, in 20 subjects. The intensity of voluntary exercise was 30% and 60% of the maximal voluntary effort (MVE). At the start period of both voluntary cranking tasks, the oxygenation increased (P < 0.05) only in the lateral and dorsal part of the dorsolateral prefrontal cortex (DLPFC), ventrolateral prefrontal cortex (VLPFC) and sensorimotor cortices. Then, the oxygenation increased gradually in all cortical areas during cranking at 60% MVE, while oxygenation increased only in the frontoparietal area and some of the frontal area during cranking at 30% MVE. The rating of perceived exertion to the cranking tasks correlated (P < 0.05) with the oxygenation responses on the right side of the lateral-DLPFC (r = 0.46) and VLPFC (r = 0.48) and the frontopolar areas (r = 0.47-0.49). Motor-driven passive one-armed cranking decreased the oxygenation in most cortical areas, except the contralateral frontoparietal areas. Accordingly, the lateral-DLPFC and VLPFC on the right side would respond in a feedforward- and motor effort-dependent manner during voluntary exercise with the right arm. Afferent inputs from mechanosensitive afferents may decrease the cortical oxygenation.