Identifying cardiorespiratory neurocircuitry involved in central command during exercise in humans

Effect of optogenetic excitation of non-orexinergic neurons in the hypothalamic perifornical area on motor behaviors and cardiovascular parameters in rats

Central command, a motor volition originating in the rostral part of the brain, plays a pivotal role in the precise regulation of autonomic nervous and cardiovascular systems. Central neuronal substrates responsible for transmitting central command signals remain incompletely understood. This study aimed to investigate the effect of optogenetic excitation of non-orexinergic (NOrx) neurons in the hypothalamic perifornical area (PeFA), where orexinergic neurons are densely distributed, on motor behaviors and cardiovascular parameters in rats. An adeno-associated viral serotype 2 vector carrying the human synapsin promoter encoding channelrhodopsin 2 (ChR2) fused to EYFP was injected into the PeFA of Sprague-Dawley rats, resulting in selective expression of ChR2-EYFP in NOrx PeFA neurons. In conscious rats, optogenetic excitation of NOrx PeFA neurons rapidly elicited walking or biting behavior, simultaneously causing pressor and tachycardiac responses regardless of the observed behavioral patterns. Under anesthesia, this excitation rapidly increased renal sympathetic nerve activity, immediately followed by sympathoinhibition. These findings suggest that NOrx PeFA neurons transmit central command signals, concurrently regulating somatomotor and autonomic nervous systems for locomotor exercise or biting behavior.

Inaugural Review Prize 2023: The exercise hyperpnoea dilemma: A 21st-century perspective

During mild or moderate exercise, alveolar ventilation increases in direct proportion to metabolic rate, regulating arterial CO2 pressure near resting levels. Mechanisms giving rise to the hyperpnoea of exercise are unsettled despite over a century of investigation. In the past three decades, neuroscience has advanced tremendously, raising optimism that the 'exercise hyperpnoea dilemma' can finally be solved. In this review, new perspectives are offered in the hope of stimulating original ideas based on modern neuroscience methods and current understanding. We first describe the ventilatory control system and the challenge exercise places upon blood-gas regulation. We highlight relevant system properties, including feedforward, feedback and adaptive (i.e., plasticity) control of breathing. We then elaborate a seldom explored hypothesis that the exercise ventilatory response continuously adapts (learns and relearns) throughout life and ponder if the memory 'engram' encoding the feedforward exercise ventilatory stimulus could reside within the cerebellum. Our hypotheses are based on accumulating evidence supporting the cerebellum's role in motor learning and the numerous direct and indirect projections from deep cerebellar nuclei to brainstem respiratory neurons. We propose that cerebellar learning may be obligatory for the accurate and adjustable exercise hyperpnoea capable of tracking changes in life conditions/experiences, and that learning arises from specific cerebellar microcircuits that can be interrogated using powerful techniques such as optogenetics and chemogenetics. Although this review is speculative, we consider it essential to reframe our perspective if we are to solve the till-now intractable exercise hyperpnoea dilemma.

Pulmonary gas exchange and ventilatory efficiency during exercise in health and diseases

INTRODUCTION

Cardiopulmonary exercise testing (CPET) is nowadays used to study the exercise response in healthy subjects and in disease. Ventilatory efficiency is one of the main determinants in exercise tolerance, and its main variables are a useful tool to guide pathophysiologists toward specific diagnostic pathways, providing prognostic information and improving disease management, treatment, and outcomes.

AREAS COVERED

This review will be based on today's available scientific evidence, describing the main physiological determinants of ventilatory efficiency at rest and during exercise, and focusing also on how CPET variables are modified in specific diseases, leading to the possibility of early diagnosis and management.

EXPERT OPINION

Growing knowledge on CPET interpretation and a wider use of this clinical tool is expected in order to offer more precise diagnostic and prognostic information to patients and clinicians, helping in the management of therapeutic decisions. Future research could be able to identify new and more simple markers of ventilatory efficiency, and to individuate new interventions for the improvement of symptoms, such as exertional dyspnea.

Exercise pressor responses are exaggerated relative to force production during, but not following, thirty-minutes of rhythmic handgrip exercise

PURPOSE

This study tested the hypothesis that blood pressure responses would increase relative to force production in response to prolonged bouts of muscular work.

METHODS

Fifteen individuals performed two minutes of static handgrip (SHG; 35% MVC), followed by three minutes of post-exercise-cuff-occlusion (PECO), before and after thirty minutes of rest (control), or rhythmic handgrip exercise (RHG) of the contralateral and ipsilateral forearms. Beat-by-beat recordings of mean arterial pressure (MAP), heart rate (HR), and handgrip force (kg) were averaged across one-minute periods at baseline, and minutes 5, 10, 15, 20, 25, and 30 of RHG. MAP was also normalized to handgrip force, providing a relative measure of exercise pressor responses (mmHg/kg). Hemodynamic responses to SHG and PECO were also compared before and after contralateral RHG, ipsilateral RHG, and control, respectively. Similar to the RHG trial, areas under the curve were calculated for MAP (blood pressure index; BPI) and normalized to the time tension index (BPInorm).

RESULTS

HR and MAP significantly increased during RHG (15.3 ± 1.4% and 20.4 ± 3.2%, respectively, both p < 0.01), while force output decreased by up to 36.6 ± 8.0% (p < 0.01). This resulted in a 51.6 ± 9.4% increase in BPInorm during 30 min of RHG (p < 0.01). In contrast, blood pressure responses to SHG and PECO were unchanged following RHG (all p ≥ 0.07), and only the mean HR (4.2 ± 1.5%, p = 0.01) and ΔHR (67.2 ± 18.1%, p < 0.01) response to SHG were exaggerated following ipsilateral RHG.

CONCLUSIONS

The magnitude of exercise pressor responses relative to force production progressively increases during, but not following, prolonged bouts of muscular work.

Orexinergic neurons contribute to autonomic cardiovascular regulation for locomotor exercise

While the hypothalamic orexinergic nervous system is established as having a pivotal role in the long-term regulation of various organismic functions, including wakefulness, metabolism and hypertensive states, whether this system contributes to the rapid autonomic cardiovascular regulation during physical activity remains elusive. This study aimed to elucidate the role of the orexinergic nervous system in transmitting volitional motor signals, i.e. central command, to drive somatomotor and sympathetic cardiovascular responses. We first found that this system is activated by voluntary locomotor exercise as evidenced by an increased expression of Fos, a marker of neural activation, in the orexinergic neurons of Sprague-Dawley rats engaged in spontaneous wheel running. Next, using transgenic Orexin-Cre rats for optogenetic manipulation of orexinergic neurons, we found that optogenetic excitation of orexinergic neurons caused sympathoexcitation on a subsecond timescale under anaesthesia. In freely moving conscious rats, this excitatory stimulation rapidly elicited exploration-like behaviours, predominantly locomotor activity, along with pressor and tachycardiac responses. Meanwhile, optogenetic inhibition of orexinergic neurons during spontaneous wheel running immediately suppressed locomotor activities and blood pressure elevation without affecting basal cardiovascular homeostasis. Taken together, these findings demonstrate the essential role of the orexinergic nervous system in the central circuitry that transmits central command signals for locomotor exercise. This study not only offers insights into the brain circuit mechanisms precisely regulating autonomic cardiovascular systems during voluntary exercise but also likely contributes to our understanding of brain mechanisms underlying abnormal cardiovascular adjustments to exercise in pathological conditions, such as hypertension. KEY POINTS: The hypothalamic orexinergic nervous system plays various roles in the long-term regulation of autonomic and endocrine functions, as well as motivated behaviours. We present a novel, rapid role of the orexinergic nervous system, revealing its significance as a crucial substrate in the brain circuit mechanisms that coordinate somatomotor and autonomic cardiovascular controls for locomotor exercise. Our data demonstrate that orexinergic neurons relay volitional motor signals, playing a necessary and sufficient role in the autonomic cardiovascular regulation required for locomotor exercise in rats. The findings contribute to our understanding of how the brain precisely regulates autonomic cardiovascular systems during voluntary exercise, providing insights into the central neural mechanisms that enhance physical performance moment-by-moment during exercise.

Exercise Physiology and Cardiopulmonary Exercise Testing

Aerobic, or endurance, exercise is an energy requiring process supported primarily by energy from oxidative adenosine triphosphate synthesis. The consumption of oxygen and production of carbon dioxide in muscle cells are dynamically linked to oxygen uptake (V̇O2) and carbon dioxide output (V̇CO2) at the lung by integrated functions of cardiovascular, pulmonary, hematologic, and neurohumoral systems. Maximum oxygen uptake (V̇O2max) is the standard expression of aerobic capacity and a predictor of outcomes in diverse populations. While commonly limited in young fit individuals by the capacity to deliver oxygen to exercising muscle, (V̇O2max) may become limited by impairment within any of the multiple systems supporting cellular or atmospheric gas exchange. In the range of available power outputs, endurance exercise can be partitioned into different intensity domains representing distinct metabolic profiles and tolerances for sustained activity. Estimates of both V̇O2max and the lactate threshold, which marks the upper limit of moderate-intensity exercise, can be determined from measures of gas exchange from respired breath during whole-body exercise. Cardiopulmonary exercise testing (CPET) includes measurement of V̇O2 and V̇CO2 along with heart rate and other variables reflecting cardiac and pulmonary responses to exercise. Clinical CPET is conducted for persons with known medical conditions to quantify impairment, contribute to prognostic assessments, and help discriminate among proximal causes of symptoms or limitations for an individual. CPET is also conducted in persons without known disease as part of the diagnostic evaluation of unexplained symptoms. Although CPET quantifies a limited sample of the complex functions and interactions underlying exercise performance, both its specific and global findings are uniquely valuable. Some specific findings can aid in individualized diagnosis and treatment decisions. At the same time, CPET provides a holistic summary of an individual's exercise function, including effects not only of the primary diagnosis, but also of secondary and coexisting conditions.

Additive influence of exercise pressor reflex activation on Valsalva responses in white and black adults

AIM

This study aimed to determine if activation of the exercise pressor reflex exerts additive or redundant influences on the autonomic responses to the Valsalva maneuver (VL), and if these responses differ between White and Black or African American (B/AA) individuals.

METHODS

Twenty participants (B/AA n = 10, White n = 10) performed three separate experimental trials. In the first trial, participants performed two VLs in a resting condition. In a second trial, participants performed 5 min of continuous handgrip (HG) exercise at 35% of the predetermined maximal voluntary contraction. In a third and final trial, participants repeated the 5-min bout of HG while also performing two VLs during the 4th and 5th minutes. Beat by beat blood pressure and heart rate (HR) were recorded continuously and the absolute systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), pulse pressure (PP), and heart rate (HR) responses were reported for phases I-IV of each VL.

RESULTS

No significant group by trial interactions or main effects of group were observed for any phase of the VL (all p ≥ 0.36). However, significant main effects of time were observed for blood pressure and heart rate during phases IIa-IV (all p ≤ 0.02). Specifically, the addition of HG exercise exaggerated the hypertensive responses during phases IIb and IV (all p ≤ 0.04) and blunted the hypotensive responses during phases IIa and III (all p ≤ 0.01).

CONCLUSIONS

These results suggest that activation of the exercise pressor reflex exerts an additive influence on autonomic responses to the VL maneuver in both White and B/AA adults.

Increased hip flexion gait as an exercise modality for individuals with obesity

PURPOSE

Exercise is a critical element for the management of body weight and improvement of quality of life of individuals with obesity. Due to its convenience and accessibility, running is a commonly used exercise modality to meet exercise guidelines. However, the weight-bearing component during high impacts of this exercise modality might limit the participation in exercise and reduce the effectiveness of running-based exercise interventions in individuals with obesity. The hip flexion feedback system (HFFS) assists participants in meeting specific exercise intensities by giving the participant specific increased hip flexion targets while walking on a treadmill. The resulting activity involves walking with increased hip flexion which removes the high impacts of running. The purpose of this study was to compare physiological and biomechanical parameters during an HFFS session and an independent treadmill walking/running session (IND).

METHODS

Heart rate, oxygen consumption (VO2), heart rate error, and tibia peak positive accelerations (PPA) were investigated for each condition at 40% and 60% of heart rate reserve exercise intensities.

RESULTS

VO2 was higher for IND despite no differences in heart rate. Tibia PPAs were reduced during the HFFS session. Heart rate error was reduced for HFFS during non-steady state exercise.

CONCLUSION

While demanding lower energy consumption compared to running, HFFS exercise results in lower tibia PPAs and more accurate monitoring of exercise intensity. HFFS might be a valid exercise alternative for individuals with obesity or individuals that require low-impact forces at the lower limbs.

Differential control of respiratory frequency and tidal volume during exercise

The lack of a testable model explaining how ventilation is regulated in different exercise conditions has been repeatedly acknowledged in the field of exercise physiology. Yet, this issue contrasts with the abundance of insightful findings produced over the last century and calls for the adoption of new integrative perspectives. In this review, we provide a methodological approach supporting the importance of producing a set of evidence by evaluating different studies together-especially those conducted in 'real' exercise conditions-instead of single studies separately. We show how the collective assessment of findings from three domains and three levels of observation support the development of a simple model of ventilatory control which proves to be effective in different exercise protocols, populations and experimental interventions. The main feature of the model is the differential control of respiratory frequency (f_R) and tidal volume (V_T); f_R is primarily modulated by central command (especially during high-intensity exercise) and muscle afferent feedback (especially during moderate exercise) whereas V_T by metabolic inputs. Furthermore, V_T appears to be fine-tuned based on f_R levels to match alveolar ventilation with metabolic requirements in different intensity domains, and even at a breath-by-breath level. This model reconciles the classical neuro-humoral theory with apparently contrasting findings by leveraging on the emerging control properties of the behavioural (i.e. f_R) and metabolic (i.e. V_T) components of minute ventilation. The integrative approach presented is expected to help in the design and interpretation of future studies on the control of f_R and V_T during exercise.

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.

Physiological role of anticipatory cardiorespiratory responses to exercise

This study aimed to investigate whether anticipatory cardiorespiratory responses vary depending on the intensity of the subsequent exercise bout, and whether anticipatory cardiorespiratory adjustments contribute importantly to enhancing exercise performance during high-intensity exercise. Eleven healthy men were provided advance notice of the exercise intensity and a countdown to generate anticipation during 10 min prior to exercise at 0, 50, 80 or 95% maximal work-rate (Experiment 1). A different group of subjects (n = 15) performed a time to exhaustion trial with or without anticipatory countdown (Experiment 2). In Experiment 1, heart rate (HR), oxygen uptake (V_O2 ) and minute ventilation (V_E ) during pre-exercise resting period increased over time and depended on the subsequent exercise intensity. Specifically, there was already a 7.4% increase in HR from more than 5 min prior to the start of exercise at 95% maximal work-rate, followed by progressively augmented increases of 12.5% between 2 and 3 min before exercise, 24.4% between 0 and 1 min before exercise. In Experiment 2, the initial HR for the first 10 s of exercise in the task with anticipation was 11.4% larger compared to without anticipation (p < 0.01), and the difference in HR between the two conditions decreased in a time-dependent manner. In contrast, the initial increases in V_O2 and V_E were significantly lower in the task with anticipation than that without anticipation. The time to exhaustion during high-intensity exercise was 14.6% longer under anticipation condition compared to no anticipation (135 ± 26 s vs. 119 ± 26 s, p = 0.003). In addition, the enhanced exercise performance correlated positively with increased HR response just before and immediately after exercise onset (p < 0.01). These results showed that anticipatory cardiorespiratory adjustments (feedforward control) via the higher brain that operate before starting exercise may play an important role in minimizing the time delay of circulatory response and enhancing performance after onset of high-intensity exercise in man.

Keeping the Breath in Mind: Respiration, Neural Oscillations, and the Free Energy Principle

Scientific interest in the brain and body interactions has been surging in recent years. One fundamental yet underexplored aspect of brain and body interactions is the link between the respiratory and the nervous systems. In this article, we give an overview of the emerging literature on how respiration modulates neural, cognitive and emotional processes. Moreover, we present a perspective linking respiration to the free-energy principle. We frame volitional modulation of the breath as an active inference mechanism in which sensory evidence is recontextualized to alter interoceptive models. We further propose that respiration-entrained gamma oscillations may reflect the propagation of prediction errors from the sensory level up to cortical regions in order to alter higher level predictions. Accordingly, controlled breathing emerges as an easily accessible tool for emotional, cognitive, and physiological regulation.

The effect of pedalling cadence on respiratory frequency: passive vs. active exercise of different intensities

PURPOSE

Pedalling cadence influences respiratory frequency (f_R) during exercise, with group III/IV muscle afferents possibly mediating its effect. However, it is unclear how exercise intensity affects the link between cadence and f_R. We aimed to test the hypothesis that the effect of cadence on f_R is moderated by exercise intensity, with interest in the underlying mechanisms.

METHODS

Ten male cyclists performed a preliminary ramp incremental test and three sinusoidal experimental tests on separate visits. The experimental tests consisted of 16 min of sinusoidal variations in cadence between 115 and 55 rpm (sinusoidal period of 4 min) performed during passive exercise (PE), moderate exercise (ME) and heavy exercise (HE). The amplitude (A) and phase lag (φ) of the dependent variables were calculated.

RESULTS

During PE, f_R changed in proportion to variations in cadence (r = 0.85, P < 0.001; A = 3.9 ± 1.4 breaths·min^-1; φ = - 5.3 ± 13.9 degrees). Conversely, the effect of cadence on f_R was reduced during ME (r = 0.73, P < 0.001; A = 2.6 ± 1.3 breaths·min^-1; φ = - 25.4 ± 26.3 degrees) and even more reduced during HE (r = 0.26, P < 0.001; A = 1.8 ± 1.0 breaths·min^-1; φ = - 70.1 ± 44.5 degrees). No entrainment was found in any of the sinusoidal tests.

CONCLUSION

The effect of pedalling cadence on f_R is moderated by exercise intensity-it decreases with the increase in work rate-and seems to be mediated primarily by group III/IV muscle afferents, at least during passive exercise.

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.

Using Deep Brain Stimulation to Unravel the Mysteries of Cardiorespiratory Control

This article charts the history of deep brain stimulation (DBS) as applied to alleviate a number of neurological disorders, while in parallel mapping the electrophysiological circuits involved in generating and integrating neural signals driving the cardiorespiratory system during exercise. With the advent of improved neuroimaging techniques, neurosurgeons can place small electrodes into deep brain structures with a high degree accuracy to treat a number of neurological disorders, such as movement impairment associated with Parkinson's disease and neuropathic pain. As well as stimulating discrete nuclei and monitoring autonomic outflow, local field potentials can also assess how the neurocircuitry responds to exercise. This technique has provided an opportunity to validate in humans putative circuits previously identified in animal models. The central autonomic network consists of multiple sites from the spinal cord to the cortex involved in autonomic control. Important areas exist at multiple evolutionary levels, which include the anterior cingulate cortex (telencephalon), hypothalamus (diencephalon), periaqueductal grey (midbrain), parabrachial nucleus and nucleus of the tractus solitaries (brainstem), and the intermediolateral column of the spinal cord. These areas receive afferent input from all over the body and provide a site for integration, resulting in a coordinated efferent autonomic (sympathetic and parasympathetic) response. In particular, emerging evidence from DBS studies have identified the basal ganglia as a major sub-cortical cognitive integrator of both higher center and peripheral afferent feedback. These circuits in the basal ganglia appear to be central in coupling movement to the cardiorespiratory motor program. © 2020 American Physiological Society. Compr Physiol 10:1085-1104, 2020.

Control of exercise hyperpnoea: Contributions from thin-fibre skeletal muscle afferents

NEW FINDINGS

What is the topic of this review? In this review, we examine the evidence for control mechanisms underlying exercise hyperpnoea, giving attention to the feedback from thin-fibre skeletal muscle afferents, and highlight the frequently conflicting findings and difficulties encountered by researchers using a variety of experimental models. What advances does it highlight? There has been a recent resurgence of interest in the role of skeletal muscle afferent involvement, not only as a mechanism of healthy exercise hyperpnoea but also in the manifestation of breathlessness and exercise intolerance in chronic disease.

ABSTRACT

The ventilatory response to dynamic submaximal exercise is immediate and proportional to metabolic rate, which maintains isocapnia. How these respiratory responses are controlled remains poorly understood, given that the most tightly controlled variable (arterial partial pressure of CO₂ /H⁺ ) provides no error signal for arterial chemoreceptors to trigger reflex increases in ventilation. This review discusses evidence for different postulated control mechanisms, with a focus on the feedback from group III/IV skeletal muscle mechanosensitive and metabosensitive afferents. This concept is attractive, because the stimulation of muscle mechanoreceptors might account for the immediate increase in ventilation at the onset of exercise, and signals from metaboreceptors might be proportional to metabolic rate. A variety of experimental models have been used to establish the contribution of thin-fibre muscle afferents in ventilatory control during exercise, with equivocal results. The inhibition of afferent feedback via the application of lumbar intrathecal fentanyl during exercise suppresses ventilation, which provides the most compelling supportive evidence to date. However, stimulation of afferent feedback at rest has no consistent effect on respiratory output. However, evidence is emerging for synergistic interactions between muscle afferent feedback and other stimulatory inputs to the central respiratory neuronal pool. These seemingly hyperadditive effects might explain the conflicting findings encountered when using different experimental models. We also discuss the increasing evidence that patients with certain chronic diseases exhibit exaggerated muscle afferent activation during exercise, resulting in enhanced cardiorespiratory responses. This might provide a neural link between the well-established limb muscle dysfunction and the associated exercise intolerance and exertional dyspnoea, which might offer therapeutic targets for these patients.

The pedunculopontine region and breathing in Parkinson's disease

Objective

Respiratory abnormalities such as upper airway obstruction are common in Parkinson's disease (PD) and are an important cause of mortality and morbidity. We tested the effect of pedunculopontine region (PPNr) stimulation on respiratory maneuvers in human participants with PD, and separately recorded PPNr neural activity reflected in the local field potential (LFP) during these maneuvers.

Methods

Nine patients with deep brain stimulation electrodes in PPNr, and seven in globus pallidus interna (GPi) were studied during trials of maximal inspiration followed by forced expiration with stimulation OFF and ON. Local field potentials (LFPs) were recorded in the unstimulated condition.

Results

PEFR increased from 6.41 ± 0.63 L/sec in the OFF stimulation state to 7.5 L ± 0.65 L/sec in the ON stimulation state (z = −2.666, df = 8, P = 0.024). Percentage improvement in PEFR was strongly correlated with proximity of the stimulated electrode contact to the mesencephalic locomotor region in the rostral PPN (r = 0.814, n = 9, P = 0.008). Mean PPNr LFP power increased within the alpha band (7–11 Hz) during forced respiratory maneuvers (1.63 ± 0.16 μV²/Hz) compared to resting breathing (0.77 ± 0.16 μV²/Hz; z = −2.197, df = 6, P = 0.028). No changes in alpha activity or spirometric indices were seen with GPi recording or stimulation. Percentage improvement in PEFR was strongly positively correlated with increase in alpha power (r = 0.653, n = 14 (7 PPNr patients recorded bilaterally), P = 0.0096).

Interpretation

PPNr stimulation in PD improves indices of upper airway function. Increased alpha‐band activity is seen within the PPNr during forced respiratory maneuvers. Our findings suggest a link between the PPNr and respiratory performance in PD.

The midbrain periaqueductal gray as an integrative and interoceptive neural structure for breathing

The periaqueductal gray (PAG) plays a critical role in autonomic function and behavioural responses to threatening stimuli. Recent evidence has revealed the PAG's potential involvement in the perception of breathlessness, a highly threatening respiratory symptom. In this review, we outline the current evidence in animals and humans on the role of the PAG in respiratory control and in the perception of breathlessness. While recent work has unveiled dissociable brain activity within the lateral PAG during perception of breathlessness and ventrolateral PAG during conditioned anticipation in healthy humans, this is yet to be translated into diseases dominated by breathlessness symptomology, such as chronic obstructive pulmonary disease. Understanding how the sub-structures of the PAG differentially interact with interoceptive brain networks involved in the perception of breathlessness will help towards understanding discordant symptomology, and may reveal treatment targets for those debilitated by chronic and pervasive breathlessness.

Direct neurophysiological evidence for a role of the human anterior cingulate cortex in central command

INTRODUCTION

The role of the anterior cingulate cortex (ACC) is still controversial. The ACC has been implicated in such diverse functions as cognition, arousal and emotion in addition to motor and autonomic control. Therefore the ACC is the ideal candidate to orchestrate cardiovascular performance in anticipation of perceived skeletal activity. The aim of this experiment was to investigate whether the ACC forms part of the neural network of central command whereby cardiovascular performance is governed by a top-down mechanism.

METHODS & RESULTS

Direct local field potential (LFP) recordings were made using intraparenchymal electrodes in six human ACC's to measure changes in neuronal activity during performance of a motor task in which anticipation of exercise was uncoupled from skeletal activity itself. Parallel cardiovascular arousal was indexed by electrocardiographic changes in heart rate. During anticipation of exercise, ACC LFP power within the 25-60 Hz frequency band increased significantly by 21% compared to rest (from 62.7 μV2/Hz (±SE 4.94) to 76.0μV2/Hz (±SE 7.24); p = 0.004). This 25-60 Hz activity increase correlated with a simultaneous heart rate increase during anticipation (Pearson's r = 0.417, p = 0.016).

CONCLUSIONS/SIGNIFICANCE

We provide the first invasive electrophysiological evidence to support the role of the ACC in both motor preparation and the top-down control of cardiovascular function in exercise. This further implicates the ACC in the body's response to the outside world and its possible involvement in such extreme responses as emotional syncope and hyperventilation. In addition we describe the frequency at which the neuronal ACC populations perform these tasks in the human.

Sensitivity of the human ventilatory response to muscle metaboreflex activation during concurrent mild hypercapnia

NEW FINDINGS

What is the central question of this study? What is the relationship between the level of systemic hypercapnia and the magnitude of the additional hyperpnoea produced in response to a standardized level of muscle metaboreflex activation? What is the main finding and its importance? When a standardized activation of the muscle metaboreflex was combined with exposure to increasing levels of hypercapnia, the hyperpnoea this caused increased linearly. The concept of a synergistic interaction between the muscle metaboreflex and the central chemoreflex in humans is supported by this finding.

ABSTRACT

Ventilation increases during muscle metaboreflex activation when postexercise circulatory occlusion (PECO) traps metabolites in resting human muscle, but only in conditions of concurrent systemic hypercapnia. We hypothesize that a linear relationship exists between the level of hypercapnia and the magnitude of the additional hyperpnoea produced in response to a standardized level of muscle metaboreflex activation. Fifteen male subjects performed four trials, in which the end-tidal partial pressure of carbon dioxide ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>P</mml:mi> <mml:mrow> <mml:mrow><mml:mi>ET</mml:mi> <mml:mo>,</mml:mo> <mml:mi>C</mml:mi></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> ) was elevated by 1, 3, 7 or 10 mmHg above resting values using a dynamic end-tidal forcing system. In each trial, subjects were seated in an isometric dynamometer designed to measure ankle plantar flexor force. Rest for 2 min in room air was followed by 15 min of exposure to one of the four levels of hypercapnia, at which 5 min further rest was followed by 2 min of sustained isometric calf muscle contraction at 50% of predetermined maximal voluntary strength. Immediately before cessation of exercise, a cuff around the upper leg was inflated to a suprasystolic pressure to cause PECO for 3 min, before its deflation and a further 5 min of rest, concluding exposure to hypercapnia. The PECO consistently elevated mean arterial blood pressure by ∼10 mmHg in all trials, indicating similar levels of metaboreflex activation. Increased ventilation during PECO was related to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>P</mml:mi> <mml:mrow> <mml:mrow><mml:mi>ET</mml:mi> <mml:mo>,</mml:mo> <mml:mi>C</mml:mi></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> as described by the following linear regression equation: Change in minute ventilation (l min^-1 ) = 0.85 ×  <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>P</mml:mi> <mml:mrow> <mml:mrow><mml:mi>ET</mml:mi> <mml:mo>,</mml:mo> <mml:mi>C</mml:mi></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> (mmHg) + 0.80 (l min^-1 ). This finding supports our hypothesis and furthers the idea of a synergistic interaction between muscle metaboreflex activation and central chemoreflex stimulation.

Respiratory frequency and tidal volume during exercise: differential control and unbalanced interdependence

Differentiating between respiratory frequency (fR ) and tidal volume (VT ) may improve our understanding of exercise hyperpnoea because fR and VT seem to be regulated by different inputs. We designed a series of exercise manipulations to improve our understanding of how fR and VT are regulated during exercise. Twelve cyclists performed an incremental test and three randomized experimental sessions in separate visits. In two of the three experimental visits, participants performed a moderate-intensity sinusoidal test followed, after recovery, by a moderate-to-severe-intensity sinusoidal test. These two visits differed in the period of the sinusoid (2 min vs. 8 min). In the third experimental visit, participants performed a trapezoidal test where the workload was self-paced in order to match a predefined trapezoidal template of rating of perceived exertion (RPE). The results collectively reveal that fR changes more with RPE than with workload, gas exchange, VT or the amount of muscle activation. However, fR dissociates from RPE during moderate exercise. Both VT and minute ventilation ( V ˙ E ) showed a similar time course and a large correlation with V ˙ CO 2 in all the tests. Nevertheless, V ˙ CO 2 was associated more with V ˙ E than with VT because VT seems to adjust continuously on the basis of fR levels to match V ˙ E with V ˙ CO 2 . The present findings provide novel insight into the differential control of fR and VT - and their unbalanced interdependence - during exercise. The emerging conceptual framework is expected to guide future research on the mechanisms underlying the long-debated issue of exercise hyperpnoea.

The control of ventilation during exercise: a lesson in critical thinking

Learning the basic competencies of critical thinking are very important in the education of any young scientist, and teachers must be prepared to help students develop a valuable set of analytic tools. In my experience, this is best achieved by encouraging students to study areas with little scientific consensus, such as the control mechanisms of the exercise ventilatory response, as it can allow greater objectivity when evaluating evidence, while also giving students the freedom to think independently and problem solve. In this article, I discuss teaching strategies by which physiology, biomedical science, and sport science students can simultaneously develop their understanding of respiratory control mechanisms and learn to critically analyze evidence thoroughly. This can be best achieved by utilizing both teacher-led and student-led learning environments, the latter of which encourages the development of learner autonomy and independent problem solving. In this article, I also aim to demonstrate a systematic approach of critical assessment that students can be taught, adapt, and apply independently. Among other things, this strategy involves: 1) defining the precise phenomenon in question; 2) understanding what investigations must demonstrate to explain the phenomenon and its underlying mechanisms; 3) evaluating the explanations/mechanisms of the phenomenon and the evidence for them; and 4) forming strategies to produce strong evidence, if none exists.

Placebo response in pain, fatigue, and performance: Possible implications for neuromuscular disorders

The placebo response in neuromuscular disorders is not well understood. The only available data regarding its underlying mechanisms are related to neuropathic pain. In this review, we describe the factors that contribute to improved outcomes in the placebo arm, with specific attention to pain and fatigue, as well as some of the most important psychobiological mechanisms that may explain such a response. This approach may also improve our insight into the symptomatology and therapeutic responses of other neuromuscular disorders. The fact that >90% of tested analgesics for neuropathic pain have failed in advanced phases of clinical trials should prompt a greater investment of effort and resources into understanding the mechanisms and impact of placebos in clinical research. Such an endeavor will help improve the design of clinical trials and will provide information that informs clinical neuromuscular practice. Muscle Nerve 56: 358-367, 2017.

Ventilatory responses to muscle metaboreflex activation in chronic obstructive pulmonary disease

KEY POINTS

Recent evidence indicates a role for group III/IV muscle afferents in reflex control of the human ventilatory response to exercise. Dyspnoea in chronic obstructive pulmonary disease (COPD) may be linked to this reflex response. This study shows that activation of the muscle metaboreflex causes a ventilatory response in COPD patients but not in healthy controls. This indicates abnormal involvement of muscle afferents in the control of ventilation in COPD which may be a contributing factor to exercise dyspnoea.

ABSTRACT

Blockade of thin fibre muscle afferent feedback during dynamic exercise reduces exercise hyperpnoea in health and chronic obstructive pulmonary disease (COPD). Therefore, we hypothesised that activation of the muscle metaboreflex at rest would cause hyperpnoea. We evaluated the effect of muscle metaboreflex activation on ventilation, in resting COPD patients and healthy participants. Following a bout of rhythmic hand grip exercise, post exercise circulatory occlusion (PECO) was applied to the resting forearm to sustain activation of the muscle metaboreflex, in 18 COPD patients (FEV₁ /FVC ratio < 70%), 9 also classified as chronically hypercapnic, and 9 age- and gender-matched controls. The cardiovascular response to exercise and the sustained blood pressure elevation during PECO was similar in patients and controls. During exercise ventilation increased by 6.64 ± 0.84 in controls and significantly (P < 0.05) more, 8.38 ± 0.81 l min^-1 , in patients. During PECO it fell to baseline levels in controls but remained significantly (P < 0.05) elevated by 2.78 ± 0.51 l min^-1 in patients until release of circulatory occlusion, with no significant difference in responses between patient groups. Muscle metaboreflex activation causes increased ventilation in COPD patients but not in healthy participants. Chronic hypercapnia in COPD patients does not exaggerate this response. The muscle metaboreflex appears to be abnormally involved in the control of ventilation in COPD and may be a contributing factor to exercise dyspnoea.

Cardiovascular Reflexes Activity and Their Interaction during Exercise

Cardiac output and arterial blood pressure increase during dynamic exercise notwithstanding the exercise-induced vasodilation due to functional sympatholysis. These cardiovascular adjustments are regulated in part by neural reflexes which operate to guarantee adequate oxygen supply and by-products washout of the exercising muscles. Moreover, they maintain adequate perfusion of the vital organs and prevent excessive increments in blood pressure. In this review, we briefly summarize neural reflexes operating during dynamic exercise with particular emphasis on their interaction.

Central and Peripheral Hemodynamic Adaptations During Cardiopulmonary Exercise Test in Heart Failure Patients With Exercise Periodic Breathing

Some heart failure (HF) patients develop ventilatory oscillation which is composed of exercise periodic breathing (EPB) and sleep apnea. The ventilatory oscillation is associated with exercise intolerance. This study employed an integrated monitoring system to elucidate the way of central and peripheral hemodynamic adaption responding to exercise. This study recruited 157 HF patients to perform exercise testing using a bicycle ergometer. A noninvasive bio-reactance device was adopted to measure cardiac hemodynamics, whereas a near-infrared spectroscopy (NIRS) was used to assess perfusion and O2 extraction in the frontal cerebral lobe (FC) and vastus lateralis muscle (VL) during exercise respectively. Furthermore, quality of life (QoL) was measured with the Short Form-36 (SF-36) and the Minnesota Living with Heart Failure questionnaires (MLHFQ). The patients were divided into an EPB group (n = 65) and a non-EPB group (n = 92) according to their ventilation patterns during testing. Compared to their non-EPB counterparts, the patients with EPB exhibited 1) impaired aerobic capacity with a smaller peak oxygen consumption (VO2peak) and oxygen uptake efficiency slopes; 2) impaired circulatory and ventilatory efficiency with relatively high cardiac output and ventilation per unit workload; 3) impaired ventilatory/hemodynamic adaptation in response to exercise with elevated deoxyhemoglobin levels in the FC region; and 4) impaired QoL with lower physical component scores on the SF-36 and higher scores on the MLHFQ. In conclusion, EPB may reduce circulatory-ventilatory-hemodynamic efficiency during exercise, thereby impairing functional capacity in patients with HF.

Autonomic responses to exercise: where is central command?

A central command is thought to involve a signal arising in a central area of the brain eliciting a parallel activation of the autonomic nervous system and skeletal muscle contraction during exercise. Although much of the neural circuitry involved in autonomic control has been identified, defining the specific higher brain region(s) serving in a central command capacity has proven more challenging. Investigators have been faced with redundancies in regulatory systems, feedback mechanisms and the complexities ofhuman neural connectivity. Several studies have attempted to address these issues and provide more definitive neuroanatomical information. However, none have clearly answered the question, "where is central command?"

Habituation to experimentally induced electrical pain during voluntary-breathing controlled electrical stimulation (BreEStim)

Objective

Painful peripheral electrical stimulation to acupuncture points was found to cause sensitization if delivered randomly (EStim), but induced habituation if triggered by voluntary breathing (BreEStim). The objective was to systematically compare the effectiveness of BreEStim and EStim and to investigate the possible mechanisms mediating the habituation effect of BreEStim.

Methods

Eleven pain-free, healthy subjects (6 males, 5 females) participated in the study. Each subject received the BreEStim and EStim treatments in a random order at least three days apart. Both treatments consisted of 120 painful but tolerable stimuli to the ulnar nerve at the elbow on the dominant arm. BreEStim was triggered by voluntary breathing while EStim was delivered randomly. Electrical sensation threshold (EST) and electrical pain threshold (EPT) were measured from the thenar and hypothenar eminences on both hands at pre-intervention and 10-minutes post-intervention.

Results

There was no difference in the pre-intervention baseline measurement of EST and EPT between BreEStim and EStim. BreEStim increased EPT in all tested sites on both hands, while EStim increased EPT in the dominant hypothenar eminence distal to the stimulating site and had no effect on EPT in other sites. There was no difference in the intensity of electrical stimulation between EStim and BreEStim.

Conclusion

Our findings support the important role human voluntary breathing plays in the systemic habituation effect of BreEStim to peripheral painful electrical stimulation.

The midbrain periaqueductal grey has no role in the generation of the respiratory motor pattern, but provides command function for the modulation of respiratory activity

It has previously been shown that stimulation of cell-columns in the periaqueductal grey (PAG) triggers site-specific cardiorespiratory effects. These are believed to facilitate changes in behaviour through coordinated changes in autonomic outflow. Here, we investigated whether PAG-evoked respiratory commands can be studied in situ using the decerebrate perfused brainstem preparation. Phrenic, vagus and abdominal iliohypogastric nerves were recorded before and after microinjection of L-glutamate (30-50 nl, 10 mM) or isoguvacine (GABA-receptor agonist, 30-50 nl, 10 mM) into the PAG. L-glutamate microinjection triggered a range of site-specific respiratory modulations (n = 17 preparations). Subsequent microinjection of isoguvacine into the same PAG sites had no effect on the baseline respiratory motor pattern or rhythm. We conclude that while the PAG has no function in respiratory pattern generation, PAG-evoked respiratory modulations can be evoked in situ in the absence of higher brain centres and while homeostatic parameters that may affect respiratory drive are held static.

Autonomic neural control of heart rate during dynamic exercise: revisited

The accepted model of autonomic control of heart rate (HR) during dynamic exercise indicates that the initial increase is entirely attributable to the withdrawal of parasympathetic nervous system (PSNS) activity and that subsequent increases in HR are entirely attributable to increases in cardiac sympathetic activity. In the present review, we sought to re-evaluate the model of autonomic neural control of HR in humans during progressive increases in dynamic exercise workload. We analysed data from both new and previously published studies involving baroreflex stimulation and pharmacological blockade of the autonomic nervous system. Results indicate that the PSNS remains functionally active throughout exercise and that increases in HR from rest to maximal exercise result from an increasing workload-related transition from a 4 : 1 vagal-sympathetic balance to a 4 : 1 sympatho-vagal balance. Furthermore, the beat-to-beat autonomic reflex control of HR was found to be dependent on the ability of the PSNS to modulate the HR as it was progressively restrained by increasing workload-related sympathetic nerve activity.

IN CONCLUSION

(i) increases in exercise workload-related HR are not caused by a total withdrawal of the PSNS followed by an increase in sympathetic tone; (ii) reciprocal antagonism is key to the transition from vagal to sympathetic dominance, and (iii) resetting of the arterial baroreflex causes immediate exercise-onset reflexive increases in HR, which are parasympathetically mediated, followed by slower increases in sympathetic tone as workloads are increased.

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.

Differentiated baroreflex modulation of sympathetic nerve activity during deep brain stimulation in humans

Targeted electric deep brain stimulation in midbrain nuclei in humans alters cardiovascular parameters, presumably by modulating autonomic and baroreflex function. Baroreflex modulation of sympathetic outflow is crucial for cardiovascular regulation and is hypothesized to occur at 2 distinct brain locations. The aim of this study was to evaluate sympathetic outflow in humans with deep brain stimulating electrodes during ON and OFF stimulation of specific midbrain nuclei known to regulate cardiovascular function. Multiunit muscle sympathetic nerve activity was recorded in 17 patients undergoing deep brain stimulation for treatment of chronic neuropathic pain (n=7) and Parkinson disease (n=10). Sympathetic outflow was recorded during ON and OFF stimulation. Arterial blood pressure, heart rate, and respiratory frequency were monitored during the recording session, and spontaneous vasomotor and cardiac baroreflex sensitivity were assessed. Head-up tilt testing was performed separately in the patients with Parkinson disease postoperatively. Stimulation of the dorsal most part of the subthalamic nucleus and ventrolateral periaqueductal gray resulted in improved vasomotor baroreflex sensitivity, decreased burst frequency and blood pressure, unchanged burst amplitude distribution, and a reduced fall in blood pressure after tilt. Stimulation of the dorsolateral periaqueductal gray resulted in a shift in burst amplitude distribution toward larger amplitudes, decreased spontaneous beat-to-beat blood pressure variability, and unchanged burst frequency, baroreflex sensitivity, and blood pressure. Our results indicate that a differentiated regulation of sympathetic outflow occurs in the subthalamic nucleus and periaqueductal gray. These results may have implications in our understanding of abnormal sympathetic discharge in cardiovascular disease and provide an opportunity for therapeutic targeting.

Dyspnea as a side effect of subthalamic nucleus deep brain stimulation for Parkinson's disease

Bilateral subthalamic nucleus deep brain stimulation for Parkinson's disease improves limb function. Unpublished observations from our clinic noted that some subthalamic nucleus deep brain stimulation patients complain of post-operative dyspnea. Therefore, we designed a prospective, longitudinal study to characterize this in greater depth. We used specific questionnaires to assess dyspnea in patients with electrodes in the subthalamic nucleus (n=13) or ventral intermediate thalamus (n=7). St. George's Hospital Respiratory Questionnaire symptom subscale scores were greater in subthalamic nucleus patients (median=18.60, interquartile range=40.80) than ventral intermediate thalamus patients (median = 0.00, interquartile range=15.38) at greater than 6 months post-operatively (p<0.05). Several of the subthalamic nucleus patients exhibited functional impairments as judged by the St. George's Hospital Respiratory Questionnaire impact subscale, the Medical Research Council Dyspnoea Scale, and the Dyspnoea-12 Questionnaire. There was no correlation between limb function ratings, stimulation parameters, or precise electrode position and dyspnea severity. We have shown, for the first time, that dyspnea can be a side effect of subthalamic nucleus deep brain stimulation, and that this dyspnea may be highly disabling.

Control of the lungs via the human brain using neurosurgery

Neurosurgery can alter cardiorespiratory performance via central networks and includes deep brain stimulation (DBS), a routinely employed therapy for movement disorders and chronic pain syndromes. We review the established cardiovascular effects of DBS and the presumed mechanism by which they are produced via the central autonomic network. We then review the respiratory effects of DBS, including modulation of respiratory rate and lung function indices, and the mechanisms via which these may occur. We conclude by highlighting the potential future therapeutic applications of DBS for intractable airway diseases.

Response of end tidal CO2 pressure to impulse exercise

The purpose of the present study was to examine how end tidal CO(2) pressure (PETCO(2)) is controlled in impulse exercise. After pre-exercise at 25 watts for 5 min, impulse exercise for 10 sec with 200 watts followed by post exercise at 25 watts was performed. Ventilation (VE) significantly increased until the end of impulse exercise and significantly re-increased after a sudden decrease. Heart rate (HR) significantly increased until the end of impulse exercise and then decreased to the pre-exercise level. PETCO(2) remained constant during impulse exercise. PETCO(2) significantly increased momentarily after impulse exercise and then significantly decreased to the pre-exercise level. PETCO(2) showed oscillation. The average peak frequency of power spectral density in PETCO(2) appeared at 0.0078 Hz. Cross correlations were obtained after impulse exercise. The peak cross correlations between VE and PETCO(2), HR and PETCO(2), and VE and HR were 0.834 with a time delay of -7 sec, 0.813 with a time delay of 7 sec and 0.701 with a time delay of -15 sec, respectively. We demonstrated that PETCO(2) homeodynamics was interactively maintained by PETCO(2) itself, CO(2) transportation (product of cardiac output and mixed venous CO(2) content) into the lungs by heart pumping and CO(2) elimination by ventilation, and it oscillates as a result of their interactions.

A set of vertically integrated inquiry-based practical curricula that develop scientific thinking skills for large cohorts of undergraduate students

Science graduates require critical thinking skills to deal with the complex problems they will face in their 21st century workplaces. Inquiry-based curricula can provide students with the opportunities to develop such critical thinking skills; however, evidence suggests that an inappropriate level of autonomy provided to underprepared students may not only be daunting to students but also detrimental to their learning. After a major review of the Bachelor of Science, we developed, implemented, and evaluated a series of three vertically integrated courses with inquiry-style laboratory practicals for early-stage undergraduate students in biomedical science. These practical curricula were designed so that students would work with increasing autonomy and ownership of their research projects to develop increasingly advanced scientific thinking and communication skills. Students undertaking the first iteration of these three vertically integrated courses reported learning gains in course content as well as skills in scientific writing, hypothesis construction, experimental design, data analysis, and interpreting results. Students also demonstrated increasing skills in both hypothesis formulation and communication of findings as a result of participating in the inquiry-based curricula and completing the associated practical assessment tasks. Here, we report the specific aspects of the curricula that students reported as having the greatest impact on their learning and the particular elements of hypothesis formulation and communication of findings that were more challenging for students to master. These findings provide important implications for science educators concerned with designing curricula to promote scientific thinking and communication skills alongside content acquisition.

Acute exercise-induced activation of Phox2b-expressing neurons of the retrotrapezoid nucleus in rats may involve the hypothalamus

The rat retrotrapezoid nucleus (RTN) contains neurons that have a well-defined phenotype characterized by the presence of vesicular glutamate transporter 2 (VGLUT2) mRNA and a paired-like homeobox 2b (Phox2b)-immunoreactive (ir) nucleus and the absence of tyrosine hydroxylase (TH). These neurons are important to chemoreception. In the present study, we tested the hypothesis that the chemically-coded RTN neurons (ccRTN) (Phox2b(+)/TH(-)) are activated during an acute episode of running exercise. Since most RTN neurons are excited by the activation of perifornical and lateral hypothalamus (PeF/LH), a region that regulates breathing during exercise, we also tested the hypothesis that PeF/LH projections to RTN neurons contribute to their activation during acute exercise. In adult male Wistar rats that underwent an acute episode of treadmill exercise, there was a significant increase in c-Fos immunoreactive (c-Fos-ir) in PeF/LH neurons and RTN neurons that were Phox2b(+)TH(-) (p<0.05) compared to rats that did not exercise. Also the retrograde tracer Fluoro-Gold that was injected into RTN was detected in c-Fos-ir PeF/LH (p<0.05). In summary, the ccRTN neurons (Phox2b(+)TH(-)) are excited by running exercise. Thus, ccRTN neurons may contribute to both the chemical drive to breath and the feed-forward control of breathing associated with exercise.

Control of breathing during exercise

During exercise by healthy mammals, alveolar ventilation and alveolar-capillary diffusion increase in proportion to the increase in metabolic rate to prevent PaCO2 from increasing and PaO2 from decreasing. There is no known mechanism capable of directly sensing the rate of gas exchange in the muscles or the lungs; thus, for over a century there has been intense interest in elucidating how respiratory neurons adjust their output to variables which can not be directly monitored. Several hypotheses have been tested and supportive data were obtained, but for each hypothesis, there are contradictory data or reasons to question the validity of each hypothesis. Herein, we report a critique of the major hypotheses which has led to the following conclusions. First, a single stimulus or combination of stimuli that convincingly and entirely explains the hyperpnea has not been identified. Second, the coupling of the hyperpnea to metabolic rate is not causal but is due to of these variables each resulting from a common factor which link the circulatory and ventilatory responses to exercise. Third, stimuli postulated to act at pulmonary or cardiac receptors or carotid and intracranial chemoreceptors are not primary mediators of the hyperpnea. Fourth, stimuli originating in exercising limbs and conveyed to the brain by spinal afferents contribute to the exercise hyperpnea. Fifth, the hyperventilation during heavy exercise is not primarily due to lactacidosis stimulation of carotid chemoreceptors. Finally, since volitional exercise requires activation of the CNS, neural feed-forward (central command) mediation of the exercise hyperpnea seems intuitive and is supported by data from several studies. However, there is no compelling evidence to accept this concept as an indisputable fact.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Are type III-IV muscle afferents required for a normal steady-state exercise hyperpnoea in humans?

When tested in isolation, stimuli associated with respiratory CO₂ exchange, feedforward central command and type III–IV muscle afferent feedback have each been shown to be capable of eliciting exercise-like cardio-ventilatory responses, but their relative contributions in a setting of physiological exercise remains controversial. We reasoned that in order to determine whether any of these regulators are obligatory to the exercise hyperpnoea each needs to be removed or significantly diminished in a setting of physiological steady-state exercise, during which all recognized stimuli (and other potential modulators) are normally operative. In the past few years we and others have used intrathecal fentanyl, a μ-opiate receptor agonist, in humans to reduce the input from type III–IV opiate-sensitive muscle afferents. During various types of intensities and durations of exercise a sustained hypoventilation, as well as reduced systemic pressure and cardioacceleration, were consistently observed with this blockade. These data provide the basis for the hypothesis that type III–IV muscle afferents are obligatory to the hyperpnoea of mild to moderate intensity rhythmic, large muscle, steady-state exercise. We discuss the limitations of these studies, the reasons for their disagreement with previous negative findings, the nature of the muscle afferent feedback stimulus and the need for future investigations.

Defining the neurocircuitry of exercise hyperpnoea

One hundred years ago in this journal, Krogh and Lindhard published a seminal paper highlighting the importance of the brain in the control of breathing during exercise. This symposium report reviews the historical developments that have taken place since 1913, and attempts to place the detailed neurocircuitry thought to underpin exercise hyperpnoea into context by focusing on key structures that might form the command network. With the advent of enhanced neuroimaging and functional neurosurgical techniques, a unique window of opportunity has recently arisen to target potential circuits in humans. Animal studies have identified a priori sites of interest in mid-brain structures, in particular the subthalamic locomotor region (subthalamic nucleus, STN) and the periaqueductal grey (PAG), which have now been recorded from in humans during exercise. When all data are viewed in an integrative manner, the PAG, in particular the lateral PAG, and aspects of the dorsal lateral PAG, appear to be key communicating circuitry for 'central command'. Moreover, the PAG also fulfils many requirements of a command centre. It has functional connectivity to higher centres (dorsal lateral prefrontal cortex) and the basal ganglia (in particular, the STN), and receives a sensory input from contracting muscle, but, importantly, it sends efferent information to brainstem nuclei involved in cardiorespiratory control.

Mechanism of augmented exercise hyperpnea in chronic heart failure and dead space loading

Patients with chronic heart failure (CHF) suffer increased alveolar VD/VT (dead-space-to-tidal-volume ratio), yet they demonstrate augmented pulmonary ventilation such that arterial [Formula: see text] ( [Formula: see text] ) remains remarkably normal from rest to moderate exercise. This paradoxical effect suggests that the control law governing exercise hyperpnea is not merely determined by metabolic CO2 production ( [Formula: see text] ) per se but is responsive to an apparent (real-feel) metabolic CO2 load ( [Formula: see text] ) that also incorporates the adverse effect of physiological VD/VT on pulmonary CO2 elimination. By contrast, healthy individuals subjected to dead space loading also experience augmented ventilation at rest and during exercise as with increased alveolar VD/VT in CHF, but the resultant response is hypercapnic instead of eucapnic, as with CO2 breathing. The ventilatory effects of dead space loading are therefore similar to those of increased alveolar VD/VT and CO2 breathing combined. These observations are consistent with the hypothesis that the increased series VD/VT in dead space loading adds to [Formula: see text] as with increased alveolar VD/VT in CHF, but this is through rebreathing of CO2 in dead space gas thus creating a virtual (illusory) airway CO2 load within each inspiration, as opposed to a true airway CO2 load during CO2 breathing that clogs the mechanism for CO2 elimination through pulmonary ventilation. Thus, the chemosensing mechanism at the respiratory controller may be responsive to putative drive signals mediated by within-breath [Formula: see text] oscillations independent of breath-to-breath fluctuations of the mean [Formula: see text] level. Skeletal muscle afferents feedback, while important for early-phase exercise cardioventilatory dynamics, appears inconsequential for late-phase exercise hyperpnea.

Tendon vibration attenuates superficial venous vessel response of the resting limb during static arm exercise

BACKGROUND

The superficial vein of the resting limb constricts sympathetically during exercise. Central command is the one of the neural mechanisms that controls the cardiovascular response to exercise. However, it is not clear whether central command contributes to venous vessel response during exercise. Tendon vibration during static elbow flexion causes primary muscle spindle afferents, such that a lower central command is required to achieve a given force without altering muscle force. The purpose of this study was therefore to investigate whether a reduction in central command during static exercise with tendon vibration influences the superficial venous vessel response in the resting limb.

METHODS

Eleven subjects performed static elbow flexion at 35% of maximal voluntary contraction with (EX + VIB) and without (EX) vibration of the biceps brachii tendon. The heart rate, mean arterial pressure, and rating of perceived exertion (RPE) in overall and exercising muscle were measured. The cross-sectional area (CSAvein) and blood velocity of the basilic vein in the resting upper arm were assessed by ultrasound, and blood flow (BFvein) was calculated using both variables.

RESULTS

Muscle tension during exercise was similar between EX and EX + VIB. However, RPEs at EX + VIB were lower than those at EX (P <0.05). Increases in heart rate and mean arterial pressure during exercise at EX + VIB were also lower than those at EX (P <0.05). CSAvein in the resting limb at EX decreased during exercise from baseline (P <0.05), but CSAvein at EX + VIB did not change during exercise. CSAvein during exercise at EX was smaller than that at EX + VIB (P <0.05). However, BFvein did not change during the protocol under either condition. The decreases in circulatory response and RPEs during EX + VIB, despite identical muscle tension, showed that activation of central command was less during EX + VIB than during EX. Abolishment of the decrease in CSAvein during exercise at EX + VIB may thus have been caused by a lower level of central command at EX + VIB rather than EX.

CONCLUSION

Diminished central command induced by tendon vibration may attenuate the superficial venous vessel response of the resting limb during sustained static arm exercise.