Reflex inputs to the cardiovascular and respiratory centers from dynamically working canine muscles. Some evidence for involvement of group III or IV nerve fibers

Control of Breathing

Substantial advances have been made recently into the discovery of fundamental mechanisms underlying the neural control of breathing and even some inroads into translating these findings to treating breathing disorders. Here, we review several of these advances, starting with an appreciation of the importance of V̇A:V̇CO2:PaCO2 relationships, then summarizing our current understanding of the mechanisms and neural pathways for central rhythm generation, chemoreception, exercise hyperpnea, plasticity, and sleep-state effects on ventilatory control. We apply these fundamental principles to consider the pathophysiology of ventilatory control attending hypersensitized chemoreception in select cardiorespiratory diseases, the pathogenesis of sleep-disordered breathing, and the exertional hyperventilation and dyspnea associated with aging and chronic diseases. These examples underscore the critical importance that many ventilatory control issues play in disease pathogenesis, diagnosis, and treatment.

Muscle stretching induces the mechanoreflex response in human arterial blood pressure

The muscle mechanoreflex has been considered to make a small contribution to the cardiovascular response to exercise in healthy humans because no pressor response has been observed during stimulation of mechanosensitive receptors, such as static passive stretching, during many human studies. There is room for rethinking this consideration since the pressor response to upper limb exercise is greater than that to lower limb exercise. We examined whether static passive stretching of the forearm muscles causes a muscle mechanoreflex-induced pressor response in humans. Eighteen healthy men were recruited for this study. After a 15-min rest period in the supine position with a neutral (0°) wrist joint angle, all participants completed static passive stretching of the forearm for 60 s at four different intensities: minimal painful passive stretching (PPS), moderate-intensity passive stretching (MPS), low-intensity passive stretching (LPS), and no load (NL). During the procedure, beat-to-beat arterial blood pressure was measured using finger photoplethysmography. The force generated between the passively stretched hand and the experimenter's hands was recorded using a force transducer. Mean arterial pressure (MAP) during PPS and MPS significantly increased from baseline during the last 40 s (P < 0.05). MAP was significantly greater at 50 s and 60 s, depending on the intensity. MPS induced a greater peak response in MAP than lower intensities (P < 0.05). None of the subjects reported pain during the MPS and LPS trials. Static passive stimulation of the forearm is an effective method of isolating the muscle mechanoareflex-induced pressor response in humans.NEW & NOTEWORTHY The muscle mechanoreflex was considered to have a small contribution to cardiovascular regulation during exercise in healthy humans. In contrast, the results of this study indicate that static stretching of the forearm induces a pressor response in healthy humans and suggest that the mechanoreflex explicitly induces the pressor response during exercise in humans. The methods applied are useful for evaluating the pressor response to the mechanoreflex regardless of health, aging, and disease.

The physiology and pathophysiology of exercise hyperpnea

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

The muscle reflex and chemoreflex interaction: ventilatory implications for the exercising human

We examined the interactive influence of the muscle reflex (MR) and the chemoreflex (CR) on the ventilatory response to exercise. Eleven healthy subjects (5 women/6 men) completed three bouts of constant-load single-leg knee-extension exercise in a control trial and an identical trial conducted with lumbar intrathecal fentanyl to attenuate neural feedback from lower-limb group III/IV muscle afferents. The exercise during the two trials was performed while breathing ambient air ([Formula: see text] ~97%, [Formula: see text]~84 mmHg, [Formula: see text] ~32 mmHg, pH ~7.39), or under normocapnic hypoxia ([Formula: see text] ~79%, [Formula: see text] ~43 mmHg, [Formula: see text] ~33 mmHg, pH ~7.39) or normoxic hypercapnia ([Formula: see text] ~98%, [Formula: see text] ~105 mmHg, [Formula: see text] ~50 mmHg, pH ~7.26). During coactivation of the MR and the hypoxia-induced CR (O₂-CR), minute ventilation (V̇e) and tidal volume (V_T) were significantly greater compared with the sum of the responses to the activation of each reflex alone; there was no difference between the observed and summated responses in terms of breathing frequency (f_B; P = 0.4). During coactivation of the MR and the hypercapnia-induced CR (CO₂-CR), the observed ventilatory responses were similar to the summated responses of the reflexes (P ≥ 0.1). Therefore, the interaction between the MR and the O₂-CR exerts a hyperadditive effect on V̇e and V_T and an additive effect on f_B, whereas the interaction between the MR and the CO₂-CR is simply additive for all ventilatory parameters. These findings reveal that the MR:CR interaction further augments the ventilatory response to exercise in hypoxia.NEW & NOTEWORTHY Although the muscle reflex and the chemoreflex are recognized as independent feedback mechanisms regulating breathing during exercise, the ventilatory implications resulting from their interaction remain unclear. We quantified the individual and interactive effects of these reflexes during exercise and revealed differential modes of interaction. Importantly, the reflex interaction further amplifies the ventilatory response to exercise under hypoxemic conditions, highlighting a potential mechanism for optimizing arterial oxygenation in physically active humans at high altitude.

A murine model of the exercise pressor reflex

KEY POINTS

The decerebrate mouse provides a novel working model of the exercise pressor reflex (EPR). The decerebrate mouse model of the EPR is similar to the previously described decerebrate rat model. Studying the EPR in transgenic mouse models can define exact mechanisms of the EPR in health and disease.

ABSTRACT

The exercise pressor reflex (EPR) is defined by a rise in mean arterial pressure (MAP) and heart rate (HR) in response to exercise and is necessary to match metabolic demand and prevent premature fatigue. While this reflex is readily tested in humans, mechanistic studies are largely infeasible. Here, we have developed a novel murine model of the EPR to allow for mechanistic studies in various mouse models. We observed that ventral root stimulation (VRS) in an anaesthetized mouse causes a depressor response and a reduction in HR. In contrast, the same stimulation in a decerebrate mouse causes a rise in MAP and HR which is abolished by dorsal rhizotomy or by neuromuscular blockade. Moreover, we demonstrate a reduced MAP response to VRS using TRPV1 antagonism or in Trpv1 null mice while the response to passive stretch remains intact. Additionally, we demonstrate that intra-arterial infusion of capsaicin results in a dose-related rise in MAP and HR that is significantly reduced by a selective and potent TRPV1 antagonist or is completely abolished in Trpv1 null mice. These data serve to validate the development of a decerebrate mouse model for the study of cardiovascular responses to exercise and further define the role of the TRPV1 receptor in mediating the EPR. This novel model will allow for extensive study of the EPR in unlimited transgenic and mutant mouse lines, and for an unprecedented exploration of the molecular mechanisms that control cardiovascular responses to exercise in health and disease.

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.

Modulation of Respiratory System by Limb Muscle Afferents in Intact and Injured Spinal Cord

Breathing constantly adapts to environmental, metabolic or behavioral changes by responding to different sensory information, including afferent feedback from muscles. Importantly, not just respiratory muscle feedback influences respiratory activity. Afferent sensory information from rhythmically moving limbs has also been shown to play an essential role in the breathing. The present review will discuss the neuronal mechanisms of respiratory modulation by activation of peripheral muscles that usually occurs during locomotion or exercise. An understanding of these mechanisms and finding the most effective approaches to regulate respiratory motor output by stimulation of limb muscles could be extremely beneficial for people with respiratory dysfunctions. Specific attention in the present review is given to the muscle stimulation to treat respiratory deficits following cervical spinal cord injury.

Evaluation of the Electromyography Test for the Analysis of the Aerobic-Anaerobic Transition in Elite Cyclists during Incremental Exercise

(1) Background: The aim of this study was to investigate the validity and reliability of surface electromyography (EMG) for automatic detection of the aerobic and anaerobic thresholds during an incremental continuous cycling test using 1 min exercise periods in elite cyclists. (2) Methods: Sixteen well-trained cyclists completed an incremental exercise test (25 W/1 min) to exhaustion. Surface bipolar EMG signals were recorded from the vastus lateralis, vastus medialis, biceps femoris, and gluteus maximus, and the root mean square (RMS) were assessed. The multi-segment linear regression method was used to calculate the first and second EMG thresholds (EMGT1 and EMGT2). During the test, gas exchange data were collected to determine the first and second ventilatory thresholds (VT1 and VT2). (3) Results: Two breakpoints (thresholds) were identified in the RMS EMG vs. time curve for all muscles in 75% of participants. The two breakpoints, EMGT1 and EMGT2, were detected at around 70%–80% and 90%–95% of VO2MAX, respectively. No significant differences were found between the means of VT1 and EMGT1 for the vastii and biceps femoris muscles (p > 0.05). There were no significant differences between means of EMGT2 and VT2 (p > 0.05). (4) Conclusions: It is concluded that the multi-segment linear regression algorithm is a valid non-invasive method for analyzing the aerobic-anaerobic transition during incremental tests with 1 min stage durations.

GsMTx4 reduces the reflex pressor response during dynamic hindlimb skeletal muscle stretch in decerebrate rats

Mechanical signals within contracting skeletal muscles contribute to the generation of the exercise pressor reflex; an important autonomic and cardiovascular control mechanism. In decerebrate rats, the mechanically activated channel inhibitor GsMTx4 was found to reduce the pressor response during static hindlimb muscle stretch; a maneuver used to investigate specifically the mechanical component of the exercise pressor reflex (i.e., the mechanoreflex). However, the effect was found only during the initial phase of the stretch when muscle length was changing and not during the later phase of stretch when muscle length was relatively constant. We tested the hypothesis that in decerebrate, unanesthetized rats, GsMTx4 would reduce the pressor response throughout the duration of a 30 sec, 1 Hz dynamic hindlimb muscle stretch protocol that produced repetitive changes in muscle length. We found that the injection of 10 μg of GsMTx4 into the arterial supply of a hindlimb reduced the peak pressor response (control: 15 ± 4, GsMTx4: 5 ± 2 mmHg, P < 0.05, n = 8) and the pressor response at multiple time points throughout the duration of the stretch. GsMTx4 had no effect on the pressor response to the hindlimb arterial injection of lactic acid which indicates the lack of local off-target effects. Combined with the recent finding that GsMTx4 reduced the pressor response only initially during static stretch in decerebrate rats, the present findings suggest that GsMTx4-sensitive channels respond primarily to mechanical signals associated with changes in muscle length. The findings add to our currently limited understanding of the channels that contribute to the activation of the mechanoreflex.

Sciatic nerve stimulation and its effects on upper airway resistance in the anesthetized rabbit model relevant to sleep apnea

Obstructive sleep apnea (OSA) is a disorder characterized by collapse of the velopharynx and/or oropharynx during sleep when drive to the upper airway is reduced. Here, we explore an indirect approach for activation of upper airway muscles that might affect airway dynamics, namely, unilateral electrical stimulation of the afferent fibers of the sciatic nerve, in an anesthetized rabbit model. A nerve cuff electrode was placed around the sciatic and hypoglossal nerves to deliver stimulus while airflow, air pressure, and alae nasi electromyogram (EMG) were monitored both before and after sciatic transection. Sciatic nerve stimulation increased respiratory effort, rate, and alae nasi EMG, which persisted for seconds after stimulation; however, upper airway resistance was unchanged. Hypoglossal stimulation reduced resistance without altering drive. Although sciatic nerve stimulation is not ideal for treating OSA, it remains a target for altering respiratory drive. NEW & NOTEWORTHY Previously, sciatic nerve stimulation has been shown to activate upper airway and chest wall muscles. The supposition that resistance through the upper airway would be reduced with this afferent reflex was disproven. Findings were in contrast with the effect of hypoglossal nerve stimulation, which was shown to decrease resistance without changing muscle activation or ventilatory drive.

Chronic femoral artery ligation exaggerates the pressor and sympathetic nerve responses during dynamic skeletal muscle stretch in decerebrate rats

Mechanical and metabolic signals arising during skeletal muscle contraction reflexly increase sympathetic nerve activity and blood pressure (i.e., the exercise pressor reflex). In a rat model of simulated peripheral artery disease in which a femoral artery is chronically (~72 h) ligated, the mechanically sensitive component of the exercise pressor reflex during 1-Hz dynamic contraction is exaggerated compared with that found in normal rats. Whether this is due to an enhanced acute sensitization of mechanoreceptors by metabolites produced during contraction or involves a chronic sensitization of mechanoreceptors is unknown. To investigate this issue, in decerebrate, unanesthetized rats, we tested the hypothesis that the increases in mean arterial blood pressure and renal sympathetic nerve activity during 1-Hz dynamic stretch are larger when evoked from a previously "ligated" hindlimb compared with those evoked from the contralateral "freely perfused" hindlimb. Dynamic stretch provided a mechanical stimulus in the absence of contraction-induced metabolite production that closely replicated the pattern of the mechanical stimulus present during dynamic contraction. We found that the increases in mean arterial blood pressure (freely perfused: 14 ± 1 and ligated: 23 ± 3 mmHg, P = 0.02) and renal sympathetic nerve activity were significantly greater during dynamic stretch of the ligated hindlimb compared with the increases during dynamic stretch of the freely perfused hindlimb. These findings suggest that the exaggerated mechanically sensitive component of the exercise pressor reflex found during dynamic muscle contraction in this rat model of simulated peripheral artery disease involves a chronic sensitizing effect of ligation on muscle mechanoreceptors and cannot be attributed solely to acute contraction-induced metabolite sensitization. NEW & NOTEWORTHY We found that the pressor and sympathetic nerve responses during dynamic stretch were exaggerated in rats with a ligated femoral artery (a model of peripheral artery disease). Our findings provide mechanistic insights into the exaggerated exercise pressor reflex in this model and may have important implications for peripheral artery disease patients.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

Group III/IV muscle afferents limit the intramuscular metabolic perturbation during whole body exercise in humans

KEY POINTS

The purpose of this study was to determine the role of group III/IV muscle afferents in limiting the endurance exercise-induced metabolic perturbation assayed in muscle biopsy samples taken from locomotor muscle. Lumbar intrathecal fentanyl was used to attenuate the central projection of μ-opioid receptor-sensitive locomotor muscle afferents during a 5 km cycling time trial. The findings suggest that the central projection of group III/IV muscle afferent feedback constrains voluntary neural 'drive' to working locomotor muscle and limits the exercise-induced intramuscular metabolic perturbation. Therefore, the CNS might regulate the degree of metabolic perturbation within locomotor muscle and thereby limit peripheral fatigue. It appears that the group III/IV muscle afferents are an important neural link in this regulatory mechanism, which probably serves to protect locomotor muscle from the potentially severe functional impairment as a consequence of severe intramuscular metabolic disturbance.

ABSTRACT

To investigate the role of metabo- and mechanosensitive group III/IV muscle afferents in limiting the intramuscular metabolic perturbation during whole body endurance exercise, eight subjects performed 5 km cycling time trials under control conditions (CTRL) and with lumbar intrathecal fentanyl impairing lower limb muscle afferent feedback (FENT). Vastus lateralis muscle biopsies were obtained before and immediately after exercise. Motoneuronal output was estimated through vastus lateralis surface electromyography (EMG). Exercise-induced changes in intramuscular metabolites were determined using liquid and gas chromatography-mass spectrometry. Quadriceps fatigue was quantified by pre- to post-exercise changes in potentiated quadriceps twitch torque (ΔQTsingle ) evoked by electrical femoral nerve stimulation. Although motoneuronal output was 21 ± 12% higher during FENT compared to CTRL (P < 0.05), time to complete the time trial was similar (∼8.8 min). Compared to CTRL, power output during FENT was 10 ± 4% higher in the first half of the time trial, but 11 ± 5% lower in the second half (both P < 0.01). The exercise-induced increase in intramuscular inorganic phosphate, H(+) , adenosine diphosphate, lactate and phosphocreatine depletion was 55 ± 30, 62 ± 18, 129 ± 63, 47 ± 14 (P < 0.001) and 27 ± 14% (P < 0.01) greater in FENT than CTRL. ΔQTsingle was greater following FENT than CTRL (-52 ± 2 vs -31 ± 1%, P < 0.001) and this difference was positively correlated with the difference in inorganic phosphate (r(2)  = 0.79; P < 0.01) and H(+) (r(2)  = 0.92; P < 0.01). In conclusion, during whole body exercise, group III/IV muscle afferents provide feedback to the CNS which, in turn, constrains motoneuronal output to the active skeletal muscle. This regulatory mechanism limits the exercise-induced intramuscular metabolic perturbation, preventing an abnormal homeostatic challenge and excessive peripheral fatigue.

Autonomic responses to exercise: group III/IV muscle afferents and fatigue

Group III and IV muscle afferents originating in exercising limb muscle play a significant role in the development of fatigue during exercise in humans. Feedback from these sensory neurons to the central nervous system (CNS) reflexively increases ventilation and central (cardiac output) and peripheral (limb blood flow) hemodynamic responses during exercise and thereby assures adequate muscle blood flow and O2 delivery. This response depicts a key factor in minimizing the rate of development of peripheral fatigue and in optimizing aerobic exercise capacity. On the other hand, the central projection of group III/IV muscle afferents impairs performance and limits the exercising human via its diminishing effect on the output from spinal motoneurons which decreases voluntary muscle activation (i.e. facilitates central fatigue). Accumulating evidence from recent animal studies suggests the existence of two subtypes of group III/IV muscle afferents. While one subtype only responds to physiological and innocuous levels of endogenous intramuscular metabolites (lactate, ATP, protons) associated with 'normal', predominantly aerobic exercise, the other subtype only responds to higher and concurrently noxious levels of metabolites present in muscle during ischemic contractions or following, for example, hypertonic saline infusions. This review discusses the mechanisms through which group III/IV muscle afferent feedback mediates both central and peripheral fatigue in exercising humans. We also briefly summarize the accumulating evidence from recent animal and human studies documenting the existence of two subtypes of group III/IV muscle afferents and the relevance of this discovery to the interpretation of previous work and the design of future studies.

Effects of gas exchange on acid-base balance

This paper describes the interactions between ventilation and acid-base balance under a variety of conditions including rest, exercise, altitude, pregnancy, and various muscle, respiratory, cardiac, and renal pathologies. We introduce the physicochemical approach to assessing acid-base status and demonstrate how this approach can be used to quantify the origins of acid-base disorders using examples from the literature. The relationships between chemoreceptor and metaboreceptor control of ventilation and acid-base balance summarized here for adults, youth, and in various pathological conditions. There is a dynamic interplay between disturbances in acid-base balance, that is, exercise, that affect ventilation as well as imposed or pathological disturbances of ventilation that affect acid-base balance. Interactions between ventilation and acid-base balance are highlighted for moderate- to high-intensity exercise, altitude, induced acidosis and alkalosis, pregnancy, obesity, and some pathological conditions. In many situations, complete acid-base data are lacking, indicating a need for further research aimed at elucidating mechanistic bases for relationships between alterations in acid-base state and the ventilatory responses.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Pulmonary gas exchange and acid-base balance during exercise

As the first step in the oxygen-transport chain, the lung has a critical task: optimizing the exchange of respiratory gases to maintain delivery of oxygen and the elimination of carbon dioxide. In healthy subjects, gas exchange, as evaluated by the alveolar-to-arterial PO2 difference (A-aDO2), worsens with incremental exercise, and typically reaches an A-aDO2 of approximately 25 mmHg at peak exercise. While there is great individual variability, A-aDO2 is generally largest at peak exercise in subjects with the highest peak oxygen consumption. Inert gas data has shown that the increase in A-aDO2 is explained by decreased ventilation-perfusion matching, and the development of a diffusion limitation for oxygen. Gas exchange data does not indicate the presence of right-to-left intrapulmonary shunt developing with exercise, despite recent data suggesting that large-diameter arteriovenous shunt vessels may be recruited with exercise. At the same time, multisystem mechanisms regulate systemic acid-base balance in integrative processes that involve gas exchange between tissues and the environment and simultaneous net changes in the concentrations of strong and weak ions within, and transfer between, extracellular and intracellular fluids. The physicochemical approach to acid-base balance is used to understand the contributions from independent acid-base variables to measured acid-base disturbances within contracting skeletal muscle, erythrocytes and noncontracting tissues. In muscle, the magnitude of the disturbance is proportional to the concentrations of dissociated weak acids, the rate at which acid equivalents (strong acid) accumulate and the rate at which strong base cations are added to or removed from muscle.

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

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

Muscle afferent activation causes ventilatory and cardiovascular responses during concurrent hypercapnia in humans

Respiratory and cardiovascular responses to muscle mechanoreflex (passive calf stretch) and metaboreflex activation (local circulatory occlusion) were examined during inhalation of a hypercapnic gas mixture in four trials. These controlled for the effects of central command, metabolite sensitization of muscle afferents and hypercapnia-induced elevation of central respiratory drive. In an isokinetic dynamometer, with circulation through the right leg occluded by inflation of a thigh cuff, 13 participants either rested (control trial; CON) or plantarflexed their ankle at 50% maximal force for 1.5 min (voluntary exercise trial; EX). Thereafter, circulatory occlusion was maintained and the calf passively stretched before return to the resting position. Both trials were performed while breathing air, as well as while breathing a normoxic, hypercapnic (5% CO(2)) gas mixture (CO(2) trial and CO(2)+EX trial). Hypercapnic gas inhalation increased baseline minute ventilation (V), heart rate and mean arterial pressure (+27.67 ± 1.74 l min(-1), +7 ± 0.85 beats min(-1) and +13 ± 3.41 mmHg, respectively; means ± SEM) above control values (9.78 ± 0.86 l min(-1), 62 ± 2.3 beats min(-1) and 88 ± 2.6 mmHg, respectively). Voluntary exercise further increased these variables from baseline during both trials (P < 0.05). During the continued circulatory occlusion after voluntary exercise, mean arterial pressure remained significantly elevated (P < 0.05). Minute ventilation returned to baseline during circulatory occlusion following exercise in the EX trial, but in the CO(2)+EX trial the V remained elevated at end-exercise levels during this period (+7.12 ± 1.13 l min(-1)). Passive stretch caused further increases in V during CO(2)+EX and CO(2) trials but not in CON and EX. These results indicate that in the absence of central command, either muscle metaboreflex and/or mechanoreflex activation stimulates ventilation during concurrent hypercapnia.

Group III and IV muscle afferents contribute to ventilatory and cardiovascular response to rhythmic exercise in humans

We investigated the role of somatosensory feedback on cardioventilatory responses to rhythmic exercise in five men. In a double-blind, placebo-controlled design, subjects performed the same leg cycling exercise (50/100/150/325 ± 19 W, 3 min each) under placebo conditions (interspinous saline, L(3)-L(4)) and with lumbar intrathecal fentanyl impairing central projection of spinal opioid receptor-sensitive muscle afferents. Quadriceps strength was similar before and after fentanyl administration. To evaluate whether a cephalad migration of fentanyl affected cardioventilatory control centers in the brain stem, we compared resting ventilatory responses to hypercapnia (HCVR) and cardioventilatory responses to arm vs. leg cycling exercise after each injection. Similar HCVR and minor effects of fentanyl on cardioventilatory responses to arm exercise excluded direct medullary effects of fentanyl. Central command during leg exercise was estimated via quadriceps electromyogram. No differences between conditions were found in resting heart rate (HR), ventilation [minute ventilation (VE)], or mean arterial pressure (MAP). Quadriceps electromyogram, O(2) consumption (VO(2)), and plasma lactate were similar in both conditions at the four steady-state workloads. Compared with placebo, a substantial hypoventilation during fentanyl exercise was indicated by the 8-17% reduction in VE/CO(2) production (VCO(2)) secondary to a reduced breathing frequency, leading to average increases of 4-7 Torr in end-tidal PCO(2) (P < 0.001) and a reduced hemoglobin saturation (-3 ± 1%; P < 0.05) at the heaviest workload (∼90% maximal VO(2)) with fentanyl. HR was reduced 2-8%, MAP 8-13%, and ratings of perceived exertion by 13% during fentanyl vs. placebo exercise (P < 0.05). These findings demonstrate the essential contribution of muscle afferent feedback to the ventilatory, cardiovascular, and perceptual responses to rhythmic exercise in humans, even in the presence of unaltered contributions from other major inputs to cardioventilatory control.

Peripheral mu-opioid receptors attenuate the augmented exercise pressor reflex in rats with chronic femoral artery occlusion

Recently, opioid receptors have been shown to be expressed on group III and IV afferents, which comprise the sensory arm of the exercise pressor reflex. Although the stimulation of opioid receptors in the central nervous system has been shown to attenuate the exercise pressor reflex, the effect on the reflex of their stimulation in the periphery is unknown. We therefore tested the hypothesis that the activation of peripheral mu-opioid receptors attenuates the exercise pressor reflex. The pressor responses to static contraction were compared before and after the injection of the mu-opioid receptor agonist [d-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO; 1 microg) into the abdominal aorta of decerebrated rats in which one femoral artery had been occluded 72 h previously (n = 10) and in control rats whose femoral arteries were freely perfused (n = 8). DAMGO attenuated the peak pressor response to contraction in rats whose femoral arteries had been occluded (before: increase of 34 + or - 3 mmHg and after: increase of 22 + or - 2 mmHg, P = 0.008); the inhibitory effect of DAMGO was prevented by the injection of naloxone (100 microg) into the abdominal aorta (before: increase of 29 + or - 5 mmHg and after: increase of 29 + or - 5 mmHg, P = 0.646, n = 7). An intravenous injection of DAMGO (1 microg, n = 6) had no effect on the peak pressor response to contraction in both groups of rats. DAMGO had no effect on the peak pressor response to contraction in rats whose femoral arteries were freely perfused (before: Delta 23 + or - 4 mmHg, after: Delta 23 + or - 3 mmHg, n = 6) but appeared to have a small effect on topography of the response. DAMGO had no effect on the peak pressor response to tendon stretch in both groups of rats (both P > 0.05). We conclude that the stimulation of peripheral mu-opioid receptors attenuates the exercise pressor reflex in rats whose femoral arteries have been ligated for 72 h.

Neural mechanism of bradycardiac responses elicited by acupuncture-like stimulation to a hind limb in anesthetized rats

The effects of acupuncture-like stimulation of a hind limb on heart rate were examined in anesthetized rats. An acupuncture needle, having a diameter of either 160 or 340 microm, was inserted into the skin and underlying muscles at a depth of about 5 mm and twisted right and left twice every second for 1 min. Stimulation by a needle with a diameter of either 160 or 340 microm produced a decrease in heart rate. Severance of the femoral and sciatic nerves ipsilateral to the hind-limb stimulation completely abolished the bradycardiac response. Also, heart rate was significantly decreased by acupuncture-like stimulation of the hind-limb muscles alone, but was not significantly influenced by the stimulation of the hind-limb skin alone. The bradycardiac response induced by acupuncture-like stimulation was not influenced by bilateral severance of the vagal nerves at the cervical level, but was abolished by bilateral stellectomy. Acupuncture-like stimulation of the hind limb induced a decrease in the activity of the cardiac sympathetic efferent nerve as well as a decrease in heart rate. These results indicate that the decrease in heart rate induced by acupuncture-like stimulation of a hind limb is a reflex response. The afferent pathway is composed of hind-limb muscle afferents, and the efferent pathway is composed of cardiac sympathetic nerves.

Physiological responses of elite Laser sailors to 30 minutes of simulated upwind sailing

In this study, we tested the hypothesis that elite dinghy sailing is a whole-body, dynamic, repeated-effort sport, and that increased heart rate and oxygen consumption reflect its dynamic element. Six elite male Laser sailors (mean age 19.7 years, s = 1.82; height 1.81 m, s = 0.03; body mass 78.0 kg, s = 4.1) performed a cycle ergometer test to volitional exhaustion to determine peak oxygen uptake (VO(2peak)) and a simulated 30-min upwind leg sail on a specially constructed Laser sailing ergometer. The simulation protocol was based on video analysis of previous Laser World Championships. Expired gases were collected in Douglas bags, heart rate recorded at rest and after every 5 min, and pre- and post-simulation capillary blood samples taken for blood lactate analysis. Results were analysed with a one-way analysis of variance. Mean VO(2peak) was 4.32 l . min(-1) (s = 0.16). Mean simulation VO(2) was 2.51 l . min(-1) (s = 0.24) and peaked at 2.58 l . min(-1) (s = 0.25) during the 5th minute. Mean simulation heart rate was 156 beats . min(-1) (s = 8), peaking during the final minute at 160 beats . min(-1) (s = 10). These results suggest that, unlike pseudo-isometric static hiking, elite dinghy sailing demands a substantial proportion (58%VO(2peak), s = 5.6) of aerobic capacity.

Theories on the nature of the coupling between ventilation and gas exchange during exercise

For over a century of creative research, many theories on the possible mechanisms controlling respiration during exercise have been developed and discussed. One of the most enduring questions is certainly related to the mechanisms that can prevent P(a)(CO(2)) rising when CO(2) production increases. As multiple systems and structures are capable of increasing ventilation (V (E)), not all the mechanisms controlling respiration can provide a proper answer to this question. Indeed, exercise is a complex physiological condition combining motor activity with a change in metabolic rate. The most intriguing aspect of exercise is that when the changes in metabolism are dissociated from the motor and locomotor activity, the strategy 'chosen' by the respiratory control system is to follow the metabolic rate (or more precisely factors temporally associated with the pulmonary gas exchange rate) regardless of the motor act. The strategy used by the respiratory system during exercise therefore appears to select from among various sources of information the most relevant to follow the rate at which CO(2) is ultimately exchanged by the lungs. Yet, the nature of the signal(s) which prevents CO(2)/H(+) disturbance during exercise is the fundamental question addressed by this simple observation and remains to be clarified. This review illustrates the attempts of many physiologists to collect experimental evidence for theories which could provide satisfactory mechanisms accounting for the matching between ventilation and the rate at which CO(2) leaves the tissues and is exchanged at the lungs. More recent models based on somatic information of circulatory origin are presented and discussed.

Mechanism behind Augmentation in Baroreflex Sensitivity after Acute Exercise in Spontaneously Hypertensive Rats

A single bout of dynamic exercise increases baroreflex sensitivity (BRS) in spontaneously hypertensive rats (SHR). We examined whether change in hemodynamics (increases in blood pressure and heart rate) associated with dynamic exercise contribute to the post-exercise modulation of BRS. SHR aged 12 weeks were chronically instrumented with a carotid artery catheter and jugular vein catheter. They were then allocated to three groups submitted to 40 min of 1) running on a treadmill at 12 m/min (Run), 2) concomitant infusion of isoproterenol and a relatively high dose of phenylephrine (Iso+Phe(high)), or 3) concomitant infusion of isoproterenol and a relatively low dose of phenylephrine (Iso+Phe(low)). Arterial pressure and heart rate were continuously recorded throughout the experiments. BRS estimated by heart rate responses to phenylephrine injection and systolic blood pressure-low frequency power amplitude (SBP-LFamp) evaluated by power spectral analysis of SBP, a marker of sympathetic activity, were examined before and after running (Run group), or administration of drugs (Iso+Phe(high) or Iso+Phe(low) groups). BRS increased significantly from 1.4 to 1.9 bpm/mmHg after running, but not after administration of Iso+Phe(high) or Iso+Phe(low). Blood pressure and SBP-LFamp significantly decreased in each of the Run, Iso+Phe(high) and Iso+Phe(low) groups. These results suggest that hemodynamic change alone does not contribute to post-exercise modulation of BRS, while hemodynamic change or sympathetic activation during exercise contributes to post-exercise hypotension associated with a reduction of sympathetic activity.

Muscle reflex control of sympathetic nerve activity in heart failure: the role of exercise conditioning

Muscle reflex control of sympathetic nerve activity has been an area of considerable investigation. During exercise, the capacity of the peripheral vasculature to dilate far exceeds the maximal attainable levels of cardiac output. The activation of sympathetic nervous system and engagement of the myogenic reflex serve as the controlling influence between the heart and the muscle vasculature to maintain blood pressure (BP). Two basic theories of neural control have evolved. The first termed "central command", suggests that a volitional signal emanating from central motor areas leads to increased sympathetic activation during exercise. According to the second theory the stimulation of mechanical and chemical afferents in exercising muscle lead to engagement of the "exercise pressor reflex". Some earlier studies suggested that group III muscle afferent fibers are predominantly mechanically sensitive whereas unmyelinated group IV muscle afferents respond to chemical stimuli. In recent years new evidence is emerging which challenges the concept of functional differentiation of muscle afferents as well as the classic description of muscle "mechano" and "metabo" receptors. Studies measuring concentrations of interstitial substances during exercise suggest that K(+) and phosphate, but not H(+) and lactate, may be important muscle afferent stimulants. The role of adenosine as a muscle afferent stimulant remains an area of debate. There is strong evidence that sympathetic vasoconstriction due to muscle reflex engagement plays an important role in restricting blood flow to the exercising muscle. In heart failure (HF), exercise leads to premature fatigue and accumulation of muscle metabolites resulting in a greater degree of muscle reflex engagement and in the process further decreasing the muscle blood flow. Conditioning leads to an increased ability of the muscle to maintain aerobic metabolism, lower interstitial accumulation of metabolites, less muscle reflex engagement and a smaller sympathetic response. Beneficial effects of physical conditioning may be mediated by a direct reduction of muscle metaboreflex activity or via reduction of metabolic signals activating these receptors. In this review, we will discuss concepts of flow and reflex engagement in normal human subjects and then contrast these findings with those seen in heart failure (HF). We will then examine the effects of exercise conditioning on these parameters in normal subjects and those with congestive heart failure (CHF).

A perspective on the muscle reflex: implications for congestive heart failure

In this review we examine the exercise pressor reflex in health and disease. The role of metabolic and mechanical stimulation of thin fiber muscle afferents is discussed. The role ATP and lactic acid play in stimulating and sensitizing these afferents is examined. The role played by purinergic receptors subdivision 2, subtype X, vanilloid receptor subtype 1, and acid-sensing ion channels in mediating the effects of ATP and H+ are discussed. Muscle reflex activation in heart failure is then examined. Data supporting the concept that the metaboreflex is attenuated and that the mechanoreflex is accentuated are presented. The role the muscle mechanoreflex plays in evoking renal vasoconstriction is also described.

Sensing vascular distension in skeletal muscle by slow conducting afferent fibers: neurophysiological basis and implication for respiratory control

This review examines the evidence that skeletal muscles can sense the status of the peripheral vascular network through group III and IV muscle afferent fibers. The anatomic and neurophysiological basis for such a mechanism is the following: 1) a significant portion of group III and IV afferent fibers have been found in the vicinity and the adventitia of the arterioles and the venules; 2) both of these groups of afferent fibers can respond to mechanical stimuli; 3) a population of group III and IV fibers stimulated during muscle contraction has been found to be inhibited to various degrees by arterial occlusion; and 4) more recently, direct evidence has been obtained showing that a part of the group IV muscle afferent fibers is stimulated by venous occlusion and by injection of vasodilatory agents. The physiological relevance of sensing local distension of the vascular network at venular level in the muscles is clearly different from that of the large veins, since the former can directly monitor the degree of tissue perfusion. The possible involvement of this sensing mechanism in respiratory control is discussed mainly in the light of the ventilatory effects of peripheral vascular occlusions during and after muscular exercise. It is proposed that this regulatory system anticipates the chemical changes that would occur in the arterial blood during increased metabolic load and attempts to minimize them by adjusting the level of ventilation to the level of muscle perfusion, thus matching the magnitudes of the peripheral and pulmonary gas exchange.

Electromyographic signs of neuromuscular fatigue are concomitant with further increase in ventilation during static handgrip

We questioned if a non-linear increase in ventilation defining a ventilatory threshold (V(Th)) accompanied the electromyographic (EMG) signs of neuromuscular fatigue. Indeed, the intramuscular accumulation of metabolites may activate the afferent nervous pathways responsible for both the 'muscle wisdom' phenomenon and the respiratory centre activation. During inframaximal (50%) handgrip sustained until exhaustion, minute ventilation (V(E)), V(E)/V(O2) and V(E)/V(CO2) ratios were measured simultaneously with surface EMG of the 'flexor digitorum' muscle. V(Th) was defined as a non-linear V(E) increase and/or an abrupt V(E)/V(O2) increase without any concomitant increase in the V(E)/V(CO2) ratio. Handgrip was repeated during complete arterial blood flow interruption in order to suppress any venous return from the exercising forearm. In both control and blood flow interruption conditions, an abrupt increase in the V(E)/V(O2) ratio was measured in the majority of trials (13 of 15 and 14 of 15, respectively) and the EMG signs of neuromuscular fatigue (a decline in median frequency and/or a non-linear increase in low-frequency EMG energies, E(L)) were concomitant with the V(Th) determination. Thus, V(Th) occurs during sustained static contraction and is concomitant with EMG signs of neuromuscular fatigue. Neurogenic factors seem to be responsible for the two responses which persist despite the absence of any release of metabolites in the circulation.

CO2 does not affect passive exercise ventilatory decline

Breathing increases abruptly at the start of passive exercise, stimulated by afferent feedback from the moving limbs, and declines toward a steady-state hyperpnea as exercise continues. This decline has been attributed to decreased arterial CO2 levels and adaptation in afferent feedback; however, the relative importance of these two mechanisms is unknown. To address this issue, we compared ventilatory responses to 5 min of passive leg extension exercise performed on 10 awake human subjects (6 men and 4 women) in isocapnic and poikilocapnic conditions. End-tidal Pco2 decreased significantly during poikilocapnic (Delta = -1.5 +/- 0.5 Torr, P < 0.001), but not isocapnic, passive exercise. Despite this difference, the ventilatory responses to passive exercise were not different between the two conditions. Using the fast changes in ventilation at the start (5.46 +/- 0.40 l/min, P < 0.001) and end (3.72 +/- 0.33 l/min, P < 0.001) of passive exercise as measures of the drive to breathe from afferent feedback, we found a decline of 68%. We conclude that the decline in ventilation during passive exercise is due to an adaptation in the afferent feedback from the moving limbs, not a decline in CO2 levels.

EMG signs of neuromuscular fatigue related to the ventilatory threshold during cycling exercise

We questioned whether electromyographic (EMG) signs of neuromuscular fatigue accompany the changes in respiratory variables measured at the ventilatory threshold (VTh) during exercise on a cycloergometer. This was based on the assumption that the activation of muscle afferents sensitive to accumulation of lactate and potassium is suspected to elicit both the EMG signs of fatigue and hyperventilation. In 39 subjects performing an incremental cycling, the EMG estimates of neuromuscular fatigue in vastus lateralis were a non-linear increase in root mean square (RMS), a decrease in median frequency (MF), a non-linear increase in low-frequency EMG energies (EL), and/or a decrease in high-frequency energies (EH). VTh was determined from a non-linear increase in VCO2 [VTh(VCO2 slope)] and an increased value of the respiratory equivalent for oxygen [VTh(VE/VO2)]. We measured a significant increase in venous blood concentration of lactate and potassium, and a significant pHv fall at VTh. One EMG estimate of fatigue was detected in 33/39 individuals and two EMG estimates in 17 subjects. Highly significant positive correlations were found between the oxygen uptakes corresponding to each EMG estimate and to each detection criterion of VTh. These observations suggest that the activation of muscle sensory pathways contribute to the mechanism of VTh.

Cardiovascular and sympathetic neural responses to handgrip and cold pressor stimuli in humans before, during and after spaceflight

Astronauts returning to Earth have reduced orthostatic tolerance and exercise capacity. Alterations in autonomic nervous system and neuromuscular function after spaceflight might contribute to this problem. In this study, we tested the hypothesis that exposure to microgravity impairs autonomic neural control of sympathetic outflow in response to peripheral afferent stimulation produced by handgrip and a cold pressor test in humans. We studied five astronauts approximately 72 and 23 days before, and on landing day after the 16 day Neurolab (STS-90) space shuttle mission, and four of the astronauts during flight (day 12 or 13). Heart rate, arterial pressure and peroneal muscle sympathetic nerve activity (MSNA) were recorded before and during static handgrip sustained to fatigue at 40 % of maximum voluntary contraction, followed by 2 min of circulatory arrest pre-, in- and post-flight. The cold pressor test was applied only before (five astronauts) and during flight (day 12 or 13, four astronauts). Mean (+/- S.E.M.) baseline heart rates and arterial pressures were similar among pre-, in- and post-flight measurements. At the same relative fatiguing force, the peak systolic pressure and mean arterial pressure during static handgrip were not different before, during and after spaceflight. The peak diastolic pressure tended to be higher post- than pre-flight (112 +/- 6 vs. 99 +/- 5 mmHg, P = 0.088). Contraction-induced rises in heart rate were similar pre-, in- and post-flight. MSNA was higher post-flight in all subjects before static handgrip (26 +/- 4 post- vs. 15 +/- 4 bursts min(-1) pre-flight, P = 0.017). Contraction-evoked peak MSNA responses were not different before, during, and after spaceflight (41 +/- 4, 38 +/- 5 and 46 +/- 6 bursts min(-1), all P > 0.05). MSNA during post-handgrip circulatory arrest was higher post- than pre- or in-flight (41 +/- 1 vs. 33 +/- 3 and 30 +/- 5 bursts min(-1), P = 0.038 and 0.036). Similarly, responses of MSNA and blood pressure to the cold pressor test were well maintained in-flight. We conclude that modulation of muscle sympathetic neural outflow by muscle metaboreceptors and skin nociceptors is preserved during short duration spaceflight.

Heart rate at the onset of muscle contraction and during passive muscle stretch in humans: a role for mechanoreceptors

Previous evidence suggests that the heart rate (HR) increase observed with isometric exercise is dependent on different afferent mechanisms to those eliciting the increase in blood pressure (BP). Central command and muscle metaboreceptors have been shown to contribute to this differential effect. However, in experimental animals passive stretch of the hindlimb increases HR suggesting that small fibre mechanoreceptors could also have a role. This has not been previously shown in humans and was investigated in this study. Healthy human volunteers were instrumented to record BP, ECG, respiration, EMG of rectus femoris and gastrocnemius and contraction force of triceps surae. Voluntary isometric contraction of triceps surae elicited a significant HR change in the first three respiratory cycles at 40 % of maximum voluntary contraction whereas BP did not change significantly until after 30 s. This suggests that different mechanisms are involved in the initiation of the cardiovascular changes. Sustained passive stretch of triceps surae for 1 min, by dorsiflexion of the foot, caused a significant (P < 0.05) increase in HR (5 +/- 2.6 beats min(-1)) with no significant change in BP. A time domain measure of cardiac vagal activity was reduced significantly during passive stretch from 69.7 +/- 12.9 to 49.6 +/- 8.9 ms. Rapid rhythmic passive stretch (0.5 Hz for 1 min) was without significant effect suggesting that large muscle proprioreceptors are not involved. We conclude that in man small fibre muscle mechanoreceptors responding to stretch, inhibit cardiac vagal activity and thus increase HR. These afferents could contribute to the initial cardiac acceleration in response to muscle contraction.

Control of breathing and muscle perfusion in humans

Very brief and intense exercise triggers a biphasic metabolic and respiratory response with a second phase that occurs after the cessation of the muscular activity. The effects on minute ventilation (V(E)) produced by manipulation of the peripheral circulation in metabolically active muscles could thus be studied without the confounding effects of painful contractions. The second phase of breath-by-breath V(E) and pulmonary gas exchange responses to a brief change in work rate (400 W for 12 s) were studied in six healthy male subjects on four occasions (24 tests). An upper thigh cuff inflation was randomly applied either above or below the systolic blood pressure (200 or 90 Torr, respectively) for 90 s just after the cessation of the contractions prior to the delayed rise in pulmonary gas exchange (eight tests in each subject). Total occlusion produced a significant reduction in the delayed rise in V(E) (-29 +/- 3 %) which normally occurred 20-25 s after the cessation of the contractions. In contrast, cuff inflation at a level predominantly impeding venous return while partially maintaining the arterial supply reduced the rise in pulmonary gas exchange in similar proportion to that during total obstruction but with a slight but not significant reduction in ventilation (-9 +/- 5 %). V(E) during partial occlusion was if anything higher than in control tests with similar oxygen uptake (280 W), despite a higher blood pressure (BP) during occlusion (+7 Torr). It is concluded that the factors resulting from a reduction in venous return or from the involvement of the arterial baroreflex are not responsible for the changes in V(E) produced by the obstruction of the circulation to and from metabolically active muscles. It is proposed that factors related to the level of the perfusion pressure in hyperaemic muscles, possibly located at the venular end of the microcirculation, could account for the changes in V(E) observed.

Contribution of skeletal muscle 'ergoreceptors' in the human leg to respiratory control in chronic heart failure

The role of skeletal muscle ergoreceptors (afferents sensitive to muscle contraction, differentiated into metaboreceptors, sensitive to metabolic changes, and mechanoreceptors, sensitive to mechanical changes) in the genesis of the increased ventilatory drive in chronic heart failure is controversial. We have aimed to clarify the contribution of muscle metaboreceptors in the leg to ventilation and to compare this with the contribution of mechanoreceptors. Eighteen heart failure patients and 12 controls were studied. Metaboreceptor and mechanoreceptor responses were measured in the leg by bicycle exercise with and without regional circulatory occlusion during recovery, and by active and equivalent passive limb movement, respectively.Patients, in comparison with controls, had a lower peak VO2 (Oxygen uptake) (18.1+/-1.6 vs. 24.5+/-2.5 ml min(-1) kg(-1), P< 0.05), and an evident metaboreceptor contribution to the ventilatory response (3.5+/-1.6 vs. -4.0+/-1.3 l min(-1), P<0.001). Passive limb movement increased ventilation in both patients and controls (+3.7+/-0.4 and +2.9+/-0.5 l min(-1) from baseline, P<0.003), but this was associated with an increase in VO2 (+0.1+/-0.01 and +0.1+/-0.02 l min(-1) from baseline, P<0.001). The ratio of the increase in ventilation to the increase in VO2 during passive movement was not significantly higher than that during active exercise for either patients or controls, suggesting a limited contribution from the mechanoreceptors. In chronic heart failure the presence of a muscle metaboreceptor reflex is also demonstrated in the leg, while mechanoreceptors exhibited a non-significant contribution in both patients and controls. The hypothesis of a peripheral origin of symptoms of exertional intolerance in this syndrome is confirmed as being mainly due to metabolic stimulation of the muscle metaboreceptors.

Limb congestion enhances the synchronization of sympathetic outflow with muscle contraction

In this report, we examined if the synchronization of muscle sympathetic nerve activity (MSNA) with muscle contraction is enhanced by limb congestion. To explore this relationship, we applied signal-averaging techniques to the MSNA signal obtained during short bouts of forearm contraction (2-s contraction/3-s rest cycle) at 40% maximal voluntary contraction for 5 min. We performed this analysis before and after forearm venous congestion; an intervention that augments the autonomic response to sustained static muscle contractions via a local effect on muscle afferents. There was an increased percentage of the MSNA noted during second 2 of the 5-s contraction/rest cycles. The percentage of total MSNA seen during this particular second increased from minute 1 to 5 of contraction and was increased further by limb congestion (control minute 1 = 25.6 +/- 2.0%, minute 5 = 32.8 +/- 2.2%; limb congestion minute 1 = 29.3 +/- 2.1%, minute 5 = 37.8 +/- 3.9%; exercise main effect <0.005; limb congestion main effect P = 0.054). These changes in the distribution of signal-averaged MSNA were seen despite the fact that the mean number of sympathetic discharges did not increase over baseline. We conclude that synchronization of contraction and MSNA is seen during short repetitive bouts of handgrip. The sensitizing effect of contraction time and limb congestion are apparently due to feedback from muscle afferents within the exercising muscle.

Group III muscle afferents evoke reflex depressor responses to repetitive muscle contractions in rabbits

Repetitive-twitch contraction of the hindlimb muscles in anesthetized rabbits consistently evokes a reflex depressor response, whereas this type of contraction in anesthetized cats evokes a reflex pressor response in about one-half of the preparations tested. Rapidly conducting group III fibers appear to comprise the afferent arm of the reflex arc, evoking the depressor response to twitch contraction in rabbits because electrical stimulation of their axons reflexly decreases arterial pressure. In contrast, electrical stimulation of the axons of slowly conducting group III and group IV afferents reflexly increases arterial pressure in rabbits. In the present study, we examined the discharge properties of group III and IV muscle afferents and found that the former (i.e., 13 of 20), but not the latter (i.e., 0 of 10), were stimulated by 5 min of repetitive-twitch contraction (1 Hz) of the rabbit triceps surae muscles. Moreover, most of the group III afferents responding to contraction appeared to be mechanically sensitive, discharging in synchrony with the muscle twitch. On average, rapidly conducting group III afferents responded for the 5-min duration of 1-Hz repetitive-twitch contraction, whereas slowly conducting group III afferents responded only for the first 2 min of contraction. We conclude that rapidly conducting group III afferents, which are mechanically sensitive, are primarily responsible for evoking the reflex depressor response to repetitive-twitch contractions in anesthetized rabbits.

Responses of group III and IV muscle afferents to distension of the peripheral vascular bed

This study was undertaken to test the hypothesis that group III and IV afferents with endings in skeletal muscle signal the distension of the peripheral vascular network. The responses of these slowly conducting afferents to pharmacologically induced vasodilation and to acute obstruction of the venous drainage of the hindlimbs were studied in barbiturate-anesthetized cats. Afferent impulses arising from endings in the triceps surae muscles were recorded from the L(7) and S(1) dorsal roots. Fifteen of the 48 group IV and 3 of the 19 group III afferents tested were stimulated by intra-aortic injections of papaverine (2-2.5 mg/kg). Sixty-two percent of the afferents that responded to papaverine also responded to isoproterenol (50 microg/kg). Seven of the 36 group IV and 2 of the 12 group III afferents tested were excited by acute distension of the hindlimb venous system. Four of the seven group IV afferents responding to venous distension also responded to papaverine (57 vs. 13% for the nonresponding). Finally, we observed that most of the group IV afferents that were excited by dynamic contractions of the triceps surae muscles also responded either to venous distension or to vasodilatory agents. These results are consistent with the histological findings that a large number of group IV endings have their receptive fields close to the venules and suggest that they can be stimulated by the deformation of these vascular structures when peripheral conductance increases. Moreover, such a mechanism offers the possibility of encoding both the effects of muscle contraction through intramuscular pressure changes and the distension of the venular system, thereby monitoring the activity of the veno-muscular pump.