Cortical and brain stem changes in neural activity during static handgrip and postexercise ischemia in humans

Sweating response to passive stretch of the calf muscle during activation of forearm muscle metaboreceptors in heated humans

Activation of muscle metaboreceptors and mechanoreceptors has been shown to independently influence the sweating response, while their integrative control effects remain unclear. We examined the sweating response when the two muscle receptors are concurrently activated in different limbs, as well as the blood pressure response. In total, 27 young males performed passive calf muscle stretches (muscle mechanoreceptor activation) for 30 s in a semisupine position with and without postisometric handgrip exercise muscle ischemia (PEMI, muscle metaboreceptor activation) at exercise intensities of 35 and 50% of maximum voluntary contraction (MVC) under hot conditions (ambient temperature, 35°C, relative humidity, 50%). Passive calf muscle stretching alone increased the mean sweating rate significantly on the forehead, chest, and thigh (SRmean) and mean arterial blood pressure (MAP), but not the heart rate (HR), from prestretching levels by 0.04 ± 0.01 mg·cm(2)·min(-1), 4.0 ± 1.3 mmHg (P < 0.05), and -1.0 ± 0.5 beats/min (P > 0.05), respectively. The SRmean and MAP during PEMI were significantly higher than those at rest. The passive calf muscle stretch during PEMI increased MAP significantly by 3.4 ± 1.0 and 2.0 ± 0.7 mmHg for 35 and 50% of MVC, respectively (P < 0.05), but not that of SRmean or HR at either exercise intensity. These results suggest that sweating and blood pressure responses to concurrent activation of the two muscle receptors in different limbs differ and that the influence of calf muscle mechanoreceptor activation alone on the sweating response disappears during forearm muscle metaboreceptor activation.

Effect of Dyspnea Induced by Breath-holding on Maximal Muscular Strength of Patients with COPD

[Purpose] The purpose of the present study was to clarify the effect of dyspnea induced by breath-holding on maximum muscular strength of patients with COPD. [Subjects] This study recruited 14 COPD subjects via public posting. [Methods] Dyspnea was assessed by the modified Borg scale. The subject asked to stop breathing at end-expiration and to hold their breath with a nose clip for as long as possible. Both total breath-holding time and threshold time of dyspnea were measured with a chronograph. Dyspnea reserve time (DRT) was defined by subtracting the threshold time of perception dyspnea from total breath-holding time in order to calculate the 50% DRT. The muscular strengths of maximal handgrip contraction were measured at baseline, 50% threshold time of dyspnea (subliminal point of dyspnea), and the 50% DRT (supraliminal point of dyspnea). [Results] The maximal handgrip at the supraliminal point of dyspnea was significantly lower than the baseline and subliminal point of dyspnea values. There was no statistically significant difference in maximal muscular strength between baseline and the subliminal point of dyspnea value. [Conclusion] The present results demonstrate that dyspnea induced by breath-holding aggravates weakness in the maximum muscular strength of patients with COPD.

Apnea-induced cortical BOLD-fMRI and peripheral sympathoneural firing response patterns of awake healthy humans

End-expiratory breath-holds (BH) and Mueller manoeuvres (MM) elicit large increases in muscle sympathetic nerve activity (MSNA). In 16 healthy humans (9♀, 35±4 years) we used functional magnetic resonance imaging with blood oxygen level-dependent (BOLD) contrast to determine the cortical network associated with such sympathoexcitation. We hypothesized that increases in MSNA evoked by these simulated apneas are accompanied by BOLD contrast changes in the insular cortex, thalamus and limbic cortex. A series of 150 whole-brain images were collected during 3 randomly performed 16-second end-expiratory BHs and MMs (-30 mmHg). The identical protocol was repeated separately with MSNA recorded from the fibular nerve. The time course of the sympathoexcitatory response to both breathing tasks were correlated with whole-brain BOLD signal changes. Brain sites demonstrating both positive (activation) and negative (deactivation) correlations with the MSNA time course were identified. Sympathetic burst incidence increased (p<0.001) from 29±6 (rest) to 49±6 (BH) and 47±6 bursts/100 heartbeats (MM). Increased neural activity (Z-scores) was identified in the right posterior and anterior insular cortices (3.74, 3.64), dorsal anterior cingulate (3.42), fastigial and dentate cerebellar nuclei (3.02, 3.34). Signal intensity decreased in the left posterior insula (3.28) and ventral anterior cingulate (3.01). Apnea both activates and inhibits elements of a cortical network involved in the generation of sympathetic outflow. These findings identify a neuroanatomical substrate to guide future investigations into central mechanisms contributing to disorders characterized by elevated basal MSNA and exaggerated sympathetic responses to simulated apneas such as sleep apnea and heart failure.

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.

Functional Imaging of the Human Brainstem during Somatosensory Input and Autonomic Output

Over the past half a century, many investigations in experimental animal have explored the functional roles of specific regions in the brainstem. Despite the accumulation of a considerable body of knowledge in, primarily, anesthetized preparations, relatively few studies have explored brainstem function in awake humans. It is important that human brainstem function is explored given that many neurological conditions, from obstructive sleep apnea, chronic pain, and hypertension, likely involve significant changes in the processing of information within the brainstem. Recent advances in the collection and processing of magnetic resonance images have resulted in the possibility of exploring brainstem activity changes in awake healthy individuals and in those with various clinical conditions. We and others have begun to explore changes in brainstem activity in humans during a number of challenges, including cutaneous and muscle pain, as well as during maneuvers that evoke increases in sympathetic nerve activity. More recently we have successfully recorded sympathetic nerve activity concurrently with functional magnetic resonance imaging of the brainstem, which will allow us, for the first time to explore brainstem sites directly responsible for conditions such as hypertension. Since many pathophysiological conditions no doubt involve changes in brainstem function and structure, defining these changes will likely result in a greater ability to develop more effective treatment regimens.

The autonomic brain: an activation likelihood estimation meta-analysis for central processing of autonomic function

The autonomic nervous system (ANS) is of paramount importance for daily life. Its regulatory action on respiratory, cardiovascular, digestive, endocrine, and many other systems is controlled by a number of structures in the CNS. While the majority of these nuclei and cortices have been identified in animal models, neuroimaging studies have recently begun to shed light on central autonomic processing in humans. In this study, we used activation likelihood estimation to conduct a meta-analysis of human neuroimaging experiments evaluating central autonomic processing to localize (1) cortical and subcortical areas involved in autonomic processing, (2) potential subsystems for the sympathetic and parasympathetic divisions of the ANS, and (3) potential subsystems for specific ANS responses to different stimuli/tasks. Across all tasks, we identified a set of consistently activated brain regions, comprising left amygdala, right anterior and left posterior insula and midcingulate cortices that form the core of the central autonomic network. While sympathetic-associated regions predominate in executive- and salience-processing networks, parasympathetic regions predominate in the default mode network. Hence, central processing of autonomic function does not simply involve a monolithic network of brain regions, instead showing elements of task and division specificity.

Dynamic cerebral autoregulation changes during sub-maximal handgrip maneuver

Purpose

We investigated the effect of handgrip (HG) maneuver on time-varying estimates of dynamic cerebral autoregulation (CA) using the autoregressive moving average technique.

Methods

Twelve healthy subjects were recruited to perform HG maneuver during 3 minutes with 30% of maximum contraction force. Cerebral blood flow velocity, end-tidal CO₂ pressure (PETCO₂), and noninvasive arterial blood pressure (ABP) were continuously recorded during baseline, HG and recovery. Critical closing pressure (CrCP), resistance area-product (RAP), and time-varying autoregulation index (ARI) were obtained.

Results

PETCO₂ did not show significant changes during HG maneuver. Whilst ABP increased continuously during the maneuver, to 27% above its baseline value, CBFV raised to a plateau approximately 15% above baseline. This was sustained by a parallel increase in RAP, suggestive of myogenic vasoconstriction, and a reduction in CrCP that could be associated with metabolic vasodilation. The time-varying ARI index dropped at the beginning and end of the maneuver (p<0.005), which could be related to corresponding alert reactions or to different time constants of the myogenic, metabolic and/or neurogenic mechanisms.

Conclusion

Changes in dynamic CA during HG suggest a complex interplay of regulatory mechanisms during static exercise that should be considered when assessing the determinants of cerebral blood flow and metabolism.

Increases in muscle sympathetic nerve activity, heart rate, respiration, and skin blood flow during passive viewing of exercise

The cardiovascular and respiratory effects of exercise have been widely studied, as have the autonomic effects of imagined and observed exercise. However, the effects of observed exercise in the first person have not been documented, nor have direct recordings of muscle sympathetic nerve activity (MSNA) been obtained during observed or imagined exercise. The aim of the current study was to measure blood pressure, heart rate, respiration, skin blood flow, sweat release, and MSNA (via microelectrodes inserted into the common peroneal nerve), during observation of exercise from the first person point of view. It was hypothesized that the moving stimuli would produce robust compensatory increases in the above-mentioned parameters as effectively as those generated by mental imagery and—to a lesser extent—actual exercise. Nine subjects watched a first-person running video, allowing them to view the action from the perspective of the runner rather than viewing someone else perform the exercise. On average, statistically significant increases from baseline during the running phase were seen in heart rate, respiratory rate, skin blood flow, and burst amplitude of MSNA. These results suggest that observation of exercise in the first person is a strong enough stimulus to evoke “physiologically appropriate” autonomic responses that have a purely psychogenic origin.

Strength in cognitive self-regulation

Failures in self-regulation are predictive of adverse cognitive, academic and vocational outcomes, yet the interplay between cognition and self-regulation failure remains elusive. Two experiments tested the hypothesis that lapses in self-regulation, as predicted by the strength model, can be induced in individuals using cognitive paradigms and whether such failures are related to cognitive performance. In Experiments 1, the stop-signal task (SST) was used to show reduced behavioral inhibition after performance of a cognitively demanding arithmetic task, but only in people with low arithmetic accuracy, when compared with SST performance following a simple discrimination task. Surprisingly, and inconsistently with existing models, subjects rapidly recovered without rest or glucose. In Experiment 2, depletions of both go-signal reaction times and response inhibition were observed when a simple detection task was used as a control. These experiments provide new evidence that cognitive self-regulation processes are influenced by cognitive performance, and subject to improvement and recovery without rest.

Real-time imaging of cortical and subcortical control of muscle sympathetic nerve activity in awake human subjects

Blood pressure is controlled on a beat-to-beat basis through fluctuations in heart rate and the degree of sympathetically-mediated vasoconstriction in skeletal muscles. By recording muscle sympathetic nerve activity (MSNA) at the same time as performing functional magnetic resonance imaging (fMRI) of the brain, we aimed to identify cortical structures involved in central cardiovascular control in awake human subjects. Spontaneous bursts of MSNA were recorded via a tungsten microelectrode inserted percutaneously into the peroneal nerve of 14 healthy subjects in a 3T MRI scanner. Blood Oxygen Level Dependent (BOLD) contrast - gradient echo, echo-planar - images were continuously collected in a 4s ON, 4s OFF sampling protocol. MSNA burst amplitudes were measured during the OFF periods and BOLD signal intensity was measured during the subsequent 4s period to allow for neurovascular coupling and nerve conduction delays. Group analysis demonstrated regions showing fluctuations in BOLD signal intensity that covaried with the intensity of the concurrently recorded bursts of MSNA. Signal intensity and MSNA were positively correlated in the left mid-insula, bilateral dorsolateral prefrontal cortex, bilateral posterior cingulate cortex and bilateral precuneus. In addition, MSNA covaried with signal intensity in the left dorsomedial hypothalamus and bilateral ventromedial hypothalamus (VMH). Construction of a functional connectivity map revealed coupling between activity in VMH and the insula, dorsolateral prefrontal cortex, precuneus, and in the region of the left and right rostroventrolateral medulla (RVLM). This suggests that activity within suprabulbar regions may regulate resting MSNA by projections to the premotor sympathetic neurons in the rostroventrolateral medulla.

The chronic protective effects of limb remote preconditioning and the underlying mechanisms involved in inflammatory factors in rat stroke

We recently demonstrated that limb remote preconditioning (LRP) protects against focal ischemia measured 2 days post-stroke. Here, we studied whether LRP provides long-term protection and improves neurological function. We also investigated whether LRP transmits its protective signaling via the afferent nerve pathways from the preconditioned limb to the ischemic brain and whether inflammatory factors are involved in LRP, including the novel galectin-9/Tim-3 inflammatory cell signaling pathway, which induces cell death in lymphocytes. LRP in the left hind femoral artery was performed immediately before stroke. LRP reduced brain injury size both at 2 days and 60 days post-stroke and improved behavioral outcomes for up to 2 months. The sensory nerve inhibitors capsaicin and hexamethonium, a ganglion blocker, abolished the protective effects of LRP. In addition, LRP inhibited edema formation and blood-brain barrier (BBB) permeability measured 2 days post-stroke. Western blot and immunostaining analysis showed that LRP inhibited protein expression of both galectin-9 and T-cell immunoglobulin domain and mucin domain 3 (Tim-3), which were increased after stroke. In addition, LRP decreased iNOS and nitrotyrosine protein expression after stroke. In conclusion, LRP executes long-term protective effects against stroke and may block brain injury by inhibiting activities of the galectin-9/Tim-3 pathway, iNOS, and nitrotyrosine.

Acupuncture--deep pain with an autonomic dimension?

Stimulation of acupuncture point Pc6, located above the median nerve, has been shown to be effective in treating nausea and vomiting. It has also frequently been reported to cause a heart rate reduction. The mechanism behind this autonomic reaction has not been clarified, so far. We combined brainstem-sensitive functional magnetic resonance imaging with heart rate recording and time-resolved rating of the needling sensation to measure neuronal correlates of sensations and autonomic reactions during acupuncture. On the cortical level, needling sensation activated typical pain-related areas, of which the ventromedial and dorsolateral prefrontal cortex and perigenual anterior cingulate cortex were further involved in mediating the heart rate response. In the brainstem, needling sensation activated nuclei of the descending pain control system, in which a network of hypothalamus, periaqueductal gray, rostral ventromedial medulla, and ventrolateral medulla was identified as the source of the heart rate changes. Our findings indicate that acupuncture may be a special pain stimulus, whose autonomic concomitants could explain its non-analgesic effects and in some cases even have a therapeutic potential.

Sympathetic influence on cerebral blood flow and metabolism during exercise in humans

This review focuses on the possibility that autonomic activity influences cerebral blood flow (CBF) and metabolism during exercise in humans. Apart from cerebral autoregulation, the arterial carbon dioxide tension, and neuronal activation, it may be that the autonomic nervous system influences CBF as evidenced by pharmacological manipulation of adrenergic and cholinergic receptors. Cholinergic blockade by glycopyrrolate blocks the exercise-induced increase in the transcranial Doppler determined mean flow velocity (MCA Vmean). Conversely, alpha-adrenergic activation increases that expression of cerebral perfusion and reduces the near-infrared determined cerebral oxygenation at rest, but not during exercise associated with an increased cerebral metabolic rate for oxygen (CMRO(2)), suggesting competition between CMRO(2) and sympathetic control of CBF. CMRO(2) does not change during even intense handgrip, but increases during cycling exercise. The increase in CMRO(2) is unaffected by beta-adrenergic blockade even though CBF is reduced suggesting that cerebral oxygenation becomes critical and a limited cerebral mitochondrial oxygen tension may induce fatigue. Also, sympathetic activity may drive cerebral non-oxidative carbohydrate uptake during exercise. Adrenaline appears to accelerate cerebral glycolysis through a beta2-adrenergic receptor mechanism since noradrenaline is without such an effect. In addition, the exercise-induced cerebral non-oxidative carbohydrate uptake is blocked by combined beta 1/2-adrenergic blockade, but not by beta1-adrenergic blockade. Furthermore, endurance training appears to lower the cerebral non-oxidative carbohydrate uptake and preserve cerebral oxygenation during submaximal exercise. This is possibly related to an attenuated catecholamine response. Finally, exercise promotes brain health as evidenced by increased release of brain-derived neurotrophic factor (BDNF) from the brain.

Differential responses to sympathetic stimulation in the cerebral and brachial circulations during rhythmic handgrip exercise in humans

The sympathetic neural regulation of the cerebral circulation remains controversial. The purpose of the present study was to determine how exercise modulates the simultaneous responsiveness of the cerebral and brachial circulations to 'endogenous' sympathetic activation (cold pressor test). In nine healthy subjects, heart rate (HR) and mean arterial blood pressure (MAP) were continuously measured during cold pressor tests (4°C water) conducted at rest and during randomized bouts of rhythmic handgrip of 10, 25 and 40% of maximal voluntary contraction. Doppler ultrasound was used to examine brachial artery blood flow (FBF) and middle cerebral artery (MCA) mean blood velocity (V mean), and indices of vascular conductance were calculated for the brachial artery (forearm vascular conductance, FVC) and MCA (cerebral vascular conductance index, CVCi). End-tidal PCO2 (P ET.CO2) was evaluated on a breath-by-breath basis. Handgrip evoked increases in HR, FBF, FVC and MCA V mean (P < 0.05 versus rest), while MAP and CVCi were unchanged and P ET.CO2 fell slightly (P < 0.05 versus rest). Increases in MAP and HR during the cold pressor test were similar at rest and during all handgrip trials. Forearm vascular conductance was markedly reduced with the cold pressor test at rest (-45 ± 8%), but this vasoconstrictor effect was progressively attenuated with increasing exercise intensity (FVC -17 ± 3% during exercise at 40% of maximal voluntary contraction; P < 0.05). In contrast, the small reduction in CVCi with cold pressor test was similar at rest and during handgrip (approximately -5%). Our data indicate that while the marked vasoconstrictor responses to sympathetic activation in the skeletal muscle vasculature are blunted by handgrip exercise, the modest cerebrovascular responses to a cold pressor test remain unchanged.