Forebrain organization representing baroreceptor gating of somatosensory afferents within the cortical autonomic network

Vertebral Subluxation and Systems Biology: An Integrative Review Exploring the Salutogenic Influence of Chiropractic Care on the Neuroendocrine-Immune System

In this paper we synthesize an expansive body of literature examining the multifaceted influence of chiropractic care on processes within and modulators of the neuroendocrine-immune (NEI) system, for the purpose of generating an inductive hypothesis regarding the potential impacts of chiropractic care on integrated physiology. Taking a broad, interdisciplinary, and integrative view of two decades of research-documented outcomes of chiropractic care, inclusive of reports ranging from systematic and meta-analysis and randomized and observational trials to case and cohort studies, this review encapsulates a rigorous analysis of research and suggests the appropriateness of a more integrative perspective on the impact of chiropractic care on systemic physiology. A novel perspective on the salutogenic, health-promoting effects of chiropractic adjustment is presented, focused on the improvement of physical indicators of well-being and adaptability such as blood pressure, heart rate variability, and sleep, potential benefits that may be facilitated through multiple neurologically mediated pathways. Our findings support the biological plausibility of complex benefits from chiropractic intervention that is not limited to simple neuromusculoskeletal outcomes and open new avenues for future research, specifically the exploration and mapping of the precise neural pathways and networks influenced by chiropractic adjustment.

Rhythmic firing of neurons in the medulla of conscious freely behaving rats: rhythmic coupling with baroreceptor input

Recent investigations emphasized the importance of neural control of cardiovascular adjustments in complex behaviors, including stress, exercise, arousal, sleep-wake states, and different tasks. Baroreceptor feedback is an essential component of this system acting on different time scales from maintaining stable levels of cardiovascular parameters on the long-term to rapid alterations according to behavior. The baroreceptor input is essentially rhythmic, reflecting periodic fluctuations in arterial blood pressure. Cardiac rhythm is a prominent feature of the autonomic control system, present on different levels, including neuron activity in central circuits. The mechanism of rhythmic entrainment of neuron firing by the baroreceptor input was studied in great detail under anesthesia, but recordings of sympathetic-related neuron firing in freely moving animals remain extremely scarce. In this study, we recorded multiple single neuron activity in the reticular formation of the medulla in freely moving rats during natural behavior. Neurons firing in synchrony with the cardiac rhythm were detected in each experiment (n = 4). In agreement with prior observations in anesthetized cats, we found that neurons in this area exhibited high neuron-to-neuron variability and temporal flexibility in their coupling to cardiac rhythm in freely moving rats, as well. This included firing in bursts at multiples of cardiac cycles, but not directly coupled to the heartbeat, supporting the concept of baroreceptor input entraining intrinsic neural oscillations rather than imposing a rhythm of solely external origin on these networks. It may also point to a mechanism of maintaining the basic characteristics of sympathetic neuron activity, i.e., burst discharge and cardiac-related rhythmicity, on the background of behavior-related adjustments in their firing rate.

Brain structural and functional correlates to defense-related inhibition of muscle sympathetic nerve activity in man

An individual’s blood pressure (BP) reactivity to stress is linked to increased risk of hypertension and cardiovascular disease. However, inter- and intra-individual BP variability makes understanding the coupling between stress, BP reactivity, and long-term outcomes challenging. Previous microneurographic studies of sympathetic signaling to muscle vasculature (i.e. muscle sympathetic nerve activity, MSNA) have established a neural predictor for an individual’s BP reactivity during short-lasting stress. Unfortunately, this method is invasive, technically demanding, and time-consuming and thus not optimal for widespread use. Potential central nervous system correlates have not been investigated. We used MagnetoEncephaloGraphy and Magnetic Resonance Imaging to search for neural correlates to sympathetic response profiles within the central autonomic network and sensorimotor (Rolandic) regions in 20 healthy young males. The main correlates include (a) Rolandic beta rebound and an anterior cingulate cortex (ACC) response elicited by sudden stimulation and (b) cortical thickness in the ACC. Our findings highlight the involvement of the ACC in reactions to stress entailing peripheral sympathetic responses to environmental stimuli. The Rolandic response furthermore indicates a surprisingly strong link between somatosensory and autonomic processes. Our results thus demonstrate the potential in using non-invasive neuroimaging-based measures of stress-related MSNA reactions, previously assessed only using invasive microneurography.

The Relationship between Autism and Ehlers-Danlos Syndromes/Hypermobility Spectrum Disorders

Considerable interest has arisen concerning the relationship between hereditary connective tissue disorders such as the Ehlers-Danlos syndromes (EDS)/hypermobility spectrum disorders (HSD) and autism, both in terms of their comorbidity as well as co-occurrence within the same families. This paper reviews our current state of knowledge, as well as highlighting unanswered questions concerning this remarkable patient group, which we hope will attract further scientific interest in coming years. In particular, patients themselves are demanding more research into this growing area of interest, although science has been slow to answer that call. Here, we address the overlap between these two spectrum conditions, including neurobehavioral, psychiatric, and neurological commonalities, shared peripheral neuropathies and neuropathologies, and similar autonomic and immune dysregulation. Together, these data highlight the potential relatedness of these two conditions and suggest that EDS/HSD may represent a subtype of autism.

Cutaneous warmth, but not touch, increases muscle sympathetic nerve activity during a muscle fatigue hand-grip task

In homeostasis, somatosensory C fibre afferents are hypothesised to mediate input to the brain about interactions with external stimuli and sympathetic efference provides the output that regulates bodily functions. We aimed to test this hypothesis and whether different types of innocuous somatosensory input have differential effects. Healthy volunteers performed a muscle fatigue (hand-grip) task to exhaustion, which produces increased muscle sympathetic nerve activity (MSNA), as measured through microneurography. Participants completed the muscle fatigue task without concurrent cutaneous sensory stimulation (control) or we applied skin warming (heat pack) as a C fibre stimulation, slow brush stroking as C and Aβ fibre stimulation, or vibration as Aβ fibre stimulation, to the participant’s forearm. We also measured heart rate, the duration of the hand-grip task, and ratings of pain at the end of the task. Concurrent skin warming showed increased MSNA compared to the other conditions. Tactile stimuli (brushing, vibration) were not significantly different to the control (no intervention) condition. Warming increased the pain from the muscle contraction, whereas the tactile stimuli did not. We interpret the effect of warming on MSNA as providing relevant afferent information during muscle contraction, which needed to be counteracted via vasoconstriction to maintain homeostasis. Brushing and vibration were less homeostatically relevant stimuli for the muscle contraction and hence had no significant effect. The findings add sensory specificity to our current understanding of homeostatic regulation through somatosensory afferent and sympathetic efferent pathways.

Evidence for differential control of muscle sympathetic single units during mild sympathoexcitation in young, healthy humans

Two subpopulations of muscle sympathetic single units with opposite discharge characteristics have been identified during low-level cardiopulmonary baroreflex loading and unloading in middle-aged adults and patients with heart failure. The present study sought to determine whether similar subpopulations are present in young healthy adults during cardiopulmonary baroreflex unloading ( study 1) and rhythmic handgrip exercise ( study 2). Continuous hemodynamic and multiunit and single unit muscle sympathetic nerve activity (MSNA) data were collected at baseline and during nonhypotensive lower body negative pressure (LBNP; n = 12) and 40% maximal voluntary contraction rhythmic handgrip exercise (RHG; n = 24). Single unit MSNA responses were classified as anticipated or paradoxical based on whether changes were concordant or discordant with the multiunit MSNA response, respectively. LBNP and RHG both increased multiunit MSNA burst frequency (∆5 ± 3 bursts/min, P < 0.001; ∆5 ± 8 bursts/min, P = 0.005), burst amplitude (∆5 ± 7%, P = 0.04; ∆13 ± 14%, P < 0.001), and total MSNA (∆302 ± 191 AU/min, P = 0.001; ∆585 ± 556 AU/min, P < 0.001). During LBNP and RHG, 43 and 64 muscle single units were identified, respectively, which increased spike frequency (∆9 ± 11 spikes/min, P < 0.001; ∆10 ± 19 spikes/min, P < 0.001) and the probability of multiple spike firing (∆10 ± 12%, P < 0.001; ∆11 ± 26%, P = 0.001). During LBNP and RHG, 36 (84%) and 39 (61%) single units possessed anticipated firing responses (∆12 ± 10 spikes/min, P < 0.001; ∆19 ± 19 spikes/min, P < 0.001), whereas 7 (16%) and 25 (39%) single units exhibited paradoxical reductions (∆−3 ± 1 spikes/min, P = 0.003; ∆−4 ± 5 spikes/min, P < 0.001). The observation of divergent subpopulations of muscle sympathetic single units in healthy young humans during two mild sympathoexcitatory stressors supports differential control at the fiber level as a fundamental characteristic of human sympathetic regulation.

NEW & NOTEWORTHY The activity of muscle sympathetic single units was recorded during cardiopulmonary baroreceptor unloading and rhythmic handgrip exercise in young healthy humans. During both stressors, the majority of single units (84% and 61%) exhibited anticipated behavior concordant with the integrated muscle sympathetic response, whereas a smaller proportion (16% and 39%) exhibited paradoxical sympathoinhibition. These results support differential control of postganglionic muscle sympathetic fibers as a characteristic of human sympathetic regulation during mild sympathoexcitatory stress.

Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/differential-control-of-sympathetic-outflow-in-young-humans/ .

Midcingulate somatomotor and autonomic functions

The midcingulate cortex (MCC) is viewed as a central node within a large-scale system devoted to adjusting behavior in the face of changing environments. Whereas the role of the MCC in interfacing action and cognition is well established, its role in regulating the autonomic nervous system is poorly understood. Yet, adaptive reactions to novel or threatening situations induce coordinated changes in the sympathetic and the parasympathetic systems. The somatomotor maps in the MCC are organized dorsoventrally. A meta-analysis of the literature reveals that the dorsoventral organization might also concern connections with the autonomic nervous system. Activation of the dorsal and ventral parts of the MCC correlate with recruitments of the sympathetic and the parasympathetic systems, respectively. Data also suggest that, in the MCC, projections toward the sympathetic system are mapped along the sensory-motor system following the same cervico-sacral organization as projections on the spinal cord for skeletal motor control.

Cerebellar impairment during an orthostatic challenge in patients with neurogenic orthostatic hypotension

OBJECTIVE

Compare activation patterns within the cortical autonomic network in patients with neurogenic orthostatic hypotension (NOH) versus healthy age-matched controls during an orthostatic challenge.

METHODS

Fifteen health controls and 15 NOH patients performed 3 Valsalva maneuvers, and 5-min of lower-body negative pressure (LBNP) during a functional brain MRI.

RESULTS

Compared to controls, NOH patients had significantly less activation within the cerebellum during both LBNP and VM. Both groups had significant activation of the bilateral insula and left thalamus during LBNP. No significant differences were found during the recovery phase of LBNP.

CONCLUSIONS

The cerebellum, which plays an important role in vestibulo-sympathetic reflexes, important for blood pressure adjustments during postural changes, appear to be affected in patients with NOH. The cerebellum also appears to be affected during other baroreflex mediated stressors such as the VM.

SIGNIFICANCE

Orthostatic reflexes mediated by the cerebellum may be impaired in patients with NOH. The results suggest an additional pathological pathway in patients with autonomic failure.

The human cortical autonomic network and volitional exercise in health and disease

The autonomic nervous system elicits continuous beat-by-beat homeostatic adjustments to cardiovascular control. These modifications are mediated by sensory inputs (e.g., baroreceptors, metaboreceptors, pulmonary, thermoreceptors, and chemoreceptors afferents), integration at the brainstem control centres (i.e., medulla), and efferent autonomic neural outputs (e.g., spinal, preganglionic, and postganglionic pathways). However, extensive electrical stimulation and functional imaging research show that the brain's higher cortical regions (e.g., insular cortex, medial prefrontal cortex, anterior cingulate cortex) partake in homeostatic regulation of the cardiovascular system at rest and during exercise. We now appreciate that these cortical areas form a network, namely the "cortical autonomic network" (CAN), which operate as part of a larger central autonomic network comprising 2-way communication of cortical and subcortical areas to exert autonomic influence. Interestingly, differential patterns of CAN activity and ensuing cardiovascular control are present in disease states, thereby highlighting the importance of considering the role of CAN as an integral aspect of cardiovascular regulation in health and disease. This review discusses current knowledge on human cortical autonomic activation during volitional exercise, and the role of exercise training on this activation in both health and disease.

Impaired cortical autonomic responses during sympathetic activation in neurogenic orthostatic hypotension characterized by postganglionic autonomic dysfunction

Neurogenic orthostatic hypotension (NOH) is a cardinal feature of autonomic dysfunction. The cortical autonomic network (CAN) is a network of brain regions associated with autonomic function. Therefore, our objective was to investigate whether impairment of CAN structures is involved in the pathophysiology of NOH. Fifteen controls (63 ± 13 yr) and 15 NOH patients (67 ± 6 yr; P = 0.2) with peripheral autonomic dysfunction completed standard tests of parasympathetic [deep breathing (DB)] and sympathetic [Valsalva maneuver (VM)] activation during a functional MRI. Blood-oxygen-level dependent (BOLD) contrasts were obtained and contrasted. Compared with controls, patients had significantly smaller heart rate responses to DB (control: 15.23 ± 9.6 vs. NOH: 5.7 ± 2.1) and Valsalva ratios (control: 2.1 ± 0.47 vs. NOH: 1.2 ± 0.1; P < 0.001). NOH patients had absent adrenergic phases (late phase II and phase IV) during VM as per a qualitative analysis. During VM, controls had greater activation in the right hippocampus (T-value: 8.03), left posterior cingulate (T_L: 7.6), and bilateral thalamus (T_R: 7.41, T_L: 8.45; P < 0.05). During phase IV, controls had greater activation in the right hippocampus (T_R: 5.78l P < 0.05). Following subtraction analysis, no significant differences were evident during DB. In conclusion, NOH patients have significantly less CAN activation during sympathetic, but not parasympathetic, activation. Impaired CANs associated with sympathetic activation may be involved in the pathophysiology of NOH. NEW & NOTEWORTHY Neurogenic orthostatic hypotension (NOH) is a cardinal feature of autonomic dysfunction characterized by failure of reflexive sympathetic activation. Our result reveal that patients with autonomic dysfunction caused by postganglionic sympathetic impairment also have impaired activation of structures within the cortical autonomic network. Impaired activation is evident during a test of sympathetic, but not parasympathetic, activation. Impaired cortical autonomic networks associated with sympathetic activation may be involved in the pathophysiology of NOH.

Fifty years of microneurography: learning the language of the peripheral sympathetic nervous system in humans

As a primary component of homeostasis, the sympathetic nervous system enables rapid adjustments to stress through its ability to communicate messages among organs and cause targeted and graded end organ responses. Key in this communication model is the pattern of neural signals emanating from the central to peripheral components of the sympathetic nervous system. But what is the communication strategy employed in peripheral sympathetic nerve activity (SNA)? Can we develop and interpret the system of coding in SNA that improves our understanding of the neural control of the circulation? In 1968, Hagbarth and Vallbo (Hagbarth KE, Vallbo AB. Acta Physiol Scand 74: 96-108, 1968) reported the first use of microneurographic methods to record sympathetic discharges in peripheral nerves of conscious humans, allowing quantification of SNA at rest and sympathetic responsiveness to physiological stressors in health and disease. This technique also has enabled a growing investigation into the coding patterns within, and cardiovascular outcomes associated with, postganglionic SNA. This review outlines how results obtained by microneurographic means have improved our understanding of SNA outflow patterns at the action potential level, focusing on SNA directed toward skeletal muscle in conscious humans.

High cardiorespiratory fitness in early to late middle age preserves the cortical circuitry associated with brain-heart integration during volitional exercise

This study tested the hypothesis that high cardiorespiratory fitness (peak oxygen uptake) preserves the cortical circuitry associated with cardiac arousal during exercise in middle- to older-aged individuals. Observations of changes in heart rate (HR) and in cortical blood oxygenation level-dependent (BOLD) images were made in 52 healthy, active individuals (45-73 yr; 16 women, 36 men) across a range of fitness (26-66 ml·kg^-1·min^-1). Seven repeated bouts of isometric handgrip (IHG) at 40% maximal voluntary contraction force were performed with functional magnetic resonance imaging at 3 T, with each contraction lasting 20 s and separated by 40 s of rest. HR responses to IHG showed high variability across individuals. Linear regression revealed that cardiorespiratory fitness was not a strong predictor of the HR response (r² = 0.09). In a region-of-interest analysis both the IHG task and the HR time course correlated with increased cortical activation in the bilateral insula and decreased activation relative to baseline in the anterior and posterior cingulate and medial prefrontal cortex (MPFC). t-Test results revealed greater deactivation at the MPFC with higher fitness levels beyond that of guideline-based activity. Therefore, whereas high cardiorespiratory fitness failed to affect absolute HR responses to IHG in this age range, a select effect was observed in cortical regions known to be associated with cardiovascular arousal.NEW & NOTEWORTHY Our first observation suggests that fitness does not strongly predict the heart rate (HR) response to a volitional handgrip task in middle- to older-aged adults. Second, the BOLD response associated with the handgrip task, and with the HR time course, was associated with response patterns in the cortical autonomic network. Finally, whereas high cardiorespiratory fitness failed to affect absolute HR responses to isometric handgrip in this age range, a select effect was observed in cortical regions known to be associated with cardiovascular arousal, beyond that achieved through healthy active living.

Cutaneous Mechanoreceptor Feedback from the Hand and Foot Can Modulate Muscle Sympathetic Nerve Activity

Stimulation of high threshold mechanical nociceptors on the skin can modulate efferent sympathetic outflow. Whether low threshold mechanoreceptors from glabrous skin are similarly capable of modulating autonomic outflow is unclear. Therefore, the purpose of this study was to examine the effects of cutaneous afferent feedback from the hand palm and foot sole on efferent muscle sympathetic nerve activity (MSNA). Fifteen healthy young participants (9 male; 25 ± 3 years [range: 22-29]) underwent microneurographic recording of multi-unit MSNA from the right fibular nerve during 2 min of baseline and 2 min of mechanical vibration (150 Hz, 220 μm peak-to-peak) applied to the left hand or foot. Each participant completed three trials of both hand and foot stimulation, each separated by 5 min. MSNA burst frequency decreased similarly during the 2 min of both hand (20.8 ± 8.9 vs. 19.3 ± 8.6 bursts/minute [Δ -8%], p = 0.035) and foot (21.0 ± 8.3 vs. 19.5 ± 8.3 bursts/minute [Δ -8%], p = 0.048) vibration but did not alter normalized mean burst amplitude or area (All p > 0.05). Larger reductions in burst frequency were observed during the first 10 s (onset) of both hand (20.8 ± 8.9 vs. 17.0 ± 10.4 [Δ -25%], p < 0.001) and foot (21.0 ± 8.3 vs. 18.3 ± 9.4 [Δ -16%], p = 0.035) vibration, in parallel with decreases in normalized mean burst amplitude (hand: 0.45 ± 0.06 vs. 0.36 ± 0.14% [Δ -19%], p = 0.03; foot: 0.47 ± 0.07 vs. 0.34 ± 0.19% [Δ -27%], p = 0.02) and normalized mean burst area (hand: 0.42 ± 0.05 vs. 0.32 ± 0.12% [Δ -25%], p = 0.003; foot: 0.47 ± 0.05 vs. 0.34 ± 0.16% [Δ -28%], p = 0.01). These results demonstrate that tactile feedback from the hands and feet can influence efferent sympathetic outflow to skeletal muscle.

Prelimbic cortex GABAA receptors are involved in the mediation of restraint stress-evoked cardiovascular responses

Stress is a response of the organism to homeostasis-threatening stimuli and is coordinated by two main neural systems: the hypothalamic-pituitary-adrenal and the autonomic nervous system. Acute restraint stress (RS) is a model of unavoidable stress, which is characterized by autonomic responses including an increase in mean arterial pressure (MAP) and heart rate (HR), as well as a drop in tail temperature. The prelimbic cortex (PL) has been implicated in the modulation of functional responses caused by RS. The present study aimed to evaluate the role of PL GABAergic neurotransmission in the modulation of autonomic changes induced by RS. Bilateral microinjection of the GABA_A receptor antagonist bicuculline methiodide into the PL reduced pressor and tachycardic responses evoked by RS, in a dose-dependent manner, without affecting the tail temperature drop evoked by RS. In order to investigate which peripheral autonomic effector modulated the reduction in RS-cardiovascular responses caused by the blockade of PL GABA_A receptors, rats were intravenously pretreated with either atenolol or homatropine methylbromide. The blockade of the cardiac sympathetic nervous system with atenolol blunted the reducing effect of PL treatment with bicuculline methiodide on RS-evoked pressor and tachycardic responses. The blockade of the parasympathetic nervous system with homatropine methylbromide, regardless of affecting the beginning of the tachycardic response, did not impact on the reduction of RS-evoked tachycardic and pressor responses caused by the PL treatment with bicuculline methiodide. The present results indicate that both cardiac sympathetic and parasympathetic activities are involved in the reduction of RS-evoked cardiovascular responses evidenced after the blockade of PL GABA_A receptors by bicuculline methiodide.

Motor, cognitive, and affective areas of the cerebral cortex influence the adrenal medulla

Modern medicine has generally viewed the concept of "psychosomatic" disease with suspicion. This view arose partly because no neural networks were known for the mind, conceptually associated with the cerebral cortex, to influence autonomic and endocrine systems that control internal organs. Here, we used transneuronal transport of rabies virus to identify the areas of the primate cerebral cortex that communicate through multisynaptic connections with a major sympathetic effector, the adrenal medulla. We demonstrate that two broad networks in the cerebral cortex have access to the adrenal medulla. The larger network includes all of the cortical motor areas in the frontal lobe and portions of somatosensory cortex. A major component of this network originates from the supplementary motor area and the cingulate motor areas on the medial wall of the hemisphere. These cortical areas are involved in all aspects of skeletomotor control from response selection to motor preparation and movement execution. The second, smaller network originates in regions of medial prefrontal cortex, including a major contribution from pregenual and subgenual regions of anterior cingulate cortex. These cortical areas are involved in higher-order aspects of cognition and affect. These results indicate that specific multisynaptic circuits exist to link movement, cognition, and affect to the function of the adrenal medulla. This circuitry may mediate the effects of internal states like chronic stress and depression on organ function and, thus, provide a concrete neural substrate for some psychosomatic illness.

Functional Imaging of Autonomic Regulation: Methods and Key Findings

Central nervous system processing of autonomic function involves a network of regions throughout the brain which can be visualized and measured with neuroimaging techniques, notably functional magnetic resonance imaging (fMRI). The development of fMRI procedures has both confirmed and extended earlier findings from animal models, and human stroke and lesion studies. Assessments with fMRI can elucidate interactions between different central sites in regulating normal autonomic patterning, and demonstrate how disturbed systems can interact to produce aberrant regulation during autonomic challenges. Understanding autonomic dysfunction in various illnesses reveals mechanisms that potentially lead to interventions in the impairments. The objectives here are to: (1) describe the fMRI neuroimaging methodology for assessment of autonomic neural control, (2) outline the widespread, lateralized distribution of function in autonomic sites in the normal brain which includes structures from the neocortex through the medulla and cerebellum, (3) illustrate the importance of the time course of neural changes when coordinating responses, and how those patterns are impacted in conditions of sleep-disordered breathing, and (4) highlight opportunities for future research studies with emerging methodologies. Methodological considerations specific to autonomic testing include timing of challenges relative to the underlying fMRI signal, spatial resolution sufficient to identify autonomic brainstem nuclei, blood pressure, and blood oxygenation influences on the fMRI signal, and the sustained timing, often measured in minutes of challenge periods and recovery. Key findings include the lateralized nature of autonomic organization, which is reminiscent of asymmetric motor, sensory, and language pathways. Testing brain function during autonomic challenges demonstrate closely-integrated timing of responses in connected brain areas during autonomic challenges, and the involvement with brain regions mediating postural and motoric actions, including respiration, and cardiac output. The study of pathological processes associated with autonomic disruption shows susceptibilities of different brain structures to altered timing of neural function, notably in sleep disordered breathing, such as obstructive sleep apnea and congenital central hypoventilation syndrome. The cerebellum, in particular, serves coordination roles for vestibular stimuli and blood pressure changes, and shows both injury and substantially altered timing of responses to pressor challenges in sleep-disordered breathing conditions. The insights into central autonomic processing provided by neuroimaging have assisted understanding of such regulation, and may lead to new treatment options for conditions with disrupted autonomic function.

Forebrain neurocircuitry associated with human reflex cardiovascular control

Physiological homeostasis depends upon adequate integration and responsiveness of sensory information with the autonomic nervous system to affect rapid and effective adjustments in end organ control. Dysregulation of the autonomic nervous system leads to cardiovascular disability with consequences as severe as sudden death. The neural pathways involved in reflexive autonomic control are dependent upon brainstem nuclei but these receive modulatory inputs from higher centers in the midbrain and cortex. Neuroimaging technologies have allowed closer study of the cortical circuitry related to autonomic cardiovascular adjustments to many stressors in awake humans and have exposed many forebrain sites that associate strongly with cardiovascular arousal during stress including the medial prefrontal cortex, insula cortex, anterior cingulate, amygdala and hippocampus. Using a comparative approach, this review will consider the cortical autonomic circuitry in rodents and primates with a major emphasis on more recent neuroimaging studies in awake humans. A challenge with neuroimaging studies is their interpretation in view of multiple sensory, perceptual, emotive and/or reflexive components of autonomic responses. This review will focus on those responses related to non-volitional baroreflex control of blood pressure and also on the coordinated responses to non-fatiguing, non-painful volitional exercise with particular emphasis on the medial prefrontal cortex and the insula cortex.

Gray matter blood flow and volume are reduced in association with white matter hyperintensity lesion burden: a cross-sectional MRI study

Cerebral White Matter Hyperintensities (WMH) are associated with vascular risk factors and age-related cognitive decline. WMH have primarily been associated with global white matter and gray matter (GM) changes and less is known about regional effects in GM. The purpose of this study was to test for an association between WMH and two GM imaging measures: cerebral blood flow (CBF) and voxel-based morphometry (VBM). Twenty-six elderly adults with mild to severe WMH participated in this cross-sectional 3 Tesla magnetic resonance imaging (MRI) study. MRI measures of GM CBF and VBM were derived from arterial spin labeling (ASL) and T1-weighted images, respectively. Fluid-attenuated inversion recovery (FLAIR) images were used to quantify the WMH lesion burden (mL). GM CBF and VBM data were used as dependent variables. WMH lesion burden, age and sex were used in a regression model. Visual rating of WMH with the Fazekas method was used to compare the WMH lesion volume regression approach. WMH volume was normally distributed for this group (mean volume of 22.7 mL, range: 2.2–70.6 mL). CBF analysis revealed negative associations between WMH volume and CBF in the left anterior putamen, subcallosal, accumbens, anterior caudate, orbital frontal, anterior insula, and frontal pole (corrected p < 0.05). VBM analysis revealed negative associations between WMH and GM volume in lingual gyrus, intracalcarine, and bilateral hippocampus (corrected p < 0.05). The visual rating scale corroborated the regression findings (corrected p < 0.05). WMH lesion volume was associated with intra-group GM CBF and structural differences in this cohort of WMH adults with mild to severe lesion burden.

Coronary artery disease affects cortical circuitry associated with brain-heart integration during volitional exercise

This study tested the hypothesis that coronary artery disease (CAD) alters the cortical circuitry associated with exercise. Observations of changes in heart rate (HR) and in cortical blood oxygenation level-dependent (BOLD) images were made in 23 control subjects [control; 8 women; 63 ± 11 yr; mean arterial pressure (MAP): 90 ± 9 mmHg] (mean ± SD) and 17 similarly aged CAD patients (4 women; 59 ± 9 yr; MAP: 87 ± 10 mmHg). Four repeated bouts each of 30%, 40%, and 50% of maximal voluntary contraction (MVC) force (LAB session), and seven repeated bouts of isometric handgrip (IHG) at 40% MVC force (fMRI session), were performed, with each contraction lasting 20 s and separated by 40 s of rest. There was a main effect of group (P = 0.03) on HR responses across all IHG intensities. Compared with control, CAD demonstrated less task-dependent deactivation in the posterior cingulate cortex and medial prefrontal cortex, and reduced activation in the right anterior insula, bilateral precentral cortex, and occipital lobe (P < 0.05). When correlated with HR, CAD demonstrated reduced activation in the bilateral insula and posterior cingulate cortex, and reduced deactivation in the dorsal anterior cingulate cortex, and bilateral precentral cortex (P < 0.05). The increased variability in expected autonomic regions and decrease in total cortical activation in response to the IHG task are associated with a diminished HR response to volitional effort in CAD. Therefore, relative to similarly aged and healthy individuals, CAD impairs the heart rate response and modifies the cortical patterns associated with cardiovascular control during IHG.

Reversal of functional changes in the brain associated with obstructive sleep apnoea following 6 months of CPAP

Obstructive sleep apnoea (OSA) is associated with an increase in the number of bursts of muscle sympathetic nerve activity (MSNA), leading to neurogenic hypertension. Continuous positive airway pressure (CPAP) is the most effective and widely used treatment for preventing collapse of the upper airway in OSA. In addition to improving sleep, CPAP decreases daytime MSNA towards control levels. It remains unknown how this restoration of MSNA occurs, in particular whether CPAP treatment results in a simple readjustment in activity of those brain regions responsible for the initial increase in MSNA or whether other brain regions are recruited to over-ride aberrant brain activity. By recording MSNA concurrently with functional Magnetic Resonance Imaging (fMRI), we aimed to assess brain activity associated with each individual subject's patterns of MSNA prior to and following 6 months of CPAP treatment. Spontaneous fluctuations in MSNA were recorded via tungsten microelectrodes inserted into the common peroneal nerve in 13 newly diagnosed patients with OSA before and after 6 months of treatment with CPAP and in 15 healthy control subjects while lying in a 3 T MRI scanner. Blood Oxygen Level Dependent (BOLD) contrast gradient echo, echo-planar images were continuously collected in a 4 s ON, 4 s OFF (200 volumes) sampling protocol. MSNA was significantly elevated in newly diagnosed OSA patients compared to control subjects (55 ± 4 vs 26 ± 2 bursts/min). Fluctuations in BOLD signal intensity in multiple regions covaried with the intensity of the concurrently recorded bursts of MSNA. There was a significant fall in MSNA after 6 months of CPAP (39 ± 2 bursts/min). The reduction in resting MSNA was coupled with significant falls in signal intensity in precuneus bilaterally, the left and right insula, right medial prefrontal cortex, right anterior cingulate cortex, right parahippocampus and the left and right retrosplenial cortices. These data support our contention that functional changes in these suprabulbar sites are, via projections to the brainstem, driving the augmented sympathetic outflow to the muscle vascular bed in untreated OSA.

•

Obstructive sleep apnoea increases muscle sympathetic nerve activity (MSNA).

•

fMRI was used to identify brain sites temporally coupled to the increase in MSNA.

•

Augmented BOLD signal intensity occurred in several cortical and subcortical sites.

•

These changes were reversed following 6 months of CPAP, which reduced the MSNA.

Functional and structural changes in the brain associated with the increase in muscle sympathetic nerve activity in obstructive sleep apnoea

Muscle sympathetic nerve activity (MSNA) is greatly elevated in patients with obstructive sleep apnoea (OSA) during daytime wakefulness, leading to hypertension, but the underlying mechanisms are poorly understood. By recording MSNA concurrently with functional Magnetic Resonance Imaging (fMRI) of the brain we aimed to identify the central processes responsible for the sympathoexcitation. Spontaneous fluctuations in MSNA were recorded via tungsten microelectrodes inserted percutaneously into the common peroneal nerve in 17 OSA patients and 15 healthy controls lying in a 3 T MRI scanner. Blood Oxygen Level Dependent (BOLD) contrast gradient echo, echo-planar images were continuously collected in a 4 s ON, 4 s OFF (200 volumes) sampling protocol. Fluctuations in BOLD signal intensity covaried with the intensity of the concurrently recorded bursts of MSNA. In both groups there was a positive correlation between MSNA and signal intensity in the left and right insulae, dorsolateral prefrontal cortex (dlPFC), dorsal precuneus, sensorimotor cortex and posterior temporal cortex, and the right mid-cingulate cortex and hypothalamus. In OSA the left and right dlPFC, medial PFC (mPFC), dorsal precuneus, anterior cingulate cortex, retrosplenial cortex and caudate nucleus showed augmented signal changes compared with controls, while the right hippocampus/parahippocampus signal intensity decreased in controls but did not change in the OSA subjects. In addition, there were significant increases in grey matter volume in the left mid-insula, the right insula, left and right primary motor cortices, left premotor cortex, left hippocampus and within the brainstem and cerebellum, and significant decreases in the mPFC, occipital lobe, right posterior cingulate cortex, left cerebellar cortex and the left and right amygdala in OSA, but there was no overlap between these structural changes and the functional changes in OSA. These data suggest that the elevated muscle vasoconstrictor drive in OSA may result from functional changes within these brain regions, which are known to be directly or indirectly involved in the modulation of sympathetic outflow via the brainstem. That there was no overlap in the structural and functional changes suggests that asphyxic damage due to repeated episodes of nocturnal obstructive apnoea is not the main cause of the sympathoexcitation.

•

Obstructive sleep apnea increases muscle sympathetic nerve activity (MSNA).

•

fMRI was used to identify brain sites temporally coupled to the increase in MSNA.

•

Augmented BOLD signal intensity occurred in several cortical and subcortical sites.

•

The elevated MSNA in OSA may result from functional changes within these sites.

Cerebral blood flow links insulin resistance and baroreflex sensitivity

Insulin resistance confers risk for diabetes mellitus and associates with a reduced capacity of the arterial baroreflex to regulate blood pressure. Importantly, several brain regions that comprise the central autonomic network, which controls the baroreflex, are also sensitive to the neuromodulatory effects of insulin. However, it is unknown whether peripheral insulin resistance relates to activity within central autonomic network regions, which may in turn relate to reduced baroreflex regulation. Accordingly, we tested whether resting cerebral blood flow within central autonomic regions statistically mediated the relationship between insulin resistance and an indirect indicator of baroreflex regulation; namely, baroreflex sensitivity. Subjects were 92 community-dwelling adults free of confounding medical illnesses (48 men, 30-50 years old) who completed protocols to assess fasting insulin and glucose levels, resting baroreflex sensitivity, and resting cerebral blood flow. Baroreflex sensitivity was quantified by measuring the magnitude of spontaneous and sequential associations between beat-by-beat systolic blood pressure and heart rate changes. Individuals with greater insulin resistance, as measured by the homeostatic model assessment, exhibited reduced baroreflex sensitivity (b = -0.16, p < .05). Moreover, the relationship between insulin resistance and baroreflex sensitivity was statistically mediated by cerebral blood flow in central autonomic regions, including the insula and cingulate cortex (mediation coefficients < -0.06, p-values < .01). Activity within the central autonomic network may link insulin resistance to reduced baroreflex sensitivity. Our observations may help to characterize the neural pathways by which insulin resistance, and possibly diabetes mellitus, relates to adverse cardiovascular outcomes.