Determination of the human brainstem respiratory control network and its cortical connections in vivo using functional and structural imaging
Introduction
Rodent brainstem models have significantly furthered the understanding of respiratory control, addressing functional and structural mechanisms of rhythm generation (Feldman and Del Negro, 2006, Paton et al., 2006, St-John and Paton, 2004), chemoreception (i.e. the response to changes in pH and hypoxia) (Kang et al., 2007, Lahiri et al., 2006, Nattie, 2000), and connectivity of the brainstem respiratory control network (Bianchi et al., 1995, Rosin et al., 2006). In humans, the structural and functional neuroanatomy of the respiratory control system is less well understood due to the ethical and technical constraints that limit invasive studies. Brainstem activity has been observed in some functional magnetic resonance imaging (FMRI) studies of human respiration, relating to dyspnoea or volitional control of breathing (McKay et al., 2003McKay et al., 2008, Peiffer et al., 2001). There are, however no human studies specifically investigating brainstem activity relating to the automatic or unconscious control of respiration, a fundamental function that is essential for life.
In this study we examined responses to chemically stimulated breathing in healthy human volunteers with FMRI. As activity in chemoreceptive brainstem respiratory control centres is stimulated by CO2 (Feldman et al., 2003), we hypothesized that we would identify pontine and medullary activity in response to CO2 stimulation. We also expected to observe activity in subcortical centres previously identified in response to CO2 stimulation. To maximize resolution within the brainstem, FMRI was limited to a narrow field of view focused on the brainstem.
Although post mortem studies (Zec and Kinney, 2003) shed some light on the structural organisation of the human brainstem respiratory network, they are unable to demonstrate function. In the second part of this study, we used diffusion tractography to investigate how activations in the thalamus connect with higher centres in the cortex in order to differentiate their potential contributions to respiratory control.
Imaging studies of the respiratory system are challenging because changes in arterial CO2 (PaCO2) levels cause confounding effects on the blood oxygen level dependent (BOLD) signal. In this study we used novel methodology to dissociate the vasodilatory effects of CO2 from its neuronal, respiratory stimulant effects.
Carbon dioxide is a potent cerebral vasodilator, and spontaneous fluctuations in PaCO2 at rest are a significant source of low-frequency variations in the BOLD signal (Wise et al., 2004). The basis of the CO2 dissociation technique used in the present study was to compare the difference between signal changes from external administration of CO2 with the signal changes correlated with the natural, resting state fluctuations in CO2. Resting-state CO2-related fluctuations have recently been used as an FMRI scaling factor by Kannurpatti and Biswal (2008) to minimize the neural stimulation that can potentially be caused by CO2 challenges. In the present study we are interested in identifying this CO2 induced neural stimulation. Our work is therefore an extension of their technique.
We hypothesized that in the non-respiratory areas of the brain, the BOLD response to spontaneous resting state fluctuations in CO2 would represent a direct effect of CO2 on the cerebral vasculature, and that the approximately linear relationship between BOLD and PaCO2 would remain constant with administration of CO2 challenges.
In brain areas in which CO2 causes neuronal activation, we hypothesized that CO2 challenges would cause the relationship between BOLD and PaCO2 to become much stronger, due to an additional contribution to BOLD from neural activation. As resting fluctuations in CO2 are also correlated with fluctuations in breathing (Van den Aardweg and Karemaker, 2002), the additional BOLD response related to CO2-induced neural activation would represent neural activity above the normal baseline level. Direct recordings of gated activation of the respiratory network during hypercapnia (Chen et al., 1991, 1992), which is absent at normal CO2 levels, gives physiological support to our hypothesis. We therefore sought areas in the brain with increased BOLD sensitivity to CO2 during externally delivered CO2 challenges compared with the baseline “resting state”.
Section snippets
Methods
12 right-handed healthy volunteers, age 32 (SD(± 5)) years (3 female) participated in this study after giving written informed consent in accordance with the Oxfordshire Clinical Research Ethics Committee.
Respiratory changes
The main effect of the CO2 challenges on breathing was to increase minute ventilation from (mean (± SD)) 5.4 (± 1.5) l min− 1 during quiet breathing to 9.6 (± 3.4) l min− 1 P < 0.01). The respiratory rate also rose from 12.9 (± 3.3) to 14.0 (± 3.6) per min (P > 0.05) and the mean tidal volume from 460 (± 230) to 730 (± 360) ml (P < 0.001). The mean PETCO2 rose from 44.4 (± 1.1) mmHg to 47.7 (± 2.0) mmHg (P < 0.01). End tidal oxygen levels were 105 (± 4) mmHg during quiet respiration and 209 (± 1) mmHg during CO2
Discussion
In this study we have determined areas in the brainstem, thalamus and putamen that respond to CO2 stimulation. We have then investigated the connections between the thalamic areas and higher cortical centres with diffusion tractography. The main findings in the brainstem were signal increases in the inferior ventral pons and the rostral dorsal pons (Kölliker-Fuse, parabrachial nuclei and locus coeruleus) and the dorsal and lateral medulla. We observed signal increases in the left VPL, left VL
Discussion of method for dissociating neuronal from vascular CO2 related effects on the BOLD signal
Our method for dissociating neuronal from vascular CO2 effects has identified areas of neuronal activation caused by CO2 stimulation in the brainstem, thalamus and putamen. The results are supported by findings from animal literature and the emerging body of literature on respiratory FMRI in humans.
The cerebral circulation is exquisitely sensitive to small changes in PaCO2. Hypercapnia (raised PaCO2) dilates the cerebral vasculature (Hutchinson et al., 2006) and increases cerebral blood flow
Limitations of the study
- 1.
As the aim of this study was to measure chemically stimulated breathing, we did not take subjective measurements of respiration during the study, as we felt that asking subjects to think about their breathing during the experiment may have modulatory effects on respiration (Han et al., 1997). Based on findings from a pilot study, we used a relatively mild CO2 stimulus that was designed to increase minute ventilation no greater than a level that subjects just notice (West et al., 1975).
Conclusions
In summary, we have shown, for the first time in humans that stimulation with CO2 activates a network of brainstem areas that include the KF/PB nuclei and locus coeruleus in the rostral dorsal pons and nuclei in the inferior ventral pons and ventrolateral medulla. We suggest that afferents from these brainstem centres connect with nuclei in the thalamus and putamen, with synaptic connections to higher cortical centres. This is the first human study to describe the thalamus in detail with
Acknowledgments
KP and RW are supported by the Medical Research Council (UK). The study was supported by the Association of Anaesthetists of Great Britain and Ireland, and the International Anesthesia Research Society.
References (104)
- et al.
A genetic study of the acute anxious response to carbon dioxide stimulation in man
J. Psychiatr. Res.
(2007) - et al.
Probabilistic diffusion tractography with multiple fibre orientations: what can we gain?
Neuroimage
(2007) - et al.
The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes
Brain Res. Brain Res. Rev.
(2003) - et al.
Physiological noise modelling for spinal functional magnetic resonance imaging studies
Neuroimage
(2008) - et al.
Respiratory-associated thalamic activity is related to level of respiratory drive
Respir. Physiol.
(1992) - et al.
Hypercapnic normalization of bold fMRI: comparison across field strengths and pulse sequences
Neuroimage
(2004) - et al.
Does hypercapnia-induced cerebral vasodilation modulate the hemodynamic response to neural activation?
Neuroimage
(2001) - et al.
Localization of cardiac-induced signal change in fMRI
Neuroimage
(1999) - et al.
Subnuclear organization of the efferent connections of the parabrachial nucleus in the rat
Brain Res.
(1984) - et al.
Spatial flow-volume dissociation of the cerebral microcirculatory response to mild hypercapnia
Neuroimage
(2006)
A global optimisation method for robust affine registration of brain images
Med. Image Anal.
Improved optimization for the robust and accurate linear registration and motion correction of brain images
Neuroimage
Detection and scaling of task-induced fMRI-bold response using resting state fluctuations
Neuroimage
Oxygen sensing in the body
Prog. Biophys. Mol. Biol.
Cytoarchitectural organization of the parabrachial/Kölliker-Fuse complex in man
Brain Dev.
Sustained and transient neural modulations in prefrontal cortex related to declarative long-term memory, working memory, and attention
Cortex
A bilateral cortico-bulbar network associated with breath holding in humans, determined by functional magnetic resonance imaging
Neuroimage
Multiple sites for central chemoreception: their roles in response sensitivity and in sleep and wakefulness
Respir. Physiol.
Carbon dioxide regulates the tonic activity of locus coeruleus neurons by modulating a proton- and polyamine-sensitive inward rectifier potassium current
Neuroscience
Anatomical evidence of direct projections from the nucleus of the solitary tract to the hypothalamus, amygdala, and other forebrain structures in the rat
Brain Res.
Regional differences in the CBF and BOLD responses to hypercapnia: a combined pet and fMRI study
Neuroimage
Efferent connections of the parabrachial nucleus in the rat
Brain Res.
Effect of treatment with nasal continuous positive airway pressure on ventilatory response to hypoxia and hypercapnia in patients with sleep apnea syndrome
Chest
Role of pontile mechanisms in the neurogenesis of eupnea
Respir. Physiol. Neurobiol.
The cerebral signature and its modulation for pain perception
Neuron
On the origin of respiratory artifacts in bold-EPI of the human brain
Magn. Reson. Imaging
Resting fluctuations in arterial carbon dioxide induce significant low frequency variations in bold signal
Neuroimage
Differential responses to CO2 and sympathetic stimulation in the cerebral and femoral circulations in humans
J. Physiol.
A-type potassium channels differentially tune afferent pathways from rat solitary tract nucleus to caudal ventrolateral medulla or paraventricular hypothalamus
J. Physiol.
A hypercapnia-based normalization method for improved spatial localization of human brain activation with fMRI
NMR Biomed.
Imaging how attention modulates pain in humans using functional MRI
Brain
Breathlessness in humans activates insular cortex
Neuroreport
Non-invasive mapping of connections between human thalamus and cortex using diffusion imaging
Nat. Neurosci.
Depletion of putative chemosensitive respiratory neurons in the ventral medullary surface in multiple system atrophy
Brain
Tracheal occlusion modulation of gene expression in the medial thalamus (abstract)
Am. J. Respir. Crit. Care Med.
Locus coeruleus noradrenergic neurons and CO2 drive to breathing
Pflugers Arch.
Central control of breathing in mammals: neuronal circuitry, membrane properties, and neurotransmitters
Physiol. Rev.
Neuroimaging of cerebral activations and deactivations associated with hypercapnia and hunger for air
Proc. Natl. Acad. Sci. U. S. A.
Cerebral perfusion response to hyperoxia
J. Cereb. Blood Flow Metab.
Respiratory-associated rhythmic firing of midbrain neurones in cats: relation to level of respiratory drive
J. Physiol.
Effect of basal conditions on the magnitude and dynamics of the blood oxygenation level-dependent fMRI response
J. Cereb. Blood Flow Metab.
Evidence for limbic system activation during CO2-stimulated breathing in man
J. Physiol.
Variability in end-tidal pCO2 and blood gas values in humans
Exp. Physiol.
The Kölliker-Fuse nucleus gates the postinspiratory phase of the respiratory cycle to control inspiratory off-switch and upper airway resistance in rat
Eur. J. Neurosci.
The human brainstem and cerebellum
Functional MRI localisation of central nervous system regions associated with volitional inspiration in humans
J. Physiol.
Bold fMRI identifies limbic, paralimbic, and cerebellar activation during air hunger
J. Neurophysiol.
Looking for inspiration: new perspectives on respiratory rhythm
Nat. Rev. Neurosci.
Breathing: rhythmicity, plasticity, chemosensitivity
Annu. Rev. Neurosci.
Role of intracellular and extracellular ph in the chemosensitive response of rat locus coeruleus neurones
J. Physiol.
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2021, Journal of Neuroscience MethodsCitation Excerpt :One is the meaningful activation pattern obtained from corrected data, which can be related to the autonomic regulation network involved in breathing control. Thalamic activity in ventrolateral and ventral posterolateral nuclei has been previously shown in humans during CO2 challenges (Pattinson et al., 2009), in animals (Bernhardt et al., 2011; Chen et al., 1992) and in humans (Hermann et al., 2007) with sleep disordered breathing deriving from strokes localized in the thalamus. Activity in the putamen has been also observed during CO2 challenges (Pattinson et al., 2009).