Journal of Applied Physiology
Vol. 82, No. 2, pp. 480-490, February 1997
CONTROL OF BREATHING, CIRCULATION, AND TEMPERATURE

Putative projection of phrenic afferents to the limbic cortex in humans studied with cerebral-evoked potentials

Christian Straus^1,2, Marc Zelter², Jean-Philippe Derenne¹, Bernard Pidoux³, Jean-Claude Willer³, and Thomas Similowski¹

¹ Laboratoire de Physiopathologie Respiratoire du Service de Pneumologie, ² Service Central d'Explorations Fonctionnelles Respiratoires, and ³ Service d'Explorations Fonctionnelles du Système Nerveux, Groupe Hospitalier Pitié-Salpêtrière, 75651 Paris cédex 13, France

ABSTRACT

INTRODUCTION

METHODS

RESULTS

DISCUSSION

ACKNOWLEDGEMENTS

FOOTNOTES

REFERENCES

ABSTRACT

Straus, Christian, Marc Zelter, Jean-Philippe Derenne, Bernard Pidoux, Jean-Claude Willer, and Thomas Similowski. Putative projection of phrenic afferents to the limbic cortex in humans studied with cerebral-evoked potentials. J. Appl. Physiol. 82(2): 480-490, 1997.Respiratory sensations may rely in part on cortical integration of respiratory afferent information. In an attempt to study such projections, we recorded evoked potentials at scalp and cervical sites in 10 normal volunteers undergoing transcutaneous phrenic stimulation (0.1-ms square pulses, intensity liminal for diaphragmatic activation, series of 600 shocks at 2 Hz). A negative cerebral component of peak latency (12.79 ± 0.54 ms; N13) was constant, and a negative spinal component (7.09 ± 1.04 ms; N7) could also be recorded, all results being reproducible over time. Monitoring of cardiac frequency, skin anesthesia, and stimulation adjacent to the phrenic nerve made the phrenic origin of N7 and N13 the foremost hypothesis. Increasing stimulation frequency and comparison with median nerve stimulation provided arguments for the neural nature of the signals and their cerebral origin. Recordings from intracerebral electrodes in a patient showed a polarity reversal of the evoked potentials at the level of the cingulate gyrus. In conclusion, phrenic stimulation could allow one to study projections of phrenic afferents to the central nervous system in humans. Their exact site and physiological meaning remain to be clarified.

phrenic nerve; diaphragm; cerebral cortex; respiratory afferents; control of breathing; dyspnea

INTRODUCTION

THE ACT OF BREATHING is mainly automatic, the permanent and vital cyclic contraction of the respiratory muscles being governed by activities of brain stem neuron groups that react to different stimulations to maintain homeostasis. This automatic regulation can be disrupted for voluntary tasks that involve respiratory muscles, including voluntary apnea and various respiratory maneuvers, but disruptions are most often for nonrespiratory or indirect respiratory actions. In humans, speech gives a particularly frequent and complex example of a voluntary perturbation of automatic respiration that intimately depends on a fine tuning of respiratory muscle action and coordination (38).

The volitional control of respiratory muscles implies their motor cortical representation. This motor representation has long been described in humans (13) and can be explored by techniques such as electrical stimulation (16), magnetic stimulation (24, 30), or functional imaging (5). Several structures are involved, principally the primary motor area but also other regions within the premotor cortex and the supplementary motor area (5, 13). Respiration has also important behavioral and emotional aspects, in view of which it is interesting to note that motor responses of the diaphragm can be evoked from stimulation of deeper cortical structures such as the limbic region (27). The behavior of such a complex motor system necessarily relies on the integration of afferent information, and indeed the central nervous system permanently receives and processes messages arising from numerous respiratory-related structures, including the upper and lower airways, the lung, the chest wall, and the respiratory muscles. It has long been appreciated that this "afferent" side of control of breathing is of particular pathophysiological and clinical relevance because it is the basis for load perception and probably for some aspects of dyspnea.

The technique of evoked potentials provides a convenient tool to study afferent pathways and the corresponding cortical structures. From the respiratory point of view, cortical potentials have been related to events such as inspiratory occlusion (7) or pressure pulse stimuli applied at the mouth (33). These types of stimuli do not allow one to identify the precise source of the causative afferents. In particular, the specific muscular messages, which may well play a very peculiar role as sources of dyspnea (20), cannot be disentangled from those of other sources. However, this can be achieved by specifically stimulating either the muscles themselves (15) or the phrenic nerve, which conveys diaphragm-originating information (14). Projections of phrenic nerve-mediated afferents have been demonstrated to the cerebral cortex in the cat (2, 9), and modifications of the corresponding evoked potentials by diaphragm fatigue have been described (2). In humans, Gandevia and Macefield (15) have evoked potentials from needle electrical stimulation of intercostal muscles. They were able to record on the scalp a negative component that had a latency that ranged from 21.3 to 38 ms and had a vertex maximum. In the same study, the authors attempted to elicit potentials from phrenic nerve stimulation, but they were unable to do so in a reproducible manner. Only in one of six experiments conducted in three subjects did they observe a negative scalp component in response to phrenic nerve stimulation (Fig. 5 in Ref. 15), with an onset latency of 11 ms and a peak latency of ~13-14 ms. Recently, Zifko et al. (37) have described, in a short report, somatosensory evoked potentials, the characteristics of which suggested an afferent projection of the phrenic nerve close to the cortical motor representation of the diaphragm.

We undertook the present work to look for cerebral and spinal potentials evoked by phrenic nerve stimulation in humans in a systematic manner with a twofold aim. On one hand, we tried to examine confounding factors that could compromise the use of the technique in pathophysiological or clinical assessment of the role of the diaphragm afferents in respiratory sensations and the control of breathing. On the other hand, we tried to understand what could be the nature of the neural structures responsible for the observed potentials, which seemed rather different from previously described ones (15, 37). With regard to this investigation, some observations made in a patient with intracerebral electrodes provided support of the neural origin of the phrenic nerve stimulation-related potentials and pointed to the limbic cortex as their possible source.

METHODS

Subjects

Patients

Patient 1 was a 28-yr-old man in whom an ependymoma of the fourth ventricle had been surgically removed. After surgery, he appeared completely dependent on mechanical ventilation, with a complete absence of respiratory activity after disconnection from the ventilator. Cortical magnetic stimulation showed a normal response of the diaphragm, and the diagnosis of a complete loss of chemosensitivity with a preserved efferent pathway was retained. Phrenic nerve stimulation-related evoked potentials were recorded at this point in time. Spontaneous breathing eventually reappeared, allowing the patient to be weaned from the ventilator. Chronic hypercapnia (arterial PCO₂ 55 Torr) with a diminished CO₂ ventilatory response (0.9 ml/ Torr, hyperoxic rebreathing technique) persisted as a sequela.

Patient 2 was a 36-yr-old right-handed woman who had been referred to the Salpêtrière Epilepsy Center for severe medically intractable partial epilepsy. The final diagnosis retained was medial temporal lobe epilepsy. A computed tomography scan was normal, magnetic resonance imaging showed a left hippocampus atrophy (1), and positron emission tomography imaging revealed a hypometabolic pattern involving the left amygdala and hippocampus. The implantation of two frontal and two temporal intracerebral depth electrodes was decided because doubt remained concerning the lateralization of the electroencephalographic epileptic zone. Of note, auditory evoked potentials and somatosensory evoked potentials of the median nerve were normal on both sides. At the time of the phrenic nerve stimulation study, the patient received carbamazepine (200 mg/day) and vigabatrin (1,500 mg/day) but still had occasional focal seizures.

Ethics

Recordings

Stimulations

RESULTS

Latencies are expressed as means ± SD.

Phrenic Nerve Stimulation

Other Stimulations

Paraphrenic stimulation, which involved skin, sternomastoid, and scalene muscles, resulted in signals that were radically different in shape and latency from those evoked by phrenic stimulation (Fig. 6). This was also the case for brachial plexus stimulation at the Erb's point (Fig. 6). Paraphrenic and brachial plexus stimulations were not performed in patient 2 because, with the patient's tolerance and cooperation being good but limited, we opted to multiply recordings from the intracerebral electrodes. We were confident enough that we obtained selective and good quality phrenic nerve stimulation.

Somatosensory evoked potentials of the median nerve (subjects 2, 4, 5, and 8 and patient 1) obtained by stimulation of the right median nerve at the wrist evoked a cortical negative component N20 (peak latency 20 ms) at the recording scalp sites C3 (subject 5) or C3 and T3 (subjects 2, 4, and 8). Such results are typical when a cephalic reference electrode is used (18). No other median nerve stimulation-related component could be identified and particularly no N18 component (11; see DISCUSSION).

DISCUSSION

In this study, spinal and cerebral potentials have been reproducibly evoked by phrenic nerve stimulation. The characteristics of the cerebral potentials are dissimilar from what has recently been described by Zifko et al. (37), with sites of recording and latencies being different between the two studies. Some features of the evoked potentials described here and the polarity reversal observed between two contacts of a depth cerebral electrode placed in the vicinity of the cingulate gyrus suggest that phrenic afferents could project to the limbic cortex in humans. Before this possibility and its potential applications to pathophysiological models or to patients with acute or chronic diseases are discussed, at least two points warrant discussion. One is the reality of the neural nature the potentials described, and the other is their phrenic origin.

Nature and Origin of the Phrenic Nerve Stimulation-Related Potentials

Two points remain to be examined. The first one is the reality of the neural origin of the observed deflections. As is often the case in such studies, there is and there can be no proof that the potentials recorded are not far-field EMG potentials. The fact that the amplitude of N13 was not homogeneous (Fig. 3) is against this hypothesis, but it has to be noted that the tracings were relatively similar at the different recording sites. Under our experimental conditions, the diaphragm, the sternomastoid muscle, and muscles innervated by the brachial plexus could be suspected to create such an artifact. However, the time course of the diaphragmatic M wave induced by phrenic nerve stimulation was very different from that of N13, with an onset latency shorter by 1.5-2 ms and a peak latency shorter by >3 ms. N13 was present in subjects in whom the sternomastoid was not activated by phrenic nerve stimulation (e.g., subject 7) and despite the consistent absence of brachial plexus activation. When there was a sternomastoid contraction, the time course of the corresponding M wave was radically different from that of N13. Finally, in patient 1, who was specifically studied to address methodological issues, phrenic nerve stimulation did not evoke cerebral potentials despite a normal motor diaphragmatic response. Thus the hypothesis of a far-field EMG potential does not seem very probable. In addition, the shape and latency of the recorded potentials, as well as their presence at T3, T4, and Oz, were against a possible eye-blink artifact, be it electrically induced or of the startle type.

The second point to examine is the nature of the afferents contributing to the potentials evoked by phenic nerve stimulation. Indeed, we are well aware that, the stimulus through surface electrodes being all but selective, there can be no absolute proof that sensory fibers of structures other than the phrenic nerve are activated by the applied electrical stimulation. This is the reason why we have tried to collect as many arguments as possible against possible artifacts.

That the recorded potentials were at least in part due to phrenic nerve-conveyed diaphragmatic afferents is substantiated by the fact that phrenic nerve stimulations always induced a diaphragm contraction. The intensity of stimulation, comparable to the one reported by Zifko et al. (37) in their previously mentioned study, was thus above the threshold of the motor fibers. For motor and sensory nerves, this corresponds to two to three times the threshold of sensory fibers and is adequate to obtain somatosensory-evoked potentials (18). It should, however, be noted that Zifko et al. indicated that there was no significant difference in phrenic nerve stimulation-related potentials when stimulation was performed by using needle electrodes placed in the vicinity of the nerve and when it was performed transcutaneously.

Significance of the N7 Spinal Potential

The nature of the N7 generator will certainly need further evaluation, but it can be discussed in two ways. On one hand, N7 could be interpreted as a volley ascending the dorsal column (18). On the other hand, the fact that the spinal potential probably has a horizontal dipole, as indicated by the recording of an anterior positivity (cricoid cartilage electrode) simultaneously to the posterior negativity (C₂ electrode), suggests that N7 could correspond to postsynaptic activity generated in the posterior horn and depends on a medullary relay of the afferent information carried by the phrenic nerve. The dual nature of the N7 spinal generator can also be postulated (12).

Significance of the N13 Cerebral Potential

First, increasing stimulation frequency to 15 Hz considerably reduced the amplitude of N13 in three subjects, in whom it, in fact, disappeared. Such a phenomenon has been shown to be related to the cortical nature of a component (28), although other explanations (e.g., fiber desynchronization) are possible (28). This argument has been retained by Gandevia and Macefield (15) to support the cortical nature of the potentials evoked by intercostal muscle stimulation. The persistence of N13 with increasing phrenic nerve stimulation frequency in subject 3 was, therefore, a matter of great surprise. The only difference between this subject and the rest of the group was a considerable level of training and skillfullness with respect to voluntary control of diaphragm contraction. Although this is extremely speculative, the persistence of N13 despite increasing frequency in this subject could be the reflection of possible effects of training on some characteristics of somatosensory evoked potentials (25, 29) and on the effectiveness of certain categories of synapses (36).

Second, comparison with the reasoning and discussion found in the literature about the N18 and N20 components evoked by median nerve stimulation (10) seems to us an element in favor of the cortical or immediately subcortical origin of N13. When a cephalic reference electrode at the earlobe is used, a negative component (peak 20 ms; N20) is the first to be detected after median nerve stimulation in the parietal derivations that are contralateral to the stimulation (18). It is thought to correspond to a near-field potential, namely, to have a cortical origin. Conversely, when evoked potentials from median nerve stimulation are recorded by using a montage including a noncephalic reference, N20 is preceded by an earlier negative component (N18), which is detected all over the scalp (11). It is generally admitted that N18 is a far-field component arising from activation of neuronal structures in the brain stem or in the thalamus (18). In the four subjects in whom we stimulated the median nerve at the wrist and recorded cerebral potentials with the same montage used to detect N13 (earlobe reference), we could not find N18. It is, therefore, likely that our recording conditions were not appropriate to record far-field potentials, and thus that the source of the phrenic N13 is "nearer" than the source of N18. Of note, N13 corresponds to a rather short latency projection. It is 7-8 ms shorter than the median nerve latency of 20 ms, although, considering the differences in path length and assuming similar types of paths (polysynaptic), the difference in latencies should not be >3-5 ms (37). This could be a sign that a brain structure deeper than the somatosensory cortex is recorded. It must be emphasized that this reasoning assumes similarities between N13 and N18 and N20 in terms of origin or electrical field behavior, which may or may not exist.

Third, the observations in patient 2 point to the limbic cortex and more precisely the cingulate gyrus as a projection of phrenic-mediated afferents. In this patient, all the potentials recorded at the different electrode sites, be they positive or negative, had peak latencies similar to these observed on the scalp in healthy volunteers. Unfortunately, direct comparison could not be made because the scalp was not accessible in this patient at the time of the study because of bandages that the clinician and nurses in charge of the patient did not want to be removed for reasons of asepsis. After the electrodes had been removed, the patient did not wish to participate in a research study again. It should be noted that auditive and somatosensory evoked potentials had been studied before the implantation of depth electrodes and had been found normal on both sides. It is, therefore, likely that the patient's disease had no important effect on evoked potentials. The lower contacts of the frontal electrodes recorded a positive potential with a peak latency of ~13 ms (P13), and a synchronous polarity reversal could be observed at contact 5 (N13) (Fig. 2). Therefore, the source of the P13-N13 dipole must have laid in a plane passing between contacts 4 and 5. The fact that the four posterior contacts of the right temporal electrode recorded a negative potential with a peak latency of 11.6 ms, whereas the four anterior ones were silent, could indicate that the latter were located exactly in the plane of the electrical generator and that the former were above it. Combined with the frontal electrodes pattern, this would define the direction of the plane in such a way that it encompasses, among others, four anatomic structures that could possibly be relevant as the P13-N13 electrical generator. These structures are the thalamus, the insular cortex, the hippocampus, and the cingulate gyrus. The possibility of recording evoked potentials originating in the thalamus is generally considered not very likely (18). The absence of any positive potential ~12-13 ms at the temporal electrodes makes the insular cortex as the source of N13 a weak hypothesis. A similar reasoning applies to the hippocampus, for which a polarity reversal would have been expected between two of the contacts of the temporal electrodes. The anterior part of the cingulate gyrus, therefore, remains the main hypothesis for the origin of N13. This is substantiated by the anatomic position of the frontal electrode contacts 4 and 5.

Relevance of a Phrenic Projection to the Cingulate Gyrus

A projection of phrenic afferents to the cingulate gyrus is consistent with the current concept that respiratory muscle afferents involve two separate pathways (8). In the first one, which would correspond to the potentials recorded by Zifko et al. (37), neural information arising from mechanoreceptors enters the spinal cord, ascends in the dorsal column, relays in the brain stem, projects to the thalamus, and is transmitted through a thalamocortical pathway to the sensorimotor cortex. The function of this pathway could relate to the proprioceptive control of respiratory muscles and the integration of movements originating in the motor cortex. The second pathway involves ascending afferents conveyed by the vagus nerve and its branches and possibly phrenic afferents. They relay from the brain stem to the amygdala before projecting to the mesocortex. In line with the involvement of the amygdala in visceral nociceptive processes (4), this circuit may deal with respiratory nociception and some behavioral aspects of respiration. Several facts link the diaphragm to the mesocortex. First, stimulation of the cingulate gyrus can produce various respiratory effects in animal (19), and the existence of a respiratory excitatory zone in this region has been evidenced in humans (27). There are case reports that describe the association of respiratory events with neurological lesions in the cingulate gyrus determining emotional disorders (23, 34). Second, two recent studies using different techniques have provided further arguments for the involvement of the human limbic system in the ventilatory response to CO₂. Functional magnetic resonance imaging allowed Gozal et al. (17) to show that CO₂ response was associated with changes in brain stem pattern of activation and, in some subjects, with changes in "limbic regions of the rostral brain." Positron emission tomography also demonstrated significant increases in regional cerebral blood flow in the cingulate gyrus during CO₂-stimulated breathing in conscious humans (6). Because of the very principle of the CO₂ rebreathing method, which undissociatively tests both the metabolic afferent messages and the corresponding efferent responses, the "sensitive" or "motor" nature of the areas delineated by functional imaging techniques cannot be ascertained. With our observation taken into account, it is suggested that some of the changes depicted in the limbic system by these techniques corresponded to its afferent activation, after transduction of the hypercapnia associated changes in respiratory mechanics into a neural code by diaphragm receptors.

In conclusion, we believe that the potentials described in this study could well be the reflection of a projection of phrenic afferents to the limbic cortex in humans. Caution is obviously needed in interpreting these findings, which warrant further investigation and the use of different models. However, they could contribute to a better understanding of the role of respiratory muscles afferent in nociceptive perceptions such as dyspnea.

ACKNOWLEDGEMENTS

The authors are extremely grateful to Drs. Claude Adam and Didier Dormont for the invaluable help that they provided by allowing them to study patient 2 and for their contribution to the description of the case and of the magnetic resonance imaging data. We thank Profs. Alain Lockhart and Alain Harf for their support and Dr. François Bellemare for stimulating discussions about phrenic nerve stimulation evoked potentials a few years ago. Finally, we are truly indebted to Dr. Duncan Newton for help with English.

FOOTNOTES

Address for reprint requests: T. Similowski, Service de Pneumologie et de Réanimation, Groupe Hospitalier Pitié-Salpêtrière, 47-83 Blvd. de l'Hôpital, 75651 Paris cédex 13, France (E-mail: thomas.similowski{at}psl.ap-hop-paris.fr).

Received 6 September 1995; accepted in final form 15 October 1996.

REFERENCES