EXTERNAL INTERCOSTAL AND PHRENIC ALPHA-MOTOR RESPONSES TO CHANGES IN RESPIRATORY LOAD

Stochastic electrical stimulation of the thoracic or cervical regions with surface electrodes facilitates swallow in rats

Introduction

Aspiration pneumonia, a leading cause of mortality, poses an urgent challenge in contemporary society. Neuromuscular electrical stimulation (NMES) has been commonly used in dysphagia rehabilitation. However, given that NMES at motor threshold targets only specific muscles, it carries a potential risk of further compromising functions related to swallowing, respiration, and airway protection. Considering that the swallow motor pattern is orchestrated by the entire swallow pattern generator (the neural mechanism governing a sequence of swallow actions), a rehabilitation approach that centrally facilitates the entire circuit through sensory nerve stimulation is desirable. In this context, we propose a novel stimulation method using surface electrodes placed on the back to promote swallowing.

Methods

The efficacy of the proposed method in promoting swallowing was evaluated by electrically stimulating sensory nerves in the back or neck. Probabilistic stimulus was applied to either the back or neck of male and female rats. Swallows were evoked by an oral water stimulus, and electromyographic (EMG) activity of the mylohyoid, thyroarytenoid, and thyropharyngeus muscles served as the primary outcome measure.

Results

Gaussian frequency stimulation applied to the skin surface of the thoracic back elicited significant increases in EMG amplitude of all three swallow-related muscles. Neck stimulation elicited a significant increase in EMG amplitude of the thyroarytenoid during swallow, but not the mylohyoid or thyropharyngeus muscles.

Discussion

While the targeted thoracic spinal segments T9-T10 have been investigated for enhancing respiration, the promotion of swallowing through back stimulation has not been previously studied. Furthermore, this study introduces a new probabilistic stimulus based on Gaussian distribution. Probabilistic stimuli have been reported to excel in nerve stimulation in previous research. The results demonstrate that back stimulation effectively facilitated swallow more than neck stimulation and suggest potential applications for swallowing rehabilitation.

Diaphragm stimulation elicits phrenic afferent-induced neuromuscular plasticity

We hypothesized that activation of phrenic afferents induces diaphragm motor plasticity. In anesthetized and spontaneously breathing rats we delivered 40 Hz, low threshold (twitch and 1.5X twitch threshold), inspiratory-triggered stimulation to the left hemidiaphragm for 30 min to activate ipsilateral phrenic afferents. Diaphragm amplitude ipsilateral and contralateral to stimulation were increased for 60 min following both currents compared to time controls not receiving stimulation. Diaphragm stimulation was repeated in laminectomy controls or following a unilateral C3-C6 dorsal rhizotomy to eliminate phrenic afferent volleys. Laminectomy controls expressed neuromuscular plasticity post-stimulation. In contrast, ipsilateral and contralateral diaphragm amplitude following dorsal rhizotomy was lower than laminectomy controls and no different than time controls, suggesting diaphragm motor plasticity was not induced post-rhizotomy. Our results indicate that diaphragm stimulation induces a novel form of plasticity in the phrenic motor system which requires phrenic afferent activation. Respiratory motor plasticity elicited by diaphragm stimulation may have value as a therapeutic strategy to improve diaphragm output in neuromuscular conditions.

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Transcutaneous electromyographic respiratory muscle recordings to quantify patient-ventilator interaction in mechanically ventilated children

Background

To explore the feasibility of transcutaneous electromyographic respiratory muscle recordings to automatically quantify the synchronicity of patient–ventilator interaction in the pediatric intensive care unit.

Methods

Prospective observational study in a tertiary paediatric intensive care unit in an university hospital. Spontaneous breathing mechanically ventilated children < 18 years of age were eligible for inclusion. Patients underwent a 5-min continuous recording of ventilator pressure waveforms and transcutaneous electromyographic signal of the diaphragm. To evaluate patient–ventilator interaction, the obtained neural inspiration and ventilator pressurization timings were used to calculate trigger and cycle-off errors of each breath. Calculated errors were displayed in the dEMG-phase scale.

Results

Data of 23 patients were used for analysis. Based on the dEMG-phase scale, the median rates of synchronous, dyssynchronous and asynchronous breaths as classified by the automated analysis were 12.2% (1.9–33.8), 47.5% (36.3–63.1), and 28.9% (6.6–49.0).

Conclusions

The dEMG-phase scale quantifying patient–ventilator breath synchronicity was demonstrated to be feasible and a reliable scale for mechanically ventilated children, reflected by high intra-class correlation coefficients. As this non-invasive tool is not restricted to a type of ventilator, it could easily be clinical implemented in the ventilated pediatric population. However; correlation studies between the EMG signal measured by surface EMG and esophageal catheters have to be performed.

Effects of unilateral airway occlusion on rib motion and inspiratory intercostal activity in dogs

Unilateral bronchial occlusion, a complication of many lung diseases, causes dyspnea but the mechanism of this symptom is uncertain. In this study, electromyographic (EMG) activity in the parasternal and external intercostal muscles in the third intercostal space and inspiratory motion of the third rib on both sides of the thorax were assessed during occlusion of a main bronchus for a single breath in anesthetized dogs. Occlusion produced a 65% increase in external intercostal EMG activity in both hemithoraces without altering parasternal EMG activity. Concomitantly, the inspiratory cranial rib motion showed a 50% decrease on both sides of the thorax. These changes were unaffected by bilateral vagotomy. However, when an external, caudally oriented force was applied to the third rib on the right or left side so that its inspiratory cranial displacement was abolished, activity in the adjacent external intercostals showed a twofold increase, but rib motion and external activity in the contralateral hemithorax remained unchanged. It is concluded that during occlusion of a main bronchus, the increase in external intercostal activity is induced by the decrease in inspiratory cranial rib displacement in both hemithoraces, and that this decrease is determined by the increase in pleural pressure swings on both sides of the mediastinum. This mechanism, combined with the decrease in PaO₂, induces similar alterations when unilateral bronchial occlusion is maintained for a series of consecutive breaths.

Histological identification of phrenic afferent projections to the spinal cord

Limited data are available regarding the spinal projections of afferent fibers in the phrenic nerve. We describe a method that robustly labels phrenic afferent spinal projections in adult rats. The proximal end of the cut phrenic nerve was secured in a microtube filled with a transganglionic tracer (cholera toxin β-subunit, CT-β, or Cascade Blue) and tissues harvested 96-h later. Robust CT-β labeling occurred in C3-C5 dorsal root ganglia cell bodies and phrenic afferent projections were identified in the mid-cervical dorsal horn (laminae I-III), intermediate grey matter (laminae IV, VII) and near the central canal (laminae X). Afferent fiber labeling was reduced or absent when CT-β was delivered to the intrapleural space or directly to the hemidiaphragm. Soaking the phrenic nerve with Cascade Blue also produced robust labeling of mid-cervical dorsal root ganglia cells bodies, and primary afferent fibers were observed in spinal grey matter and dorsal white matter. Our results show that the 'nerve soak' method effectively labels both phrenic motoneurons and phrenic afferent projections, and show that primary afferents project throughout the ipsilateral mid-cervical gray matter.

Diaphragm electromyographic activity following unilateral midcervical contusion injury in rats

Contusion-type injuries to the spinal cord are characterized by tissue loss and disruption of spinal pathways. Midcervical spinal cord injuries impair the function of respiratory muscles and may contribute to significant respiratory complications. This study systematically assessed the impact of a 100-kDy unilateral C4 contusion injury on diaphragm muscle activity across a range of motor behaviors in rats. Chronic diaphragm electromyography (EMG) was recorded before injury and at 1 and 7 days postinjury (DPI). Histological analyses assessed the extent of perineuronal net formation, white-matter sparing, and phrenic motoneuron loss. At 7 DPI, ∼45% of phrenic motoneurons were lost ipsilaterally. Relative diaphragm root mean square (RMS) EMG activity increased bilaterally across a range of motor behaviors by 7 DPI. The increase in diaphragm RMS EMG activity was associated with an increase in neural drive (RMS value at 75 ms after the onset of diaphragm activity) and was more pronounced during higher force, nonventilatory motor behaviors. Animals in the contusion group displayed a transient decrease in respiratory rate and an increase in burst duration at 1 DPI. By 7 days, following midcervical contusion, there was significant perineuronal net formation and white-matter loss that spanned 1 mm around the injury epicenter. Taken together, these findings are consistent with increased recruitment of remaining motor units, including more fatigable, high-threshold motor units, during higher force, nonventilatory behaviors. Changes in diaphragm EMG activity following midcervical contusion injury reflect complex adaptations in neuromotor control that may increase the risk of motor-unit fatigue and compromise the ability to sustain higher force diaphragm efforts.

NEW & NOTEWORTHY

The present study shows that unilateral contusion injury at C4 results in substantial loss of phrenic motoneurons but increased diaphragm muscle activity across a range of ventilatory and higher force, nonventilatory behaviors. Measures of neural drive indicate increased descending input to phrenic motoneurons that was more pronounced during higher force, nonventilatory behaviors. These findings reveal novel, complex adaptations in neuromotor control following injury, suggestive of increased recruitment of more fatigable, high-threshold motor units.

Tracheal occlusion-evoked respiratory load compensation and inhibitory neurotransmitter expression in rats

Respiratory load compensation is a sensory-motor reflex generated in the brain stem respiratory neural network. The nucleus of the solitary tract (NTS) is thought to be the primary structure to process the respiratory load-related afferent activity and contribute to the modification of the breathing pattern by sending efferent projections to other structures in the brain stem respiratory neural network. The sensory pathway and motor responses of respiratory load compensation have been studied extensively; however, the mechanism of neurogenesis of load compensation is still unknown. A variety of studies has shown that inhibitory interconnections among the brain stem respiratory groups play critical roles for the genesis of respiratory rhythm and pattern. The purpose of this study was to examine whether inhibitory glycinergic neurons in the NTS were activated by external and transient tracheal occlusions (ETTO) in anesthetized animals. The results showed that ETTO produced load compensation responses with increased inspiratory, expiratory, and total breath time, as well as elevated activation of inhibitory glycinergic neurons in the caudal NTS (cNTS) and intermediate NTS (iNTS). Vagotomized animals receiving transient respiratory loads did not exhibit these load compensation responses. In addition, vagotomy significantly reduced the activation of inhibitory glycinergic neurons in the cNTS and iNTS. The results suggest that these activated inhibitory glycinergic neurons in the NTS might be essential for the neurogenesis of load compensation responses in anesthetized animals.

Asynchrony, neural drive, ventilatory variability and COMFORT: NAVA versus pressure support in pediatric patients. A non-randomized cross-over trial

Purpose

To determine if neurally adjusted ventilatory assist (NAVA) improves asynchrony, ventilatory drive, breath-to-breath variability and COMFORT score when compared to pressure support (PS).

Methods

This is a non-randomized short-term cross-over trial in which 12 pediatric patients with asynchrony (auto-triggering, double triggering or non-triggered breaths) were enrolled. Four sequential 10-min periods of data were recorded after 20 min of ventilatory stabilization (wash-out) at each of the following settings: baseline PS with the ventilator settings determined by the attending physician (1-PS_b); PS after optimization (2-PS_opt); NAVA level set so that maximum inspiratory pressure (P_max) equaled P_max in PS (3-NAVA); same settings as in 2-PS_opt (4-PS_opt).

Results

The median asynchrony index was significantly lower during NAVA (2.0 %) than during 2-PS_opt (8.5 %, p = 0.017) and 4-PS_opt (7.5 %, p = 0.008). In NAVA mode, the NAVA trigger accounted on average for 66 % of triggered breaths. The median trigger delay with respect to neural inspiratory time was significantly lower during NAVA (8.6 %) than during 2-PS_opt (25.2 %, p = 0.003) and 4-PS_opt (28.2 %, p = 0.0005). The median electrical activity of the diaphragm (EAdi) change during trigger delay normalized to maximum inspiratory EAdi difference was significantly lower during NAVA (5.3 %) than during 2-PS_opt (21.7 %, p = 0.0005) and 4-PS_opt (24.6 %, p = 0.001). The coefficient of variation of tidal volume was significantly higher during NAVA (44.2 %) than during 2-PS_opt (19.8 %, p = 0.0002) and 4-PS_opt (23.0 %, p = 0.0005). The median COMFORT score during NAVA (15.0) was lower than that during 2-PS_opt (18.0, p = 0.0125) and 4-PS_opt (17.5, p = 0.039). No significant changes for any variable were observed between 1-PS_b and 2-PS_opt.

Conclusions

Neurally adjusted ventilatory assist as compared to optimized PS results in improved synchrony, reduced ventilatory drive, increased breath-to-breath mechanical variability and improved patient comfort.

Electronic supplementary material

The online version of this article (doi:10.1007/s00134-012-2535-y) contains supplementary material, which is available to authorized users.

Loading and unloading breathing during exercise: respiratory responses and compensatory mechanisms

To characterize the ventilatory responses to resistive loading or unloading, we studied the effects of breathing 79% helium-21% oxygen (He-O₂), 79% argon-21% oxygen (Ar-O₂) and 79% SF₆-21% oxygen (SF₆-O₂) on the volume-time parameters, end-tidal partial pressure of CO₂ (P_ETCO₂), mouth pressure (P_Im), work of breathing (W_I), central inspiratory activity (dP/dt_I), and electromyographic activity of parasternal inspiratory muscles (EMGps) in 10 normal subjects at rest and during short-time steady-state exercise. There were no significant changes in tidal volume (V_T), breathing frequency (f), inspiratory (T_I) and expiratory (T_E) durations, minute ventilation (V_E), and P_ETCO₂ when air was replaced by He-O₂ or SF₆-O₂ at rest. V_E and P_ETCO₂ were not significantly different after replacement of air by He-O₂ or SF₆-O₂ during exercise. However, inhalation of He-O₂ decreased in V_T and increased in f, whereas inhalation of SF₆-O₂ led to the opposite effects compared with air during exercise. Both at rest and exercise, P_Im, W_I, dP/dt_I and EMGps were significantly less during He-O₂ breathing and higher during SF₆-O₂ breathing (P < 0.01) from the first respiratory cycle after room air was replaced by He-O₂ or SF₆-O₂. Ar-O₂ breathing did not affect the time-volume parameters both at rest and during exercise compared with air. The increase in P_Im, W_I, and dP/dt_I was observed at Ar-O₂ inhalation during exercise relatively to air conditions (P < 0.05). We conclude that internal resistive loaded (SF₆-O₂) or unloaded (He-O₂) breathing changes the neuromuscular output required to maintain constant ventilation. The mechanisms of load or unload compensation seem to be mediated by afferent information from lung and respiratory muscle receptors as well as by segmentary reflexes and properties of muscle fibers.

The distribution of monosynaptic excitation from the pyramidal tract and from primary spindle afferents to motoneurones of the baboon's hand and forearm

1. Intracellular records were obtained from motoneurones innervating muscles of the baboon's forearm and hand. Monosynaptic excitatory postsynaptic potentials (EPSPs) were elicited by stimulation of motor cortex (CM EPSPs) and peripheral nerves (Ia EPSPs).2. CM EPSPs were larger on average in motoneurones innervating intrinsic hand muscles and extensor digitorum communis (EDC) than in neurones of other forearm muscles.3. Among motoneurones of the median nerve, the CM EPSP tended to be larger for cells with more rapidly conducting axons than for those with more slowly conducting axons. Among motoneurones of EDC the opposite tendency was found.4. The afferent fibres responsible for the Ia EPSP nearly always had a lower stimulus threshold than that of motor axons in the same nerve. Some observations were made concerning the distribution of heteronymous Ia EPSPs.5. Among motoneurones of a given nerve, those with large Ia EPSPs tended to receive larger CM EPSPs than did cells in which the Ia EPSP was small.6. The results are discussed in relation to problems concerning the pyramidal control of hand and finger movement.

What muscle variable(s) does the nervous system control in limb movements?

To control force accurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable in normal limb movements.

Morton's hypothesis, that length is controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements.

The idea that feedback is used to compensate for changes in muscle stiffness has received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems for velocity or viscosity are considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint.

Force Regulation of Ankle Extensor Muscle Activity in Freely Walking Cats

To gain insight into the relative importance of force feedback to ongoing ankle extensor activity during walking in the conscious cat, we isolated the medial gastrocnemius muscle (MG) by denervating the other ankle extensors and measured the magnitude of its activity at different muscle lengths, velocities, and forces accomplished by having the animals walk up and down a sloped pegway. Mathematical models of proprioceptor dynamics predicted afferent activity and revealed that the changes in muscle activity under our experimental conditions were strongly correlated with Ib activity and not consistently associated with changes in Ia or group II activity. This allowed us to determine the gains within the force feedback pathway using a simple model of the neuromuscular system and the measured relationship between MG activity and force. Loop gain increased with muscle length due to the intrinsic force–length property of muscle. The gain of the pathway that converts muscle force to motoneuron depolarization was independent of length. To better test for a causal relationship between modulation of force feedback and changes in muscle activity, a second set of experiments was performed in which the MG muscle was perturbed during ground contact of the hind foot by dropping or lifting the peg underfoot. Collectively, these investigations support a causal role for force feedback and indicate that about 30% of the total muscle activity is due to force feedback during level walking. Force feedback's role increases during upslope walking and decreases during downslope walking, providing a simple mechanism for compensating for changes in terrain.

Dynamic and static contributions to the rhythmic y activation of primary and secondary spindle endings in external intercostal muscle

1. In cats anaesthetized with sodium pentobarbital 160 external intercostal muscle spindle afferents were identified by their pause in response to ventral root stimulation; the internal intercostal muscle was denervated. 2. In order to assignate the afferents to either primary or secondary endings they were tested for their responsiveness to vibration (Bianconi & Van Der Meulen, 1963). The maximal frequency which they were able to follow regularly for at least four cycles, termed 'critical frequency', was determined. 3. The endings fell into two groups: low-f (frequency) sensitive endings with critical frequencies below 400 c/s and high-f sensitive endings with critical frequencies above 400 c/s. The latter were regarded as primary endings and the former as secondary ones. 4. The manner in which the spindle endings resumed activity after a pause produced by shocks to the ventral root, i.e. whether 'phasic' or 'tonic' (Granit & Van Der Meulen, 1962) was studied in all the spindle afferents. 5. All the secondary (low-f sensitive) endings were 'tonic' except three for which the determination of critical frequency was questionable. Both 'tonic' and 'phasic' properties were found among the primary (high-f sensitive) endings. 6. The majority of the secondary endings (74%) showed inspiratory ythmic fusimotor activation in parallel with the skeletomotor contracton as did the primary endings (79 %). 7. Fifty-seven spindle endings which all showed marked rhythmic inspiratory gamma activation were tested for respiratory variations in their dynamic responses to steady stretch and length changes introduced at low repetition rates. 8. The results indicate that both 'dynamic' and 'static' gamma fibres are represented among the rhythmic gamma fibres controlling primary muscle spindle endings, whereas rhythmic activation of secondary endings seems to be mediated only by 'static' fibres.

Respiratory muscle activity and respiratory obstruction after abdominal surgery

BACKGROUND

Respiratory movements in patients after abdominal surgery are frequently abnormal, with associated disturbances in the pattern of inspiratory pressure generation. The reasons for these abnormalities are not clear and have been attributed to impaired action of the diaphragm. However, an alternative is that partial airway obstruction could trigger reflex activation of the inspiratory ribcage muscles, which would cause a similar pattern of inspiratory pressure change. Direct measurement of electrical activity can indicate if reflex activation of inspiratory muscles occurs when partial airway obstruction is present.

METHODS

In an open study, we implanted electrodes to measure the EMG of scalene, intercostal and external oblique abdominal muscles in patients after lower abdominal surgery. Analgesia was with morphine i.v. by patient control. We used nasal cannulae to measure nasal airflow and compared EMG activity when airway obstruction was present with activity when breathing was not obstructed.

RESULTS

The pattern of activity of the different muscles was distinct. Intercostal activity reached a maximum during inspiration, before the scalene muscles, whereas scalene activity increased in phase with increasing lung volume. Abdominal muscle activity commenced when expiratory flow had ceased and continued until the next inspiration. In all three muscle groups, partial airway obstruction did not alter muscle activity.

CONCLUSIONS

Partial airway obstruction does not activate inspiratory ribcage muscles, in patients receiving morphine for postoperative analgesia after lower abdominal surgery. Changes in respiratory pressures and abnormalities of chest wall movement described in previous studies cannot be attributed to reflex responses and probably result from increased airway resistance and abdominal muscle action.