Motoneurone synchronization for intercostal and abdominal muscles: interneurone influences in two different species

The contribution of branched-axon monosynaptic inputs in the generation of short-term synchronization of motoneurones remains uncertain. Here, synchronization was measured for intercostal and abdominal motoneurones supplying the lower thorax and upper abdomen, mostly showing expiratory discharges. Synchronization in the anaesthetized cat, where the motoneurones receive a strong direct descending drive, is compared with that in anaesthetized or decerebrate rats, where the direct descending drive is much weaker. In the cat, some examples could be explained by branched-axon monosynaptic inputs, but many others could not, by virtue of peaks in cross-correlation histograms whose widths (relatively wide) and timing indicated common inputs with more complex linkages, e.g., disynaptic excitatory. In contrast, in the rat, correlations for pairs of internal intercostal nerves were dominated by very narrow peaks, indicative of branched-axon monosynaptic inputs. However, the presence of activity in both inspiration and expiration in many of the nerves allowed additional synchronization measurements between internal and external intercostal nerves. Time courses of synchronization for these often consisted of combinations of peaks and troughs, which have never been previously described for motoneurone synchronization and which we interpret as indicating combinations of inputs, excitation of one group of motoneurones being common with either excitation or inhibition of the other. Significant species differences in the circuits controlling the motoneurones are indicated, but in both cases, the roles of spinal interneurones are emphasised. The results demonstrate the potential of motoneurone synchronization for investigating inhibition and have important general implications for the interpretation of neural connectivity measurements by cross-correlation

Effects of Age and Sex on Inspiratory Muscle Activation Patterns During Exercise

Purpose: Characterize the effects of age, sex, and their interaction on inspiratory muscle activation patterns during exercise. Methods: Twenty younger (20-30y, n=10 women) and twenty older (60-80y, n=10 women) subjects performed an incremental cycle exercise test. Electromyography of the scalene (EMGsca) and sternocleidomastoid (EMGscm) muscles were measured using skin surface electrodes, while diaphragm electromyography (EMGdi) and esophageal and transdiaphragmatic pressures were measured using an esophageal catheter. Electromyography data were transformed into root-mean-square with a 100ms time constant. Esophageal (PTPes) and diaphragmatic (PTPdi) pressure-time products were used as indices of total inspiratory muscle pressure production and diaphragmatic pressure production, respectively. Results: At absolute minute ventilations (V E), women and older subjects had greater EMGdi than men and younger subjects, respectively (all p<0.05), but no differences were noted when V E was expressed in relative terms (all p>0.05). Women had greater EMGsca activity than men at absolute and relative levels of V E (all p<0.05). Older subjects had greater EMGsca than younger subjects when V E was expressed relative (all p<0.05) but not absolute terms (all p>0.05). At absolute and relative levels of V E, women and older subjects had greater EMGscm than men and younger subjects, respectively (all p<0.05). Women and older subjects had a greater PTPdi/PTPes at a V E of 70 l·min-1 than men and younger subjects, respectively (both p<0.05), but no differences were noted when V E was expressed in relative terms (all p>0.05). No significant interactions between age and sex were noted (all p>0.05). Conclusion: Age and sex significantly affect inspiratory muscle activation patterns during exercise; however, the extent of the effects depends on whether comparisons are made at a solute or relative V E.This study was supported by the British Columbia Lung Association (BCLA). YMS, PBD, and AHR were supported by graduate scholarships from the Natural Sciences and Engineering Research Council of Canada (NSERC). PBD and MRS were supported by fellowships from the University of British Columbia and BCLA. JAG was supported by a Scholar Award from the Michael Smith Foundation for Health Research, a New Investigator Award from the Providence Health Care Research Institute and St. Paul’s Hospital Foundation, and a Canadian Institutes of Health Research Clinical Rehabilitation New Investigator Award

Manipulation of mechanical ventilatory constraint during moderate intensity exercise does not influence dyspnoea in healthy older men and women

© 2018 The Authors. The Journal of Physiology © 2018 The Physiological Society. KEY POINT SUMMARY:The perceived intensity of exertional breathlessness (i.e. dyspnoea) is higher in older women than in older men, possibly due to sex-difference respiratory system morphology. During exercise at a given absolute intensity or minute ventilation, older women have a greater degree of mechanical ventilatory constraint (i.e. work of breathing and expiratory flow limitation) than their male counterparts, which may lead to a greater perceived intensity of dyspnoea. Using a single-blind randomized study design, we experimentally manipulated the magnitude of mechanical ventilatory constraint during moderate-intensity exercise at ventilatory threshold in healthy older men and women. We found that changes in the magnitude of mechanical ventilatory constraint within the physiological range had no effect on dyspnoea in healthy older adults. When older men and women perform submaximal exercise at a moderate intensity, mechanical ventilatory constraint does not contribute significantly to the sensation of dyspnoea. We sought to determine the effect of manipulating mechanical ventilatory constraint during submaximal exercise on dyspnoea in older men and women. METHODS:Eighteen healthy subjects (60-80 y; 9 men, 9 women) completed two days of testing. On Day 1, subjects performed pulmonary function testing and a maximal incremental cycle exercise test. On Day 2, subjects performed three 6-min bouts of cycling at ventilatory threshold, in a single-blind randomized manner, while breathing: i) normoxic helium-oxygen (HEL) to reduce the work of breathing (Wb ) and alleviate expiratory flow limitation (EFL); ii) through an inspiratory resistance (RES) of ∼5 cmH2 O·l-1 ·s-1 to increase Wb ; and iii) ambient air as a control (CON). Oesophageal pressure, diaphragm electromyography, and sensory responses (using the category-ratio 10 Borg scale) were monitored throughout exercise. RESULTS:During the HEL condition, there was a significant decrease in Wb (men: -21 ± 6%, women: -17 ± 10%) relative to CON (both p < 0.01). Moreover, if EFL was present during CON (4 men, 5 women), it was alleviated during HEL. Conversely, during the RES condition, Wb (men: 42 ± 19%, women: 50 ± 16%) significantly increased relative to CON (both p < 0.01). There was no main effect of sex on Wb (p = 0.59). Across conditions, women reported significantly higher dyspnoea intensity than men (2.9 ± 0.9 vs. 1.9 ± 0.8 Borg scale units, p < 0.05). Despite significant differences in the degree of mechanical ventilatory constraint between conditions, dyspnoea intensity was unaffected, independent of sex (p = 0.46). CONCLUSION:When older men and women perform submaximal exercise at a moderate intensity, mechanical ventilatory constraint does not contribute significantly to the sensation of dyspnoea. This article is protected by copyright. All rights reserved.Natural Science and Engineering Research Council of Canada (NSERC) British Columbia Lung Association (BCLA

The effects of age and sex on mechanical ventilatory constraint and dyspnea during exercise in healthy humans

Natural Science and Engineering Research Council of Canada 590 (NSERC); British Columbia Lung Association (BCLA); University of British Columbia; Michael Smith Foundation for Health Research; Providence Health Care Research Institute; St. Paul’s Hospital Foundation; Canadian 595 Institutes of Health Research Clinical Rehabilitation

Electrophysiological and Morphological Characterization of Propriospinal Interneurons in the Thoracic Spinal Cord

Propriospinal interneurons in the thoracic spinal cord have vital roles not only in controlling respiratory and trunk muscles, but also in providing possible substrates for recovery from spinal cord injury. Intracellular recordings were made from such interneurons in anesthetized cats under neuromuscular blockade and with the respiratory drive stimulated by inhaled CO2. The majority of the interneurons were shown by antidromic activation to have axons descending for at least two to four segments, mostly contralateral to the soma. In all, 81% of the neurons showed postsynaptic potentials (PSPs) to stimulation of intercostal or dorsal ramus nerves of the same segment for low-threshold (≤5T) afferents. A monosynaptic component was present for the majority of the peripherally evoked excitatory PSPs. A central respiratory drive potential was present in most of the recordings, usually of small amplitude. Neurons depolarized in either inspiration or expiration, sometimes variably. The morphology of 17 of the interneurons and/or of their axons was studied following intracellular injection of Neurobiotin; 14 axons were descending, 6 with an additional ascending branch, and 3 were ascending (perhaps actually representing ascending tract cells); 15 axons were crossed, 2 ipsilateral, none bilateral. Collaterals were identified for 13 axons, showing exclusively unilateral projections. The collaterals were widely spaced and their terminations showed a variety of restricted locations in the ventral horn or intermediate area. Despite heterogeneity in detail, both physiological and morphological, which suggests heterogeneity of function, the projections mostly fitted a consistent general pattern: crossed axons, with locally weak, but widely distributed terminations