Control of exercise hyperpnoea: Contributions from thin‐fibre skeletal muscle afferents

Abstract

The ventilatory response to dynamic submaximal exercise is immediate and proportional to metabolic rate, which maintains isocapnia. How these respiratory responses are controlled remains poorly understood, given that the most tightly controlled variable (arterial partial pressure of CO 2 /H + ) provides no error signal for arterial chemoreceptors to trigger reflex increases in ventilation. This review discusses evidence for different postulated control mechanisms, with a focus on the feedback from group III/IV skeletal muscle mechanosensitive and metabosensitive afferents. This concept is attractive, because the stimulation of muscle mechanoreceptors might account for the immediate increase in ventilation at the onset of exercise, and signals from metaboreceptors might be proportional to metabolic rate. A variety of experimental models have been used to establish the contribution of thin-fibre muscle afferents in ventilatory control during exercise, with equivocal results. The inhibition of afferent feedback via the application of lumbar intrathecal fentanyl during exercise suppresses ventilation, which provides the most compelling supportive evidence to date. However, stimulation of afferent feedback at rest has no consistent effect on respiratory output. However, evidence is emerging for synergistic interactions between muscle afferent feedback and other stimulatory inputs to the central respiratory neuronal pool. These seemingly hyperadditive effects might explain the conflicting findings encountered when using different experimental models. We also discuss the increasing evidence that patients with certain chronic diseases exhibit exaggerated muscle afferent activation during exercise, resulting in enhanced cardiorespiratory responses. This might provide a neural link between the well-established limb muscle dysfunction and the associated exercise intolerance and exertional dyspnoea, which might offer therapeutic targets for these patients. K E Y W O R D Sexercise, muscle afferent feedback, respiratory control

The Ratio of Oxygen Uptake From Ventilatory Anaerobic Threshold to Respiratory Compensation Point Is Maintained During Incremental Exercise in Older Adults

“…Several mechanisms are believed to allow the organism to rapidly maintain homeostasis in response to dynamic exercise ( Bruce et al, 2019 ). The excretion of carbon dioxide during expiration is related to pulmonary blood flow, that is, cardiac output and ventilatory capacity.…”

Physiological Function during Exercise and Environmental Stress in Humans—An Integrative View of Body Systems and Homeostasis

“…How pulmonary VE is regulated during exercise is only partly understood and subject to much controversy [126][127][128]. Beyond humoral (blood-borne) factors, which can stimulate ventilation via activation of central (medullary neurons) and peripheral (carotid bodies) chemoreceptors, ventilatory changes during exercise seem heavily reliant on fastacting neural feed-forward (i.e., an augmented central command [129]) and feedback from mechano-and metaboreceptors (particularly from locomotor muscle group III and IV afferents [130,131]). Depending on the phase of exercise (phase 1, 2 or 3 of hyperpnea), the intensity of exercise (leading to either hyperpnea or hyperventilation) and the environment (hypoxia, cold or heat exposure), the contribution-and possibly, interaction-of different signals in the drive to breathe is likely to vary.…”

Cardiorespiratory responses to muscle metaboreflex activation in fibrosing interstitial lung disease

“…However, the control of breathing during exercise is multifactorial and incompletely understood (Dempsey et al., 2014). Although historically controversial, there is accumulating evidence supporting the role of group III and IV skeletal muscle afferent feedback in human exercise hyperpnea (Bruce et al., 2019; White & Bruce, 2020). Group III and IV muscle afferents are activated during exercise by mechanical and metabolic intramuscular stimuli, and afferent signals are transmitted to medullary cardiorespiratory centres via the spinal cord (Fisher et al., 2015; Kaufman & Forster, 1996).…”