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Abstract
This thesis examines aspects of the neural control of breathing after spinal cord injury (SCI). Spinal injury at cervical and thoracic levels disrupts the function of respiratory muscles, with serious consequences for health. Chapter One describes the respiratory muscles and their neural control. Chapter Two shows the effect of six weeks stimulated cough training in 15 people with high-level SCI. All had weak expiratory muscles and a compromised cough. Electrical stimulation (50 Hz) was applied through surface electrodes on the thoraco-abdominal wall during a voluntary cough. Stimulation acutely improved cough and six weeks of stimulated cough training further improved abdominal (Pga) and thoracic (Poes) pressures by 25% and 36% respectively, and expiratory cough flow (PEFcough) by 16%. Lung function also improved after training. Chapter Three shows the effect of stimulus intensity on expiratory pressure, flow and volume, during a voluntary cough with added stimulation. PEFcough reached a plateau at ~210 mA. Peak values for Pga, Poes and PEFcough were 83 cm H2O, 66 cm H2O and 4 l/s, respectively. The plateau in PEFcough indicates dynamic airway compression occurred. This may help to produce a more effective cough. Chapter Four examines whether the reflex response to loading of the scalene muscles by brief airway occlusion was present in people with high-level SCI above C6. Two of 14 subjects had an early inhibitory reflex. A subsequent excitatory response occurred in five subjects. The median onset and peak latencies were longer than in able-bodied subjects, but the amplitude of the responses was similar. The low incidence of the reflex in people with high-level SCI suggests an important role for ascending signals arising from the intercostal muscle afferents in the genesis of the reflex. Chapter Five describes a new short-latency inhibitory reflex which occurs bilaterally in the scalene muscles in response to single stimuli to the 8th and 10th intercostal nerves in able-bodied subjects. The reflex was absent in people with high-level SCI. Further understanding of the neural control of respiratory muscles could help reduce morbidity and mortality due to respiratory complications in people with SCI.