Cortical, descending and reflex control of human inspiratory muscles

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Of all the skeletal muscles, the breathing muscles perform the most crucial action, that is, they maintain ventilation. The neural control of respiratory muscles must be finely integrated to maintain ventilation while we are awake, asleep, exercising, eating and talking. The neural circuitry for the respiratory muscles is unique. Control of respiratory muscles differs from that of limb muscles because the respiratory motoneurones are activated rhythmically but are controlled via two descending pathways. During automatic breathing, descending neural drive arises from the central respiratory pattern generator in the brainstem. Through this system, disturbances are reflexly compensated for without conscious effort. However, in addition, to automatic control, respiration can also be controlled voluntarily from the motor cortex, for example during speech or a breath hold. However, in humans the neural connections and interactions between these two centres are poorly understood. In addition to descending signals, the activity of inspiratory muscles is affected by sensory reflex inputs. The proposed studies will investigate three aspects of the control of human inspiratory muscles that act to 'pump' air into the lungs and upper airway 'dilator' muscles that act to keep the airway open. Because expiration is usually passive during quiet breathing we will focus on the neural control of inspiration. First, we will investigate some of the reflex connections of human 'pump' and 'dilator' muscles in people with and without obstructive sleep apnoea. Second, we will study the descending control of inspiratory muscles using single motor unit recordings. In addition, we will make the first detailed analysis of single motor unit activity from the tongue. This muscle critically helps preserve breathing when we sleep. Finally, we will examine the interactions between the motor cortex and medulla in the control of breathing about which relatively little is known in awake humans.