Breathing is a rhythmic motor act, which is under both conscious and automatic control. This system maintains numerous controlled variables within their homeostatic ranges, but is also responsible for rapidly changing ventilation in response to often unpredictable stimuli. We will discuss the anatomy and physiology of the ventilatory control system, then address integrated responses and illustrative examples of adaptation and dysfunction in the setting of selected disease states.
The respiratory control system, broadly speaking, comprises a controller, sensors, and a plant (Fig. 11-1). This hierarchical structure, in which there is central processing of afferent input, is important for coordinating respiratory movements with behaviors such as eating, speaking, and moving.1 The controller is a neuronal network within the central nervous system (CNS), which is responsible for generating and modulating individual breaths and the overall breathing pattern. Often referred to as the respiratory central pattern generator (rCPG), the controller comprises reciprocally connected neuronal populations in the medulla and pons.2,3 Neural output from the rCPG drives the activity of various motor neuron pools. Motor neurons in the spinal cord (e.g., phrenic and intercostal) innervate the respiratory pump muscles, while brain stem motor neurons innervate upper airway muscles. The so-called “plant” includes the CO2 stores, which are made up of lung stores and circulating blood volume including hemoglobin, and is an important component of breathing control. Closed loop feedback to the controller is supplied by chemoreceptors and mechanoreceptors.
Block diagram of the respiratory control system.
The consistent cycling of the ventilatory pattern is generated spontaneously from the spatial and functional architecture of the rCPG. Intrinsic membrane properties of rhythmically active neurons within the rCPG are capable of producing automatic periodicity.4 In addition, reciprocal (excitatory and inhibitory) synaptic connections between neuronal populations in the medulla and pons are believed to be critical for the automatic generation of the respiratory rhythm.2,3
The neural respiratory cycle comprises three phases (Fig. 11-2).5 Inspiration (TI) involves ramp-like increases in inspiratory motor neuron firing, which drive phrenic nerve activity throughout this phase. The first phase of expiration (TE1) is often called post-inspiration, because inspiratory motor neurons are still active. Persistent inspiratory motor activity during TE1, which declines throughout this phase, acts to slow the exit of air from the lungs. Finally, during the second phase of expiration (TE2), expiratory muscles are typically electrically silent. During this phase of passive relaxation, gas is expelled as the lungs and chest wall return to their equilibrium state (i.e., functional residual capacity). However, under conditions where respiratory drive is increased, expiratory muscles including the internal intercostal and abdominal muscles become active during TE2. This notion is an example of ...