Receptor interactions in modulating ventilatory activity.

The ventilatory control system utilizes a variety of sensory receptor groups, including chemoreceptors and mechanoreceptors, to provide feedback concerning the status of controlled variables. Most ventilatory responses to altered receptor inputs generally involve a complex interaction between several receptor groups, central integrative mechanisms, and other modulatory inputs (e.g., "state," hormonal, or neurotransmitter status). Because the control system is complex, nonlinear, and dynamic, the ultimate ventilatory response elicited by a given stimulus is not easy to predict based on the reflex effects of individual receptor groups studied in isolation. A full understanding of the role that sensory receptors play in ventilatory control requires information concerning interactions among receptor groups and with other elements of the control system. The complexity of the problem and the lack of a uniform definition of the term "interaction" has hindered research in this area. An interaction is defined as a nonadditive relationship between independent inputs to the system. Within this definition, five domains of interaction are described. 1) Algebraic interactions occur in ventilation and/or its components because of their multiplicative and nonlinear relationship. 2) Closed-loop interactions occur because of the prevalence of feedback loops within the respiratory control system. 3) Neural interactions reflect central nervous system integration of simultaneous receptor inputs and are demonstrated when feedback loops are opened. Three subdomains of neural interactions are defined: modulatory, dynamic, and range-specific neural interactions. 4) Mechanical interactions result from nonlinear transformations of motoneuron output into mechanical actions. 5) Adaptive interactions occur when paired receptor or modulatory inputs alter future responses. To understand the role of any sensory receptor group in ventilatory control, it is necessary to define its interactions with other control system elements in each of these domains. Understanding the mechanisms of these interactions requires detailed information about the physical system subserving ventilatory control (mechanics and gas exchange) and the relevant properties of the neural network coordinating their actions.