Thermoregulation as a non-unified system: A difficult to teach concept

Situating homeostasis in organisms: maintaining organization through time

Since it was inspired by Bernard and developed and named by Cannon, the concept of homeostasis has been invoked by many as the central theoretical framework for physiology. It has also been the target of numerous criticisms that have elicited the introduction of a plethora of alternative concepts. We argue that many of the criticisms actually target the more restrictive account of homeostasis advanced by the cyberneticists. What was crucial to Bernard and Cannon was a focus on the maintenance of the organism as the goal of physiological regulation. We analyse how Bernard's and Cannon's broad conception of what was required to maintain the organism was narrowed to negative feedback, characterized in terms of setpoints, by the cyberneticists and demonstrate how many of the alternative concepts challenge the role of setpoints - treating them as variable in light of circumstances or in anticipation of future circumstances, or as dispensable altogether. To support our analysis, we draw on the experimental and theoretical work on thermoregulation, a phenomenon that has been considered as a paradigmatic example of homeostasis and has been a common focus of those advancing alternative concepts. To integrate the insights advanced by the original proponents of homeostasis and the theorists proposing replacement notions we advance a framework in which regulation is viewed from the perspective of maintaining the organism.

The thermoregulation system and how it works

Heat exchange processes between the body and the environment are introduced. The definition of the thermoneutral zone as the ambient temperature range within which body temperature (T_b) regulation is achieved only by nonevaporative processes is explained. Thermoreceptors, thermoregulatory effectors (both physiologic and behavioral), and neural pathways and T_b signals that connect receptors and effectors into a thermoregulation system are reviewed. A classification of thermoeffectors is proposed. A consensus concept is presented, according to which the thermoregulation system is organized as a dynamic federation of independent thermoeffector loops. While the activity of each effector is driven by a unique combination of deep (core) and superficial (shell) T_bs, the regulated variable of the system can be viewed as a spatially distributed T_b with a heavily represented core and a lightly represented shell. Core T_b is the main feedback; it is always negative. Shell T_bs (mostly of the hairy skin) represent the auxiliary feedback, which can be negative or positive, and which decreases the system's response time and load error. Signals from the glabrous (nonhairy) skin about the temperature of objects in the environment serve as feedforward signals for various behaviors. Physiologic effectors do not use feedforward signals. The system interacts with other homeostatic systems by "meshing" with their loops. Coordination between different thermoeffectors is achieved through the common controlled variable, T_b. The term balance point (not set point) is used for a regulated level of T_b. The term interthreshold zone is used for a T_b range in which no effectors are activated. Thermoregulatory states are classified, based on whether: T_b is increased (hyperthermia) or decreased (hypothermia); the interthreshold zone is narrow (homeothermic type of regulation) or wide (poikilothermic type); and the balance point is increased (fever) or decreased (anapyrexia). During fever, thermoregulation can be either homeothermic or poikilothermic; anapyrexia is always a poikilothermic state. The biologic significance of poikilothermic states is discussed. As an example of practical applications of the concept presented, thermopharmacology is reviewed. Thermopharmacology uses drugs to modulate specific temperature signals at the level of a thermoreceptor (transient receptor potential channel).