Adjustable set point: to honor Harold T. Hammel

Abstract

The value of a regulated variable in the absence of external perturbation stabilizes at the set point of the system. This set point is an information input that may be determined by an external signal to which the regulated variable is compared or may be determined by the structural characteristics of the system itself. In the case of temperature regulation the actual internal temperature is compared with the set point "wanted" by the organism. The activating signal for the regulatory responses, the "error signal," is the difference between the actual temperature and the set point. When an error signal is detected, the organism produces the available corrective responses. Yet, the notion of thermoregulatory set point has been challenged recently. Such a questioning entails that both fever and anapyrexia are useless concepts. This minireview examines the available arguments and data and concludes that to abandon the concepts of set point, fever, and anapyrexia is premature, at best.

Homeostasis: Beyond Curt Richter

“…Cabanac (Cabanac 1997) states that the "main property of a regulatory system is that a deviation of the regulated variable triggers a correcting response, which opposes the deviation" (p. 27). It is this corrective response that distinguishes regulation from a passive steady-state (Cabanac 2006).…”

“…While it is generally accepted that corrective responses triggered by a deviation are indicative of a regulated system (Cabanac 2006), accumulating evidence indicates that regulated variables commonly achieve this stability in the absence of any obvious error signal or deviation. This position was well-stated by Somjen (Somjen 1992), "The conclusion seems almost inescapable: the central nervous system (CNS) anticipates present and future needs on the basis of past experience.…”

“…Cabanac (Cabanac 2006)discusses the issue in detail, and citing Brobeck (Brobeck 1965), he notes that the distinction has long been recognized as being critical in any discussion of homeostasis. Control refers to the ability to activate effector responses in order to stabilize a regulated variable, either by reversing a perturbation or anticipating and thus helping circumvent or minimize a perturbation.…”

“…Physiologically plausible models that give regulated systems the appearance of a set-point propose that there are thresholds for activating homeostatic responses and that they exist within hierarchically arranged, and possibly interacting, physiological control systems (Bligh 1998;Bligh 2006;Brengelmann & Savage 1997;Kanosue et al 1997;Romanovsky 2004;Satinoff 1978). When intact animals encounter naturally occurring homeostatic challenges, it is typical to find that physiological responses and motivated behaviors are activated in a coordinated manner to defend a regulated variable (Cabanac 2006). However, the specific value/range that is defended at a given moment can vary according to myriad factors (Hammel 1970;Satinoff 1996).…”

Termorregulação e depressão metabólica em endotermos.

“…Desta forma, vêem como inviável a possibilidade de existência de um set-point, como definido em termos da engenharia, posto que sob condições fisiológicas normais a temperatura corpórea flutuaria dentro da zona inter-limiares. A constatação deste padrão termorregulatório em humanos, em conjunto com o fato de que tanto a temperatura esofagiana quanto a temperatura retal (comumente experimentos são conduzidos em função desta última, a qual se supõe ser mais fidedigna a uma temperatura central ("core")) não são a temperatura no cérebro, levou Cabannac a sugerir que se esta zona nula existe é de pouca relevância fisiológica e poderia significar, simplesmente, que o intervalo de regulação da temperatura é menos vital que, por exemplo, o de concentração de cálcio no sangue (Cabannac, 2006). A concentração calcêmica é sempre garantida em 10 mg/100 ml de sangue e a melhor maneira de explicar tal defesa é através de uma mecanismo de realimentação negativa onde 10 mg/100 ml é dado como o set-point do sistema de regulação (Bonnier & Cabanac, 1970;Talmage, 2000) .…”