The role of the circadian system in fractal neurophysiological control

Abstract

Many neurophysiological variables such as heart rate, motor activity, and neural activity are known to exhibit intrinsic fractal fluctuations - similar temporal fluctuation patterns at different time scales. These fractal patterns contain information about health, as many pathological conditions are accompanied by their alteration or absence. In physical systems, such fluctuations are characteristic of critical states on the border between randomness and order, frequently arising from nonlinear feedback interactions between mechanisms operating on multiple scales. Thus, the existence of fractal fluctuations in physiology challenges traditional conceptions of health and disease, suggesting that high levels of integrity and adaptability are marked by complex variability, not constancy, and are properties of a neurophysiological network, not individual components. Despite the subject's theoretical and clinical interest, the neurophysiological mechanisms underlying fractal regulation remain largely unknown. The recent discovery that the circadian pacemaker (suprachiasmatic nucleus) plays a crucial role in generating fractal patterns in motor activity and heart rate sheds an entirely new light on both fractal control networks and the function of this master circadian clock, and builds a bridge between the fields of circadian biology and fractal physiology. In this review, we sketch the emerging picture of the developing interdisciplinary field of fractal neurophysiology by examining the circadian system’s role in fractal regulation.

Lack of exercise leads to significant and reversible loss of scale invariance in both aged and young mice

“…Moreover, scale invariance is lost in the behavioral activity fluctuations of aged humans and in Alzheimer's disease patients, and this has been attributed to dysfunction of the SCN (12,28). Together, these results indicate that proper functioning of the SCN is crucial for scale-invariant patterns of activity (14). In the presence of running wheels, the scale-invariant correlation in activity fluctuations was improved in aging mice (1.5-and 2-y group).…”

“…Recent studies showed that scaleinvariant activity patterns are disrupted with aging and in patients suffering from dementia (12). The degree of the disruption is strongly correlated with the circadian neurotransmitter content in the suprachiasmatic nucleus (SCN) (14), the master clock in mammals that regulates circadian rhythms in physiology and behavior (15).…”

Ecological assessment of heart rate complexity: Differences between high- and low-anxious adolescents

“…Fractality is probably the most well-known index of such complexity (Pittman-Polletta et al, 2013). The self-similar, fractal nature of the human heartbeat was discovered thirty years ago (see the influential article by Goldberger, 1996) although not much research has been carried out to understand the meaning of this fractal feature in psychological anxiety-related fields.…”

Cycles, Arrows and Turbulence: Time Patterns in Renal Disease, a Path from Epidemiology to Personalized Medicine?

“…Under stable conditions in health, heart rate is not entirely regular [87]. Physiological heart rate variability (HRV) indicates an adequate connection between the heart and autonomous nervous system, as well as a coupling between 2 important biological oscillators, that is, respiration and heart rate [87,88].…”

“…The latter is characterized by a 1/f (f = frequency) pattern [88][89][90]. The 1/f pattern, in which the amplitude of the variations is inversely related to its frequency, remains similar over different time scales.…”