Recent advances in physiological oscillations

Recent advances in physiological oscillationsPhysiological variables exhibit oscillatory patterns on many time scales (Mackey and Glass 1988, Buzsaki and Draguhn 2004, Stefanovska 2007).Compared to random behavior these periodical patterns convey to the whole system flexibility in presence of changing demands and variable environment.Indeed, a rhythmical activity enables temporal coordination among physiological process via synchronization and entrainment, guaranties efficient hierarchies among separate parts of the system, facilitates prediction of repetitive events favoring the preparation of the organism before the occurrence of event, minimizes energy expenditure, and allows storage and transmission of an infinite amount of information via frequency encoding (Rapp 1987).The multiplicity of the observed scales is the consequence of the need of living organisms to cope with sudden demands, such as during fight, and slow adaptations, such as those driven by seasons or aging, and the result of the natural selection that might have favored individuals able to extend the aforementioned advantages over a range of frequencies as wide as possible.Periodical fluctuations of physiological variables are the result of the activity of self-sustained oscillators located in various places in our body.One of these autonomous oscillators is the heart responsible for oscillations of several physiological variables including arterial pressure and blood flow at the cardiac frequency and its multiples.Self-sustained oscillators interact with each other in such a way that their rhythmical activity might be modulated by the action of others.For example, the frequency of the heart is modulated by the respiratory centers located in the brain stem through the vagal innervation of the sinus node imposing changes to the heart period synchronous with the respiratory rate that contribute to the respiratory sinus arrhythmia (Eckberg and Karemaker 2009).The activity of these autonomous oscillators perturbs mechanisms responsible for controlling physiological variables and the resulting response might appear as a periodical activity as well.For example, the cardiac-related rhythm of the integrated muscle sympathetic nerve activity is mainly due to the periodical modification of the central inhibition induced by pulse-synchronous baroreceptor nerve activity driven by arterial pressure variations (Barman et al 2003).The oscillatory behaviors of physiological variables are not only the consequence of the incessant activity of internal rhythmical sources but also of periodical external inputs.For example, synchronization between heart rate and stride frequency, usually termed cardiolocomotor synchronization, was observed during running (Kirby et al 1989) and circadian variations are observed in many physiological variables (Millar-Craig et al 1978).Physiological fluctuations occurring at frequencies below the cardiac one (i.e.below 0.5 Hz in humans in resting supine condition) is usually referred to as variability.This term gained popularity with the spreading of the analysis of the variations of inter-beat interval from the electrocardiogram (Task Force 1996).Physiological fluctuations