Study of internal oscillations in a dynamic system through an external signal implemented in circadian rhythms

Through the analysis carried out on a dynamic model that is represented as a system of ordinary differential equations that describes the behavior of the circadian cycles; we will show and analyze in the next document what are the conditions that allow the synchronization of the circadian clock oscillator with the external modification oscillator. The implementation of this type of techniques in anatomical problems is highlighted, which are rare in the literature. The implementations will be carried out through different simulations using numerical techniques and the way in which we will determine the coupling conditions of an internal cycle of the system versus external cycles will be detailed. In the final development of this work, we will be able to see in the model without an external modification signal the existence of stable limit cycles and discover the moment in which the synchronization of the internal oscillator and the external modification signal occurs. These types of problems are common when making biological models that are described by a physical analysis.

Conditions for the emergence of circumnutations in plant roots

The plant root system shows remarkably complex behaviors driven by environmental cues and internal dynamics, whose interplay remains largely unknown. A notable example is circumnutation growth movements, which are growth oscillations from side to side of the root apex. Here we describe a model capable of replicating root growth behaviors, which we used to analyze the role of circumnuntations, revealing their emergence I) under gravitropic stress, as a combination of signal propagation and sensitivity to the signal carriers; II) as a result of the interplay between gravitropic and thigmotropic responses; and III) as a behavioral strategy to detect and react to resource gradients. The latter function requires the presence of a hypothetical internal oscillator whose parameters are regulated by the perception of environmental resources.

Roles of Direct Photoreception and the Internal Circadian Oscillator in the Regulation of Melatonin Secretion in the Pineal Organ of the Domestic Turkey: A Novel In Vitro Clock and Calendar Model

The regulation of melatonin secretion in the avian pineal organ is highly complex and shows prominent interspecies differences. The aim of this study was to determine the roles of direct photoreception and the internal oscillator in the regulation of melatonin secretion in the pineal organ of the domestic turkey. The pineal organs were collected from 12-, 13- and 14-week-old female turkeys reared under a 12 L:12 D cycle with the photophase from 07.00 to 19.00, and were incubated in superfusion culture for 3–6 days. The cultures were subjected to different light conditions including 12 L:12 D cycles with photophases between 07.00 and 19.00, 13.00 and 01.00 or 01.00 and 13.00, a reversed cycle 12 D:12 L, cycles with long (16 L:8 D) and short (8 L:16 D) photophases, and continuous darkness or illumination. The pineal organs were also exposed to light pulses of variable duration during incubation in darkness or to periods of darkness during the photophase. The secretion of melatonin was determined by direct radioimmunoassay. The turkey pineal organs secreted melatonin in a well-entrained diurnal rhythm with a very high amplitude. Direct photoreception as an independently acting mechanism was able to ensure quick and precise adaptation of the melatonin secretion rhythm to changes in light-dark conditions. The pineal organs secreted melatonin in circadian rhythms during incubation in continuous darkness or illumination. The endogenous oscillator of turkey pinealocytes was able to acquire and store information about the light-dark cycle and then to generate the circadian rhythm of melatonin secretion in continuous darkness according to the stored data. The obtained data suggest that the turkey pineal gland is highly autonomous in the generation and regulation of the melatonin secretion rhythm. They also demonstrate that the turkey pineal organ in superfusion culture is a valuable model for chronobiological studies, providing a highly precise clock and calendar. This system has several features which make it an attractive alternative to other avian pineal glands for circadian studies.

Arc Feet Effects on Stability Based on a Simple Oscillator-Driven Walking Model

Lower-extremity movement in bipedal walking is characterized by a foot-rolling motion that includes heel-strike and toe-off. We investigated the dynamical influence of this movement on walking stability using a simple walking model that has a circular arc at the end of each leg. The leg is driven by a rhythmic signal from an internal oscillator to generate walking. We focused on stability characteristics due to the arc foot based on (1) the stability region for parameters such as mass distribution and walking speed, in which the circular arc radius is optimal when it is almost the same length as the leg to maximize the stable region and (2) the rate of convergence to stable walking, which is maximized by a circular arc radius of zero. These two conflicting results imply that the optimal radius of a circular arc for local stability is a trade-off between the two criteria, reflecting a dynamic feature of bipedal walking that should be considered in biped robot design.

Sleep and its Interaction with Endocrine Rhythms

Rhythmic variations with frequences from fractions of seconds to years characterise a wide variety of biological processes (Aschoff, 1979). Biological rhythms can be observed, not only in the individual of the species, but also in the cells which comprise the organism and the populations of which it is a member. These regular fluctuations can be endogenously generated by some form of internal oscillator, or alternatively may passively reflect exogenous environmental alterations. An important group of rhythms combines both endogenous and exogenous inputs with an internal oscillator or oscillators which are capable of being influenced by some external change. In this situation, the internal rhythm is kept in harmony with an environmental cycle by a change in the outside world acting as a synchroniser or zeitgeber. In this type if the animal is artificially isolated from its normal external synchroniser, the rhythm will continue, but free running, with a periodicity which is a close approximation to the duration of the environmental cycle to which it is normally tied. These rhythms normally synchronised to an environmental cycle but capable of being self-sustaining at approximately the same rate, are termed circa rhythms: thus circadian, circannual and circalunar rhythms, according to the geophysical cycle by which they are normally entrained.

Internal synchronization among several circadian rhythms in rats under constant light

Three biological rhythms (locomotor activity, body temperature, and plasma corticosterone) were measured simultaneously in individual rats under light-dark cycles and continuous light. Spontaneous locomotor activity was recorded on an Animex and body temperature was telemetrically monitored throughout the experiments. Blood samples were obtained serially at 2-h intervals on the experimental days. Phase angles of these rhythms were calculated by a least-squares spectrum analysis. Under light-dark cycles, the acrophases of locomotor activity, body temperature, and plasma corticosterone were found at 0029, 0106, and 1940 h, respectively. When rats were exposed to 200 lx continuous light, locomotor activity and body temperature showed free-running rhythms with a period of 25.2 h on the average. Plasma corticosterone levels determined at 12 days after exposure to continuous light exhibited a circadian rhythm with the acrophase shifted to 0720. The acrophases of locomotor activity and body temperature, determined simultaneously on the same day, were found to be located at 1303 and 1358 h, respectively. Phase-angle differences among the three rhythms on the 12th day of continuous light were essentially the same with those under the light-dark cycle. These results suggest that circadian rhythms of locomotor activity, body temperature, and plasma corticosterone are most probably coupled to a common internal oscillator in the rat.