Resumen:

Las fluctuaciones periódicas en diversos parámetros fisiológicos son una propiedad general de la materia viva; cuando estas fluctuaciones ocurren a intervalos regulares se consideran como «ritmos biológicos». Los ritmos biológicos son generados por un mecanismo endógeno del organismo.

Los ritmos biológicos se presentan en un amplio intervalo en frecuencias de oscilación, desde un ciclo por milisegundo a un ciclo por año. Por otra parte, el ambiente geofísico se caracteriza también por la existencia de ciclos que derivan de movimientos de la tierra y la luna en relación con el sol. Estos ciclos ambientales o geofísicos son los días, las mareas, las fases lunares y las estaciones del año. Cuando la frecuencia de un ritmo biológico se aproxima a la de un ciclo ambiental se le denomina con el prefijo "circa"; por esta razón, a los ritmos biológicos cercanos a las 24 horas se les llama ritmos circadianos.

Los ritmos circadianos representan uno de los rasgos adaptativos más ubicuos de los organismos. Así, en los mamíferos representan un importante proceso por medio del cual eventos del medio interno se organizan en una secuencia temporal apropiada que permite una máxima adaptación al medio externo. Esta característica permite al organismo predecir cambios en el ambiente geofísico asociado con el día y la noche, y prepararse para ellos.

Para llevar a cabo este papel adaptativo, los ritmos circadianos requieren que el sistema tenga la capacidad de medir el tiempo biológico, es decir, que el ritmo circadiano sea generado endógenamente, y que se pueda ajustar al tiempo geográfico, esto es, que en condiciones ambientales usuales, el periodo del oscilador se ajuste al periodo del ciclo ambiental.

El origen endógeno de la ritmicidad biológica se basa en el hecho de que, en condiciones de aislamiento de señales ambientales temporales, la ritmicidad biológica persiste, con una ligera pero significativa variación, en el valor del periodo de la oscilación. Lo anterior indica que el ritmo observado no depende de fenómenos geofísicos cíclicos, sino que el ritmo que se mantiene en condiciones constantes refleja un proceso interno del organismo. Esta capacidad esencial de los organismos para mantener la ritmicidad circadiana, aun en ausencia de señales ambientales periódicas, es conocida como ritmo en oscilación espontánea o en corrimiento libre (free running).

Sin embargo, los organismos no se encuentran aislados de señales temporales, sino que mantienen una estrecha relación temporal con las señales ambientales. Por lo anterior la fase y el periodo del ritmo transmitido puede ajustarse a la fase y al periodo de los cambios cíclicos ambientales: proceso llamado sincronización.

Se considera que las tres propiedades fundamentales de los ritmos circadianos son la persistencia del ritmo en corrimiento libre, la compensación de temperatura y la sincronización.

La palabra sincronización significa «acción de sincronizar» y ésta: «hacer que coincidan en el tiempo dos o más movimientos o fenómenos» (el término inglés entrainment proviene de la palabra francesa entrainer, «acarrear, generar»). En este contexto, la sincronización de un reloj biológico se genera por medio de un tren de estímulos controladores con un periodo determinado, que inducen a que un reloj biológico, con un periodo endógeno diferente de 24 horas, se ajuste al periodo del ciclo ambiental periódico.

La sincronización del reloj biológico proporciona al medio interno un estimado del tiempo externo. Este proceso puede ocurrir por una modulación del periodo y/o de la fase del ritmo biológico. Es decir, el periodo endógeno del ritmo biológico se ajusta al periodo del estímulo sincronizador con una relación de fase estable (o ángulo de fase) entre el sincronizador y la oscilación sincronizada.

Palabras clave: Sincronización; ritmos biológicos; sincronización luminosa; reloj biológico

Abstract

The periodic fluctuations in diverse physiological parameters are a general property of all organisms. Furthermore, when these fluctuations occur to intervals regulates these are considered as «biological rhythms». The biological rhythms are generated by an endogenous mechanism of the organism.

The biological rhythms appear in wide interval in frequencies of oscillation, which go from a cycle by millisecond to a cycle per year. Additionally, the geophysical environment is characterized by the existence of cycles deriving from movements of the earth and the moon with regard to sun. These environmental or geophysical cycles are the days, tides, lunar phases and seasons of the year. When the frequency of a biological rhythm approaches that of an environmental cycle, the prefix "circa" is used to refer to it. Likewise, 24-hour biological rhythms are designated as circadian rhythms.

The circadian rhythms represent one of the most ubiquitous adaptive characteristics of the organism. In mammals, they represent an important process through which events of the internal milieu are organized in an appropriate temporary sequence, thus enhancing a maximum adaptation to external milieu.

This characteristic allows organisms to predict and to be prepared for changes in the geophysical environment associated with the day and the night.

To carry out this adaptive role, the circadian rhythms require the biological system having the capacity to measure the biological time. Thus, the circadian rhythm should be generated endogenously, adjusting the geographical time. Moreover, under usual environmental conditions, the period of the oscillator is adjusted to the period of the environmental cycle.

The endogenous origin of the biological rhythms is based on the fact that, in temporary environmental signs isolation conditions, the biological rhythm persists with a light but significant variation in the value of the period of oscillation.

The afore mentioned considerations suggest that the rhythm observed does not depend on cyclic geophysical phenomena. Thus, the rhythm maintained under constant conditions reflects an internal organism's process. This essential ability of the organism to maintain circadian rhythms, even in the absence of periodic environmental cues, is known as rhythm in spontaneous oscillation or free-running.

Nevertheless, the organism is never isolated from temporary signals and it keeps a narrow temporary relation with the environmental cues by which the phase and the period of the overt rhythm can be adjusted to the phase and period of the environmental cyclic changes. This process is called «entrainment».

It is considered that the three fundamental properties of circadian rhythms are the persisting free-running rhythm, the temperature compensation and the entrainment.

Literally, the word entrainment means «to get aboard a train» (from the French word entramen «to carry along»). In this context, the entrainment of a biological clock is generated through a controllers stimuli train with a specific period, which induces a biological clock with a different endogenous period from 24 hours to be adjusted for the period of the periodic environmental cycle.

The entrainment of the biological clock provides to internal milieu of a reckoned of the external time. This process can occur for a modulation of the period and/or of the phase of the biological rhythm, that is, the endogenous period of the biological rhythm is adjusted to the period of the zeitgeber with a relation phase stable (or phase angle) between the zeitgeber and the oscillation entrained.

Studies where subjects were submitted to a rigorous temporary isolation indicated that only certain environmental variables are capable of acting as temporary signals for the circadian system.

In 1951, Aschoff coined the word «Zeitgeber» from the German «given of time», which describes an environmental cycle capable of affecting the period and the phase of a biological clock.

In nature, multiple environmental cues oscillate under a daily cycle, including light, darkness, temperature, humidity, availability of food and social signals. Some of these factors may act as zeitgebers of the biological clock, but the most consistent and predictable environmental signal is the 24-hour cycle of light-darkness (L:O) (photic entrainment).

Nevertheless, organisms can be entrained for other stimuli (non-photic entrainment) such as temperature, electromagnetic fields, environmental pressure, sound, availability of food and social signals.

Researchers have developed two theoretical models to explain the mechanism(s) by which the circadian clock is entrained to an environmental cycle: the discreet model (non-parametric or phasic) and the continuous model (parametric or tonic).

The model of continuous entrainment is based on the observation that the period in free running (POE) to depend of the intensity light and suggests that the light has a continuous action on the biological clock to entrain it to a cycle light-darkness (L:O).

The mechanism suggested for this is the acceleration and deceleration of the POE (angular velocity), due to daily changes in the intensity of the light, these permit to circadian pacemaker is continuously adjusted along the environmental cycle.

The discreet model has been the most utilized model to explain the entrainment to environmental cycles. The basic premise of this model is that the circadian pacemaker entrained this in equilibrium with the cycle light: darkness (L:D), which consists of brief pulses of light (zeitgeber). When a brief pulse of light falls in a specific phase of the biological clock, this produces an phase response equal to the difference between the POE and the period of the cycle entrained.

The day-night cycles generated by the rotation of the earth around its axis influence the life of the organism to a large extension. Many organisms coordinate their activities to these cycles. Some of them are diurnal, while other ones nocturnal. Moreover other animals escape from the daily periodic environment and they organize their life in constant environments as in the depth of the ocean or in natural caverns.

It is not clear how and because biological clocks with a period of approximately 24 hours evolved in cyclic environments of exactly 24 hours. A possible explanation is that the cycles L:D provide an optimum stability for their expression.

There has been as were that the cycle L:D is the first environmental signal behind the emergency and maintenance of the circadian clocks.

A large number of cell functions are affected by the light, and is being speculated that the original organisms could have restricted some of their outstanding metabolic processes at night, thus avoiding the adverse effects of the light.

In fact, some organisms adjust several of their sensitive cell processes to the light. For example, there is an augmented replication of the DNA, at night to avoid the exposition to deleterious ultraviolet radiation.

Thus it is possible to propose a hypothesis of how the circadian clocks could evolve at phylogenetically level: the ancient organisms generated a temporary program, where sensitive processes to the light were temporarily restricted to avoid the damage induced by the sunlight; these temporary programs turned out to be advantageous and thus they were selected through evolution of species.

Key words: Entrainment; biological rhythms; photic entrainment; biological clock