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Our Biological Clocks

Help Link : Vidéo: The Human Suprachiasmatic Nucleus Link : The Biological Clock's Mastermind

The Various Speeds at Which We Perceive Time

Until the 1950s, scientists regarded the human pineal gland as a single, probably vestigial, organ. Comparative anatomical studies had shown that this gland had the same embryological origin as the parietal (“third”) eye in other, less evolved vertebrates. Several lizards, for example, have a parietal eye seated in an opening at the top of their skulls. This “third eye”consists of a cornea, a lens, and a retina and is connected to the brain by a pedicle that is comparable to the optic nerve. In amphibians, birds, and mammals, this parietal opening has closed, so the pineal gland remains inside the skull. It has thus of course lost its visual function, but continues to exert a highly significant influence on the rest of the body through the secretion of melatonin.

Histological studies have subsequently shown that the pineal gland is not really a single organ but a dual one whose two hemispheres have nearly fused.


Most human bodily functions and behaviours are not “steady-state”. Instead, they fluctuate in 24-hour cycles, such as the sleeping and waking cycle and the cycles for body temperature, hunger, and the secretion of various hormones.

The central clock that regulates all of these circadian cycles is located in two tiny structures in the brain, at the base of the left and right hypothalamus. Each of these structures is no larger than a pencil tip and contains several tens of thousands of neurons. These structures are called the suprachiasmatic nuclei because they are located just above the optic chiasma, where the left and right optic nerves cross paths.  

This strategic position enables the suprachiasmatic nuclei to receive projections from the optic nerve that tell them the intensity of the ambient light entering the eyes. The neurons of these nuclei use this information to resynchronize themselves with daylight every day, because like any clock, the human biological clock is not perfect and does need to be reset periodically.

Despite this need to resynchronize with an external cue, it has been shown that the suprachiasmatic nuclei do in fact constitute a biological clock with its own independent rhythm. First, many experiments have shown that the fluctuations of the human circadian cycle persist even when individuals are cut off from the light of day. Second, in experiments where the suprachiasmatic nuclei were destroyed in animals such as hamsters, their cyclical behaviours, such as their sleep/wake cycles, become completely disorganized. And when suprachiasmatic nuclei were then transplanted from hamster fetuses into these animals, their biological rhythms returned, but with the properties of the donors.

These findings indicate that the mammalian biological clock mechanism is in fact not only endogenous, but also of genetic origin. Scientists have now even determined that these rhythms are the result of the cyclical activity of certain genes.

Using these cyclical rhythms, the suprachiasmatic nuclei send signals along their output pathways–for example, to the pineal gland–to regulate the cycles of a number of physiological and behavioural functions.

Melatonin, the hormone secreted by the pineal gland, was not discovered until the late 1950s. Its role in regulating biological rhythms was then uncovered only gradually, making the pineal gland the last of the endocrine glands to have its function identified.

For centuries, the role of this gland had been the subject of much speculation, some of it physiological, but some of it metaphysical. The pineal gland is the only brain structure that does not come in a pair, one to the left and one to the right of the sagittal plane (a virtual surface separating the two hemispheres), Unlike the other, paired brain structures, the pineal gland has the appearance of a single structure (see sidebar to the left) and is located exactly at the centre of this plane. This distinctive trait likely helped to feed the speculation about this gland’s role.

One of the most famous of these theories is certainly that of the French philosopher René Descartes, who considered the pineal gland the “seat” of the soul. Descartes was a dualist and regarded this unique structure as the possible interface between the material body and the immaterial soul.

For example, according to Descartes, for the woman in this illustration to become aware of the presence of the arrow, the first step would be a material one, in which the light from the arrow passed through the air and her eyes and struck her retinas. The signal would then be retransmitted, again materially, through a different medium, until it reached the pineal gland. There the signal would finally become immaterial, entering the realm of consciousness. This transformation, which even Descartes could not explain, could also take place in the opposite direction. For example, if this woman wanted to aim the arrow, the decision to do so, arising initially in her immaterial mind, would have to take material form in the pineal gland, which could then initiate the required muscle movements.

Pierre Gassendi, a philosopher who was a contemporary of Descartes, located the mysterious interface between the soul and the body in a different brain structure, the corpus callosum. In contrast, modern neurobiologists work from a purely materialist perspective and reject this Cartesian dualism.

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