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Sleep and dreams
Sub-Topics
The Sleep/ Dream/ Wake Cycle

Linked
Help Rhythms of life: the biological clocks that control the daily lives of all living things Melatonin and Circadian Rhythm Disorder Suprachiasmatic nucleus
Voies nerveuses entre le NSC et la glande pinéale Sigma receptor modulation of noradrenergic-stimulated pineal melatonin biosynthesis in rats La Pinéale, la Mélatonine et les Rythmes Biologiques The pineal gland
Researcher
Giles E. Duffield
Experiment
Circadian Rhythms in the Suprachiasmatic Nucleus are Temperature-Compensated and Phase-Shifted by Heat Pulses In Vitro
History
Biological Clocks: Museum
Original modules
Tool Module: Your “Mental Stopwatch” Your “Mental Stopwatch”

Melatonin receptors are present on the neurons of the suprachiasmatic nuclei of most species. This suggests that the secretion of melatonin is regulated by a negative feedback loop. Experiments in which the administration of exogenous melatonin affected the circadian rhythm of locomotor activity in various rodents have also shown that this hormone can affect the functioning of the body’s biological clock.

Tool Module : Cybernetics
THE SUPRACHIASMATIC NUCLEI AND THE PINEAL GLAND

The source of the human body’s circadian rhythms lies in the suprachiasmatic nuclei (SCNs), which constitute the central oscillator in the human biological clock. Each of these two nuclei in the left and right anterior hypothalamus comprises several tens of thousands of especially small neurons and has its own spontaneous rhythm of biochemical and electrical activity. But this rhythm is entrained by and synchronized with daylight through a neural tract that runs from the retina to the SCNs.

From the SCNs, information is then relayed to several different brain structures, among them the pineal gland, via complex multineuronal pathways. Because the hypothalamus and the pineal gland are located close together in the diencephalon, one might expect that the neural pathways connecting them would be direct, but they are not. Instead, these pathways make a long detour from the hypothalamus via the spinal cord before returning to the pineal gland.

 

During the daytime, the activity of each SCN reduces that of another part of the hypothalamus, the paraventricular nucleus (in the diagram to the left, this inhibition is represented by a red arrow). The axons of the paraventricular nucleus then descend to the preganglionic sympathetic neurons of the lateral horn of the spinal cord. In turn, these cells modulate the excitability of the neurons of the superior cervical ganglia, whose axons finally project to the pineal gland.

This entire excitatory pathway is shown in green in the diagram above. Because this circuit contains only one inhibitory connection—the one from the SCN to the paraventricular nucleus—one can readily understand how the excitation of the SCN by daylight ultimately reduces the production of melatonin by the pineal gland. Conversely, when the sun sets, the effect of this inhibiting connection diminishes, thus enabling the excitatory connections to increase the secretion of melatonin by this gland.

The main neurotransmitter regulating the activity of the pineal gland is norepinephrine. When norepinephrine binds to its receptors, it triggers a cascade of second messengers, including adenylate cyclase and its product, cyclic AMP. This cyclic AMP contributes to the synthesis of melatonin from its precursor, tryptophane. 

This melatonin is released into the bloodstream, through which it reaches every organ in the body. That is how it participates in the modulation of the circuits of the brainstem that ultimately control the sleep-wake cycle.

The suprachiasmatic nuclei were once regarded as homogeneous structures, but now, like other nuclei, are instead regarded as sets of distinct, interconnected functional units. On the basis of the neuropeptides produced by the various neurons of the SCNs and the functional organization of the pathways leading into and out of them, brain anatomists now consider each SCN to consist of a ventral SCN and a dorsal SCN.

The neurons of the ventral SCN are now believed to function not so much as clocks but rather as the location in the SCN that receives and responds to external inputs, while the neurons of the dorsal SCN are believed to constitute the SCN’s actual robust, endogenous clock. This view is supported by certain jet-lag experiments which have shown that in rats, the process by which a light stimulus resets the internal clock occurs far more rapidly in the ventral SCN than in the dorsal SCN.

Scientists have now discovered that the neurotransmitter GABA excites the cells of the dorsal SCN but inhibits those of the ventral SCN. These opposing effects might influence the differing reaction times of these two sub-regions when someone travels across several time zones. This discovery thus opens new insights into the mechanisms behind the disturbing symptoms of jet lag.


Source: Hugues Dardente and Nicolas Cermakian, Médecines/Science,
Volume 21, Number 1 (January 2005)

Link : Les noyaux suprachiasmatiques : une horloge circadienne composée Link : The clock in the dorsal suprachiasmatic nucleus runs faster than that in the ventral Link : A GABAergic mechanism is necessary for coupling dissociable ventral and dorsal regional oscillators within the circadian clock Link : An Abrupt Shift in the Day/Night Cycle Causes Desynchrony in the Mammalian Circadian Center

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