Abstract
In mammals, the suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-h rhythms in both physiology and behavior. The SCN is a multicellular oscillator in which individual neurons function as cell-autonomous oscillators. The production of a coherent output rhythm is dependent upon mutual synchronization among single cells and requires both synaptic communication and gap junctions. Changes in phase-synchronization between individual cells have consequences on the amplitude of the SCN’s electrical activity rhythm, and these changes play a major role in the ability to adapt to seasonal changes. Both aging and sleep deprivation negatively affect the circadian amplitude of the SCN, whereas behavioral activity (i.e., exercise) has a positive effect on amplitude. Given that the amplitude of the SCN’s electrical activity rhythm is essential for achieving robust rhythmicity in physiology and behavior, the mechanisms that underlie neuronal synchronization warrant further study. A growing body of evidence suggests that the functional integrity of the SCN contributes to health, well-being, cognitive performance, and alertness; in contrast, deterioration of the 24-h rhythm is a risk factor for neurodegenerative disease, cancer, depression, and sleep disorders.
Highlights
The rotation of the Earth around its central axis causes a daily rhythm in environmental factors, including light intensity, temperature, and food availability
The suprachiasmatic nucleus (SCN) synchronizes its circadian rhythm to the external day–night cycle using light information that is projected via the retinohypothalamic tract (RHT)
During the subjective night, the neuron–glia network in the SCN undergoes morphological rearrangements [119], and during the day, axon terminal coverage of vasoactive intestinal polypeptide (VIP)-expressing neurons increases [119]. These findings indicate that the SCN is a remarkably plastic structure that can efficiently adapt its network structure in response to changes in functional needs
Summary
The rotation of the Earth around its central axis causes a daily rhythm in environmental factors, including light intensity, temperature, and food availability. In order to anticipate these 24-h changes in the environment, many species have evolved an internal clock. In mammals, this internal clock resides in the suprachiasmatic nucleus (SCN) in the ventral hypothalamus [1]. The SCN synchronizes its circadian rhythm to the external day–night cycle using light information that is projected via the retinohypothalamic tract (RHT). This information is conveyed to other regions in the central nervous system [2, 3]. The dorsal SCN receives strong input from the ventral SCN [9], whereas the ventral SCN receives little input from the dorsal SCN [10]
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