The Suprachiasmatic Nucleus

Autor: P. J. Sollars, G. E. Pickard
Rok vydání: 2008
Předmět:
DOI: 10.1016/b978-012370880-9.00283-8
Popis: The hypothalamic suprachiasmatic nucleus (SCN) is a circadian oscillator that functions as a biological clock. Retinal efferent fibers originating predominately from intrinsically photosensitive retinal ganglion cells (ipRGCs) and traveling via the retinohypothalamic tract (RHT) provide irradiance signals to the SCN that entrain the circadian oscillator to the 24 h environmental day/night cycle. Entrainment provides a predictable and appropriate phase relationship between the endogenous and the day/night cycles, in effect enabling recognition of local time. Two different models have been proposed to explain the neural mechanisms by which the SCN circadian oscillator is entrained to the day/night cycle. The widely accepted phasic or nonparametric model proposes that entrainment is accomplished by a daily resetting of the SCN clock and is critically dependent on light-induced phase shifts of the clock occurring at dusk and/or dawn transitions in the day/night cycle. The continuous or parametric model suggests that light exerts a tonic effect on the oscillator, with the clock running faster in the light and slower in darkness. Light entrains the SCN by inducing the expression of genes that are integral components of the molecular clock mechanism. The SCN receives a dense serotonergic input arising from the midbrain median raphe nucleus. Serotonin (5-HT) acting at 5-HT1B presynaptic receptors located on RHT terminals inhibits the release of glutamate from the RHT and modulates entrainment. Although considerable progress has been made in working out the functional organization of the SCN, additional work is needed to understand more completely the details of the retinal circuitry that provides the crucial irradiance signals that enable its clock-like capacity.
Databáze: OpenAIRE
Externí odkaz: