Circadian rhythm is controlled by

(A) Suprechiasmatic nuclei # CIRCADIAN RHYTHMS:> Most if not all living cells plants and animals have rhythmic fluctuations in their function that are about 24 hours in length - i.e., they are circadian.> Most of the cells in the body have circadian rythms.> In liver these are all other cells the rhythms are entrained by the paired suprachiasmatic nuclei (SCN), one on either side, above the optic chiasm.> Have rhythmic fluctuations in their function that are about 24 hours in length--ie, they are circadian (L circa "about" + dia "day").> Normally they become entrained, ie, synchronized to the day-night light cycle in the environment.> If they are not entrained, they become progressively more out of phase with light-dark cycle because they are longer or shorter than 24 hours.> These nuclei receive information about the light-dark cycle via a special neural pathway, retinohypothalamic fibers that pass from the optic chiasm to the SCN.> Efferents from the SCN initiate neural & humoral signals that entrain a wide variety of well-known circadian rhythms. These include the rhythms in the secretion of ACTH and other pituitary hormones, the sleep-wake cycle, activity patterns, and the secretion of the pineal hormone melatonin.> Neurons in the SCN discharge rhythmically when removed from the body and cultured in vitro.> Genes in these neurons are activated diurnally, and their protein products enter the cytoplasm, where they modify cell function and thus neuronal discharge.