Central neural mechanisms in diurnal rhythm regulation and neuroendocrine responses to light

(1) Two central neural mechanisms are neceessary for the regulation of diurnal rhythms in mammals. The first is a visual pathway to mediate the effects of light in the entrainment of such rhythms. The second is an endogenous ‘clock’ mechanism. (2) The retinohypothalamic projection terminating in the suprachiasmatic hypothalamic nuclei appears to be the visual pathway which mediates entrainment of rhythms. (3) Ablation of the suprachiasmatic nuclei results in loss of diurnal rhythms indicating that this region of the hypothalamus plays an essential role in the endogenous central generation of rhythmic functions.